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Title: Synthetic Tannins, Their Synthesis, Industrial Production and Application
Author: Grasser, Georg
Language: English
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Georg Crasser, Dr. Phil., Ing.
Lecturer in Tanning Chemistry
at the German Technical College, Brunn


Whilst the synthesis of the natural tannins has been successfully
outlined by Emil Fischer, it has been left to the Chemical Industry,
notably the Badische Anilin und Soda-fabrik in
Ludwigshafen-on-the-Rhine, to discover the means of making possible the
production of the synthetic tannins.

The scientific results of Fischer's researches are to-day common
knowledge, and these, together with questions arising therefrom, will
only be lightly touched upon in the book herewith presented. Even an
attempt at enumerating the present synthetic tannins has so far not been
published, and I have therefore availed myself of the opportunity of
making a brief summary of them. My work at the B.A.S.F. deepened my
insight in this new field; ample opportunity of applying these synthetic
products in practice was given me when, as a result of the war, I was
appointed technical consultant to the Austrian Hide and Leather
Commission, and in this capacity was called upon to act as general
adviser to the trade. The ultimate object of my scientific researches
was then to investigate the chemistry of this particular field, and this
has led me to present a picture, complete as far as it goes, of this
branch of chemical technology.

The intention of the present volume is to communicate to the reader what
has so far been scientifically evolved and practically applied in this
field. First of all, however, it may illustrate the extreme importance
and the universal applicability of the synthetic tannins in the making
of leather. The modern leather industry cannot, to-day, be without these
important products, but also in those tanneries, where the synthetic
tannins have not so far been regarded as indispensable, their use is
strongly recommended. Just as in the case of the coal-tar dyes, the
synthetic tannins will make us independent of foreign supplies, and thus
keep within our own borders the vast sum of money required in former
days for the purchase of foreign tanning materials. May this book prove
the means of providing an incentive for a still wider application of the
synthetic tannins.


GRAZ, _August_ 1920.


Doctor Grasser hardly needs an introduction to the leather trade of this
country in its scientific aspect, but if one be sought for, none could
serve the purpose better than a translation of the book herewith
presented to the British-speaking public.

Viewed with curiosity from their start, the synthetic tannins
needed--like many other important discoveries--an extreme emergency for
the purpose of showing their value. The Great War provided the
opportunity of which chemical industry was to avail itself, and to-day
we do not only see synthetic tannins placed upon the market as a
veritable triumph of chemical technology and a creditable triumph of
manufacturing chemistry; we also see their immensely practical qualities
established as a fact, and, as the author aptly remarks, no modern
tanner can to-day dissociate himself from the use of synthetic tannins
for the production of leather in the true sense of this word. There is
no branch of leather-making where synthetic tannins cannot help and
improve processes already established.

The immense number of substances patented by German manufacturing
chemists for the purpose of producing synthetic tanning materials is
almost staggering. In view of this fact it is doubly pleasing to see
that British chemists have found new ways, and are able to produce
equally good and more varied synthetic tannins than has hitherto been
deemed possible. The originator of these products and his acolytes
must at least share the credit with those who, in spite of the
limitations necessarily set by the former, have been able to find new
and better ways.

In his book Dr. Grasser gives a short review of the necessary forerunner
of any work upon synthetic tannins: the investigations and syntheses of
the natural tannins. It is certainly to be hoped that we may soon see
such works as those of Fischer's and Freudenberg's, recently published,
translated into English. For the guidance of the reader it may be noted
that a short account of the works of these authors may be found in the
_Journal of the Society of Leather Trades' Chemists_, vol. v. (May
issue); in addition to this some of the matter contained in the chapter
on synthesis of tanning matters appeared in the January 1921 issue of
the _Journal of the American Leather Chemists' Association._

In addition to these two sections, the last part of this book deals with
the practical applications of synthetic tannins, and it is hoped that
the tanner will find much valuable information in these pages. The main
outlines of the synthesis of tanning matters should prove of great value
to the chemist engaged in this branch of chemical technology.

The translator takes great pleasure in the acknowledging the valuable
assistance rendered him by Mr. Robin Bruce Croad, A.R.T.C., F.I.C., and
by Mr. Arthur Harvey.



Introduction: Classification of Synthetic Tannins


The Synthesis of Vegetable Tannins

1. Tannin
2. Digallic Acid
3. Ellagic Acid
4. Depsides
     Carbomethoxylation of Hydroxybenzoic Acids
     Chlorides of Carbomethoxyhydroxybenzoic Acids
     Preparation of Didepsides
     Preparation of Tridepsides
     Preparation of Tetradepsides
     Tannoid Substances of the Tannin Type
     Chart showing the Decomposition of Products of Tannin


Synthesis of Tanning Matters

1. Aromatic Sulphonic Acids
2. Condensation of Phenols
     Condensation of Hydroxybenzene
     Condensation of Dihydroxybenzene
     Trihydroxy benzene
     Phenolic Ethers
     Nitro Bodies
     Amino Bodies
     Aromatic Alcohols
     Aromatic Acids
3. Condensation of Naphthalene Derivatives
4. Condensation of the Anthracene Group
5. Di- and Triphenylmethane Groups
6. Summary



Tanning Effects of Mixtures and Natural Products

1. Mixture of Phenolsulphonic Acid and Formaldehyde
2. Mixture of Phenolsulphonic Acid and Natural Tannins
3. Tanning Effects of Different Natural Substances


Methods of Examining Tanning Matters


Synthetic Tannins: Their Industrial Production and Application

A. Condensation of Free Phenolsulphonic Acid
B. Condensation of Partly Neutralised Phenolsulphonic Acid
C. Condensation of Completely Neutralised Phenolsulphonic Acid
D. Condensation of Cresolsulphonic Acid
E. Relative Behaviour of an Alkaline Solution of Bakelite and
   Natural Tannins
F. Dicresylmethanedisulphonic Acid (Neradol D)
     1. Neradol D Reactions
     2. Electro-Chemical Behaviour of Neradol D
     3. The Influence of Salts and Acid Contents
        on the Tanning Effect of Neradol D
     4. Phlobaphene Solubilising Action of Neradols
     5. Effect of Neradol D on Pelt
     6. Reactions of Neradol D with Iron and Alkalies
     7. Reagents suitable for Demonstrating the
        Various Stages of Neradol D Tannage
     8. Combination Tannages with Neradol D
          (1) Chrome Neradol D Liquors
          (2) Aluminum Salts and Neradol
          (3) Fat Neradol D Tannage
     9. Analysis of Leather containing Neradol D
     10. Properties of Leather Tanned with Neradol D
     11. Neradol D, Free from Sulphuric Acid
     12. Neutral Neradol
G. Different Methods of Condensation as Applied to
   Phenolsulphonic Acid
     1. Condensation Induced by Heat
     2. Condensation with Sulphur Chloride
     3. Condensation with Phosphorus Compounds
     4. Condensation with Aldehydes
     5. Condensation with Glycerol





In laying down a definition of "Synthetic Tannins," it is first of all
necessary to clearly define the conception of "tannin." Primarily,
tannins may be considered those substances of vegetable origin which may
be found, as water-soluble bodies, in many plants, exhibiting certain
chemical behaviour, possessing astringent properties and being capable
of converting animal hide into leather. This latter property of the
tannins, that of converting the easily decomposable protein of animal
hide into a permanently conserved substance and imparting to this
well-defined and technically valuable properties, has become the
criterion of the practical consideration of a tannin. It appears that
different substances certainly show the chemical reactions peculiar to
the tannins, and to a certain extent also exhibit astringent character
without, however, possessing the important property peculiar to the
tannins of converting hide into leather. Such substances, in our
present-day terminology, are termed pseudo-tannins (_e.g._, the "tannin"
contained in coffee-beans). Decomposition products of the natural
tannins, to which belong, for instance, gallic acid and the
dihydroxybenzenes, exhibit the well-known reactions of the tannins
(coloration with iron salts), but they cannot be regarded as tannins
from either a technical or a physiological standpoint.

As regards their chemical constitution, the natural (true) tannins
probably belong to different groups of organic compounds, and with our
present-day scant knowledge of their chemistry, it is impossible to
classify them. One is, however, justified in assuming that both the
natural tannins and the related humic acids are ester-derivatives of
hydroxybenzoic acids. [Footnote: E. Fischer, _Ber._, 1913, 46, 3253.]

The production of synthetic tannins employs two quite distinct
methods; one is to synthesise the most simple tannin, viz., the
tannic acid contained in galls (tannin), or to build up substances
similar in character to the tannins, from hydroxybenzoic acids. The
other, entirely new way, is to produce chemical substances, which
certainly have nothing in common with the constitution of the natural
tannins, but which behave like true tannins in contact with animal pelt,
and in addition, since they can be manufactured on a commercial scale,
are of practical value.

Owing to the fact that, until recently, the constitution of tannin has
remained unknown, it is easy to comprehend that the efforts to
synthesise the latter substance, or compounds similar to it, have been
mainly attempted on similar lines. The oldest investigation in this
direction dates from H. Schiff,[Footnote: Liebig's _Ann._, 1873, 43,
170.] who prepared substances similar to tannin by dehydrating
hydroxybenzoic acids. By allowing phosphorus oxychloride to interact
with phenolsulphonic acid, he obtained a well-defined substance
possessing tanning properties, which he considered an esterified
phenolsulphonic acid anhydride, the composition of which he determined
as HO.C_6H_4.SO_2.O.C_6H_4HSO_3. It is, however, probable that this
substance is not homogeneous, but consists of a mixture of higher
condensation products.

Klepl [Footnote: _Jour. pr. Chem._, 1883, 28, 208.] obtained--by simply
heating _p_-hydroxybenzoic acid--a so-called di- and tridepside, but
this simple method is not applicable to many other hydroxybenzoic acids,
since these are decomposed by the high temperature required to induce

Amongst other attempts to produce condensation products with
characteristics similar to those possessed by the tannins, those by
Gerhardt [Footnote: Liebig's _Ann_, 1853, 87, 159.] and Loewe [Footnote:
_Jahresh. f. Chem._, 1868, 559.] must be especially noted; they treated
gallic acid with phosphorus oxychloride or arsenic acid, and thereby
obtained amorphous compounds, exhibiting the reactions characteristic of
tanning substances. E. Fischer and Freudenberg, [Footnote: Liebig's
_Ann._, 372, 45.] by treating _p_-hydroxybenzoic acid in the same way,
succeeded in obtaining a didepside, and during the last years
practically only these two investigators have demonstrated the syntheses
of these depsides and produced high-molecular polydepsides.

At the same time researches were instituted with the object of
determining the constitution of tannin, and E. Fischer succeeded in
demonstrating its probable composition as being that of a glucoside
containing 5 molecules of digallic acid per 1 molecule of glucose.

This last-named class of synthetic tannins--which may be properly termed
"tanning matters" in contradistinction to the true tannins--exhibit very
distinct tanning character when brought in contact with animal hide, but
from the point of view of chemical constitution have nothing in common
with the natural tannins. Not only are they of interest to the industry
from a practical point of view; they have also been examined very
closely from a chemical standpoint.

It is, however, necessary to differentiate with great exactitude between
the conception of _true tanning effect_ and _pickling effect_ when
considering the action of chemical substances on pelt (i.e., animal
hide, treated with lime, depilated, and the surplus flesh
removed). Whereas any _true tannage_ is characterised by the complete
penetration of the substance and its subsequent fixation by the pelt in
such a way that a thorough soaking and washing will not bring about a
reconversion (of the leather) to the pelt state; _pickling_, on the
other hand, is only characterised by the penetration of the substance in
the pelt and fixation to such an extent that a subsequent washing of the
pickled pelt will bring back the latter to a state closely approximating
that of a true pelt. Simple as such a differentiation appears, there
are still a number of cases occupying a position between the two
referred to, and which we may term _pseudo-tannage_. An example of the
latter is formaldehyde tannage; formaldehyde has for a long time been
employed in histological work for the purpose of hardening animal hide,
by which it is readily absorbed from solution whereby it hardens the
hide without, however, swelling it. A hide which has thus been treated
with formaldehyde absorbs the natural tannins with greater ease; this,
on the one hand, argues the probability of formaldehyde acting as a
pickling agent; on the other hand, it is also one of its characteristics
that it will either in neutral acid, [Footnote: R. Combret, Ger. Pat,
112, 183.] or, still better, in alkaline [Footnote: J. Pullman,
Ger. Pat, 111,408; Griffith, _Lea. Tr. Rev._, 1908.] solution, convert
pelt into leather. In a formaldehyde-tanned leather, however, no trace
of tannin can be detected; and the yield (of leather, based on the pelt
employed), which, from a practical standpoint, is so important, is so
very low that it is hardly possible to speak of it as a tannin in the
ordinary sense of the word. Formaldehyde must, therefore, be termed a

The tanning effect of formaldehyde is, according to Thuau, [Footnote:
_Collegium_, 1909, 363, 211.] increased by those salts which bring
about colloidal polymerisation of the formaldehyde, the resultant
compounds being absorbed by the hide fibre. Fahrion considers this to be
a true tannage, and is supported by Nierenstein [Footnote: _Ibid._,
1905, 157, 159.]:--

  R.NH_2             R.NH-|
           +O.C.H. =      CH_2 + H_2O
  R.NH_2     |       R.NH-|
  (Hide.)    H      (Leather.)

A peculiar combination between true tannage and pickling is to be found
in the tawing process (tannage with potash, alum, and salt), whereby,
firstly, the salt and the acid character of the alum produce a pickling
effect, and secondly, the alum at the same time is hydrolysed, and its
dissociation components partly adsorbed by the hide, thereby effecting
true tannage. This double effect is still more pronounced in the
synthetic tannins which contain colloidal bodies of pronounced tanning
intensity on the one hand, inorganic and organic salts on the other,
which then act as described above. Their real mode of action can only
be explained with the aid of experimental data. The following chapters
will deal with the different behaviour of the various groups of
synthetic tannins.




The first investigations of gall-tannin date from the year 1770, at
which time, however, no exact differentiation between tannin and gallic
acid was made. The first step in this direction was made when
Scheele,[Footnote: Grell's _Chem. Ann._, 1787, 3, I.] in 1787,
discovered gallic acid in fermented gall extract, and in the same year
Kunzemuller [Footnote:_Ibid._, 1787,3,413.] separated gallic acid (or
pyrogallol) as a crystalline body from oak galls. Dize [Footnote:
_Jour. Chim. et Phys._, 1791, 399.] continued the investigations, which
were brought to a conclusion with Deyeux' work [Footnote: _Ann. Chim._,
1793, 17, I.]; both recognised that the substance isolated was not a
single substance, but was a mixture of gallic acid, a green colouring
matter, a rosin (tannin?), and extraneous matter. Proust [Footnote:
_Ibid._, 1799, 25, 225.] was the first to differentiate the crystalline
gallic acid from the amorphous, astringent substance, which latter he
named "Tannin."

Amongst the numerous subsequent investigations of tannin must be
especially noted the one by Berzelius [Footnote: Pogg,_Ann._, 1827, 10,
257.], who purified the potash salt and decomposed this with sulphuric
acid. Pelouze [Footnote: Liebig's _Ann._, 1843, 47, 358.], later on,
observed the formation of the crystalline gallic acid from tannin, when
the latter is boiled with sulphuric acid; this had already been observed
by J. Liebig.[Footnote: _Ibid._1843, 39, 100.] Both had noticed the
absence of nitrogen. In addition to the methods of preparation of tannin
then in vogue neutral solvents were mainly employed by subsequent
investigators; Pelouze [Footnote: _Jour. Prakt. Chem._, 1834, 2, 301,
and 328.] treated powdered galls with ether containing alcohol and
water, and considered the upper layer to be a solution of gallic acid
and impurities, the bottom layer to contain the pure tannin.

The EMPIRICAL FORMULA of tannin has also been the subject of much
speculation by the different investigators, the difficulty here being
that of obtaining a pure specimen of the substance free from sugars, and
which could be submitted to elementary analysis. Whereas these early
purified substances were thought to correspond to the formula of
digallic acid (galloylgallic acid), C_14H_10O_9, Fischer and Freudenberg
[Footnote: _Ber._, 1912, 915 and 2709.] were able to show, with
approximate certainty, that the constitution of tannin is that of a
pentadigalloyl glucose.

Early attempts at _hydrolysing tannin_ gave varying results, some
investigators claiming the presence, and others the absence of
sugars. Here, again, E. Fischer and Freudenberg [Footnote: _Ibid._] were
able to conclusively prove that on hydrolysing tannin with dilute acids,
7.9 per cent. glucose is dissociated, and that hence glucose forms part
of the tannin molecule. Fischer and Freudenberg also determined the
optical activity of pure tannin in water: [Greek: a]_D was found to lie
between +58° and +70°.

Graham found [Footnote: _Phil. Transact._, 1861, 183.] that the _tannin
molecule_ is of considerable size, since its diffusion velocity is 200
times less than that of common salt. Paternò [Footnote:
_Zeits. phys. Chem._, 1890, iv. 457.] was the first to determine the
molecular weight of tannin, employing Raoult's method; he found that
tannin in aqueous solution behaves like a colloid and that hence
Raoult's method is not applicable. When, on the other hand, he dissolved
tannin in acetic acid, results concordant with the formula of
C_14H_10O_9, corresponding to a molecular weight of 322, were
obtained. Sabanajew [Footnote: _Ibid._, 1890, v. 192.] later determined
the molecular weight of tannin in aqueous solution as 1104, in acetic
acid solution as 1113-1322, Krafft [Footnote: _Ber._, 1899, 32, 1613.]
as 1587-1626 in aqueous solution. Walden [Footnote: _Ibid._, 1898,
3167.] determined the molecular weight of tannin-schuchardt as
1350-1560, tannin-merck as 753-763, digallic acid as 307-316 (calculated
322). Feist [Footnote: _Chem. Ztg._, 1908, 918.] determined the
molecular weight of tannin as 615 and one of his own preparation as 746,
Turkish tannin as 521 and Chinese tannin as 899. In this connection it
should be noted that the calculated molecular weight of pentagalloyl
glucose, which in E. Fischer's opinion forms a substantial part of the
tannin molecule, is 940, but Fischer also thinks that this compound
possesses a much higher molecular weight.

STRUCTURE OF TANNIN--The oldest structural formula of tannin is Schiff's
digallic acid formula:--[Footnote 1: _Ber_., 1871, 4, 231.]

       ^                        ^ OH
      | |                      | |
   HO | | OH              HOOC | | OH
       V                        V

A drawback to the acceptance of this formula is the absence of an
asymmetrical C-atom; the formula, therefore, does not explain the
optical activity exhibited by tannin. Schiff attempted to overcome this
difficulty by adopting a diagonal structural formula, but even when
adopting Clauss' diagonal formula for benzene the optical activity of a
number of other compounds depends upon the existence of the asymmetrical
C-atom. Biginelli [Footnote 2: _Gazz chim. Ital_., 1909, 39, 268.] also
opposed the digallic acid formula, and supported his view by referring
to the arsenic compounds obtained by him on heating arsenic acid and
gallic acid, instead of obtaining digallic acid. Walden, [Footnote 3:
_Ber_., 1898, 31, 3168.] on the other hand, found, on analysing the
digallic acid thus prepared, only slight traces of arsenic and, by the
elementary analysis, obtained figures closely corresponding to those of
digallic acid.

Bottinger [Footnote 4: _Ibid_., 1884, 17, 1476.] prepared the so-called
_[Greek: b]_-digallic acid by heating ethyl gallate with pyroracemic
acid and sulphuric acid and proposed the so-called ketone-tannin

   HO_____OH                  ______OH
         COOH                       OH

Schiff completed this formula by a diagonal, so as to explain the
optical activity observed--

   HO     OH                  ______OH
         COOH                       OH
  [Diagonal bond between HO and COOH on left.]

The ketone formula was corroborated by Nierenstein, [Footnote: _Ber._
1905, 38, 3641.] who distilled tannin with zinc dust and obtained
diphenylmethane (smell of benzene) and a crystalline product,
M.P. 7O°-71° C. (M.P. of diphenyl = 71° C.). König and Kostanecki
[Footnote: _Ibid._, 1906, 39, 4027.] sought to find the constitution of
the tannins in the leuco-compounds of the oxyketones, to which catechin
belongs. Nierenstein (see above), however, emphasises that the high
molecular weight and the optical activity speak against the digallic
acid formula, but in favour of this are the following points: (1) the
decomposition of tannin with the formation of gallic acid; (2) the
decomposition of methylotannin with the formation of di- and trimethyl
esters of gallic acid; and (3) the production of diphenylmethane on
distillation with zinc dust. The latter reaction especially illustrates
the analogous formation of fluorene from compounds of the type--

   ^ ______ ^
  | |      | |
  | |      | |
   V        V

Nierenstein gave the name "Tannophor" to the mother-substance of tannin,
phenylbenzoate, C_6H_5-COO-C_6H_5.

Dekker [Footnote: "De Looistoffen," vol. ii, p. 30 (1908).] was,
however, unable to detect diphenylmethane on distilling with zinc dust,
and did, therefore, not accept Nierenstein's views. In proposing the

  HO ^ _ __C
    | |    |
    | |     }O
    | |    |    __OH
    | |____|_C_/  \OH
  HO V         \__/
     OH       OH  OH

Dekker [Footnote: _Ber._, 1906, 34, 2497.] was enabled to account for
most of the details in the behaviour of tannin, viz.: (1) the
empirical constitution, C_14H_10O_9; (2) the almost complete
hydrolysis into gallic acid (the dotted line indicates the
decomposition of the molecule into 2 molecules gallic acid by taking
up water); (3) the formation of diphenylmethane as a result of
distillation with zinc dust; and (4) the electrical
non-conductivity. Since tannin on acetylating yields a considerable
amount of triacetylgallic acid, it should, according to Dekker,
contain at least six acetylisable hydroxyls.

Nierenstein [Footnote: _Chem. Ztg._, 1906, 31, 880.] objected to this
formula on account of its containing seven hydroxyl groups, whereas
Dekker found six, Nierenstein five, and Herzig still fewer hydroxyl
groups. The formula would also favour the conception of tinctorial
properties which could hardly be ascribed to tannin. Lloyd [Footnote:
_Chemical News_, 1908, 97, 133.] proposed a very intricate formula
containing three digallic acid groups joined into one six-ring system,
which would then explain the optical activity; it would, on the other
hand, also require an inactive cis-form.

Iljin [Footnote: _Jour. of the Russian phys. chem. Soc._, 1908, 39,
470.] prepared two phenylhydrazine derivatives of tannin (C_74 H_58 N_8
O_30 and C_98 H_82 N_14 O_96) and proposed the formula, C_58 H_40 O_33,
the constitution of which would be--

  R_1             R_1
    |            |
    |  |         |
  R_2  |          R_2
  R_1  |          R_1
    |  |         |
    |            |
  R_2             R_2

  where  R1= CO C_6 H_2 (OH)_3
  and    R2= C_6 H_2 (OH)_2

Nierenstein [Footnote: _Ber._, 1905, 38, 3841; 1907, 40, 917; 1908, 41,
77 and 3015; 1909, 42, 1122 and 3552; _Chem. Ztg._, 1907, 31, 72; 1909,
34, 15.] considers tannin to be a mixture of digallic acid and
leucotannin, the latter possessing the formula--

     ^-------CH.OH--O----^ OH
    | |                 | |
  HO V OH           HOOC V OH

The optical activity of tannin is expressed in this formula and its
probability is corroborated by Nierenstein, who was able to resolve the
acetylated tannin by fractional precipitation into pentacetyl tannin
(M.P. 203°-208° C.) and pentacetyl leucotannin (M.P. 166° C.). By
oxidation, the former is converted into ellagic acid, and on hydrolysis
with dilute sulphuric acid readily yielded gallic acid. Hydrolysis of
the pentacetyl leucotannin, however, yielded gallic aldehyde, and
oxidation yielded purpurotannin (a naphthalene derivative) in addition
to ellagic acid.

Nierenstein [Footnote: _Ber._, 1910, 43, 628.] also succeeded in
converting tannin into carboethoxytannin, the latter on saponification
yielding crystalline, inactive digallic acid. On acetylating pentacetyl
leucotannin with acetyl chloride a hexacetyl derivative (M.P. 159° C.)
is obtained, the strychnine salt of which is resolved into both of the
active components. This proves the presence of digallic acid and
leucotannin in tannin lev. pur. Schering investigated by Nierenstein.
The latter author [Footnote: Liebig's _Ann._, 1912, 386, 318; 388, 223.]
later considered tannin to be polydigalloylleucodigallic acid anhydride
and the simplest tannin to be a digalloylleucodigallic acid
anhydride. This view, however, would not stand subsequent criticisms,
being in disagreement with the earlier observations of molecular weight
and acidic properties of tannin. Manning [Footnote: _Ibid._, 1912, 34,
918.] believed to have isolated a pentethylester of the pentagalloyl
glucoside from tannin, but this was shown to be the ethyl ester of
gallic acid.

Feist [Footnote: _Ber._, 1912, 45, 1493.] had arrived at the conclusion
that tannin was a glucose compound, and maintained that tannin from
Turkish galls was a compound of glucogallic acid combined as an ester
with 2 molecules gallic acid. But Fischer and Strauss [Footnote:
_Ibid._, 1912, 45, 3773.] synthetically prepared a glucoside of gallic
acid exhibiting differences from Feist's preparation which were so great
that the latter no longer could be considered a single glucoside of
gallic acid.

Fischer and Freudenberg [Footnote: _Ibid._, 1912, 45, 2717; 1913, 46,
1127.] subsequently elaborated a method of purifying tannin, and on
investigating the purified substance, arrived at the conclusion that
no other hydroxybenzoic acid than gallic acid was present in
tannin. On repeating Strecker's hydrolysis they obtained 7-8 per cent,
sugar, and hence concluded that 1 molecule of glucose was combined
with about 10 molecules of gallic acid. Owing to the difficulty of
isolating the intermediary hydrolysis products, and the subsequent
impossibility of drawing any conclusions as to the constitution of
tannin, the latter investigators decided to adopt the methods offered
by synthesis. Their basic idea was the absence of carboxylic groups in
tannin, and that hence the total gallic acid must be present in ester
form. These conditions are fulfilled if one views tannin as being an
ester compound of 1 molecule of glucose and 5 molecules of digallic
acid, of similar construction as, for example, pentacetyl
glucose. Fischer and Freudenberg succeeded in preparing the former by
shaking a mixture of finely powdered glucose, chloroform, and
quinoline with an excess of tricarbomethoxygalloyl chloride for
twenty-four hours and precipitating the resulting product with methyl
alcohol; suitably purified, a light amorphous colourless substance was
obtained which proved to be penta-(tricarbomethoxygalloyl)
glucose. Careful saponification with excess alkali in acetone-aqueous
solution at room temperature yielded a tannin very closely resembling
tannin, identified as pentagalloyl glucose. It is doubtful, however,
whether this substance is homogeneous, and it is probably a mixture of
two stereoisomers.

Fischer and Freudenberg, therefore, further concluded that tannin is
mainly an ester compound of glucose and 5 molecules _m_-digallic
acid. Elucidation on this point offered itself advantageously in
Herzwig's methylotannin, [Footnote: _Ber._, 1905, 38, 989.] which is
obtained by the interaction of diazomethane and tannin. The first step
was then to prepare pentamethyl-_m_-digallic acid

   CH_3.O_______          ______COOH
   CH_3.O           CH_3.O      O.CH_3

from trimethylgalloyl chloride and the _m-p_-dimethyl ether of gallic
acid; the chloride of this substance, coupled with [Greek: a]- and
[Greek: b]-glucose, yields--

      | |
     |  CH.OR        H_______O.CH_3
    |   |        R=CO{_______}O.CH_3
  O{    CH.OR        H       O
    |                        | H_____O.CH_3
     |   CH                  CO{_____}O.CH_3
      |  |	               H     O.CH_3


  [Illustration: Penta-(pentamethyl-_m_-digalloyl)-glucose.]

The [Greek: a]- and [Greek: b]-derivatives thus obtained differ in their
behaviour towards polarised light, and are, again, probably mixtures of
two stereoisomers, _i.e._, mixtures of derivatives of [Greek: a]- and
[Greek: b]-glucose. Compared to methylotannin, these preparations
exhibit very close resemblance to the former, from which it may be
concluded that they are closely related to this substance, and probably
possess the same or a very similar structure; the result of the above
experiments has, therefore, brought us at least in close proximity to
the structure of tannin. It must, however, be borne in mind that the
analysis and hydrolysis of tannin does not afford an explanation of the
question as to whether tannin is a compound of glucose and 10, 9, or 11
molecules of gallic acid; it is also possible, though not probable, that
tannin would contain a polysaccharide instead of glucose
itself. Similarly to sugar, the true glucosides can be coupled with
hydroxybenzoic acids, which is proved by the preparation of
tetra-galloyl-[Greek: a]-methyl glucoside; this substance, also,
exhibits tannoid character.


Whereas, until recently, tannin had been considered to be gallic acid
anhydride, or digallic acid, closer investigations have revealed that
neither is tannin digallic acid nor is the synthetically prepared
digallic acid identical with tannin. Schiff [Footnote: _Ber._, 1871,
231 and 967.] prepared digallic acid by the interaction of phosphorus
oxychloride and gallic acid, and believed the product obtained to be
identical with tannin; to this latter he first ascribed an ether formula
(I.), later an ester formula (II.)--

     (OH)_2      (OH)_2
      ¦¦          ¦¦
      ¦            ¦
     COOH         COOH

                ¦¦     ¦
                O      COOH

Froda [Footnote: _Gasz. chim._, 1878, 9.] held that Schiff's
condensation product contained phosphorus or arsenic acid and ascribed
its tanning properties to the latter; according to this investigator,
digallic acid, when completely freed from arsenic acid, does not react
with gelatine or quinine. Biginelli [Footnote: _Ibid._, 1909, 39,
ii. 268 and 283.] did not consider the action of arsenic acid that of a
catalyst, but held that it entered into reaction; according to his
investigations products containing arsenic (C_7H_7O_8As and
C_14H_11O_12As) are obtained when gallic acid is heated with arsenic

In his preparation of digallic acid, Iljin [Footnote:
_Jour. f. prakt. Chem._, 1911, 82, 451.] could only obtain gallic acid,
and the ethyl ether of gallic acid showing no characteristics of the
tannins; when, however, he heated gallic acid with arsenic pentoxide, he
obtained bodies exhibiting the reactions given by tannins.

Bottinger [Foonote: _Ber._, 1884, 1503.] made the first attempt at
synthesising tannin; he heated gallic acid or its ethyl ester with
glyoxylic acid or pyroracemic acid, and obtained a substance of the
composition C_14H_10O_9.2H_2O, which certainly showed some of the
characteristics exhibited by tannin, but which by no means was identical
with the latter. Bottinger's preparation is probably identical with
[Greek: b]-digallic acid, one of two dibasic isomers having the


the other possible isomer having the composition

  CO |

Fischer [Footnote: _Ber_., 1908, 41, 2875.] obtained a digallic acid
(M.P. 275°-280° C) by coupling tricarbomethoxygalloyl chloride with
dicarbomethoxygallic acid.

Nierenstein [Footnote: _Ibid_., 1910, 43, 628.] obtained, from the
carbethoxy compound of tannin, a crystalline, optically active digallic
acid, M.P. 268°-270° C. The pentacetate of this substance, obtained by
reduction and acetylisation, yielded hexacetylleucotannin. A
pentamethyldigallic acid methyl ester of the composition


was obtained by Mauthner [Footnote: _Jour. f. prakt. Chem_., 1911, 84,
140.] from the chloride of trimethylgallic acid and the methyl ester of
the acid from the glucoside of syringin; on saponification with caustic
potash the former compound yielded trimethylgallic acid and syringic

Fischer [Footnote: _Ber_., 1913, 46, 1116.] synthesised the so-called
_m_-digallic acid by coupling tricarbomethoxygalloyl chloride with
carbonylgallic acid and subsequent splitting off of CO_2. The
_m_-digallic acid appears as rather thick, colourless, microscopic
needles containing about 16 per cent. water of crystallisation,
M.P. 271° C. They are slightly soluble in cold, soluble in hot water,
and very soluble in methyl and ethyl alcohols. Their aqueous solution
gives dark blue coloration with ferric chloride, and precipitates
gelatine and quinine.

Fischer and his students [Footnote 5: _Ibid_., 1912, 45, 915, 2709;
1913, 46, 1116.] prepared quite a number of digallic acid derivatives,
amongst which are the following:--

  Pentamethyl-_m_-digallic acid methyl ester, C_20H_22O_9.
  Pentacetyl-_m_-digallic acid, C_24H_20O_14.
  Pentamethyl-_m_-digallic acid, C_19H_20O_9.
  Pentamethyl-_m_-digalloyl chloride, C_19H_19O_8Cl.
  Pentamethyl-_p_-digallic acid, C_19H_20O_9.
  Pentamethyl-_p_-digallic acid methyl ester, C_20H_22O_9.

Hydrolysis of digallic acid yields gallic acid; oxidation, on the other
hand, ellagic acid and luteic acid (Luteo Säure), which can be separated
by shaking with pyridine. The reduction of digallic acid yields, by
different methods, the same reduction compound, [Footnote: Nierenstein,
Abderhalden's "Handb. d. biochem. Arbeitsm.," vi. 154.] viz., the
racemic leucodigallic acid, which differs from digallic acid by being
devoid of any tannoid properties; the latter distinction may be ascribed
to the transformation of the tannophor group--CO.O--, to the
tannoid-inactive group CH(OH)--O--.

The successful resolving of racemic leucodigallic acid into both of its
optically active components can only be brought about through the _d_-
or _l_-hexacarbethoxyleucodigallic acid on introducing the latter into a
1 per cent. pyridine solution and heating to 45°-50° C., whereby the
_d_- or _l_-acid is formed accompanied by a strong evolution of carbon

Hydrolysis of leucogallic acid yields gallic acid and gallic aldehyde;
oxidation by means of hydrogen peroxide yields ellagic acid and luteic
acid, and oxidation with potassium persulphate and sulphuric acid, in
acetic acid solution, yields purpurotannin (see below) [Footnote:
Liebig's _Ann_., 1912, 386, 318.].

Another distinct difference between digallic acid and leucodigallic acid
is the fact that the formaldehyde condensation product of the former
resembles gallic acid, whereas that of the latter resembles tannin; it
is therefore probable that the leucodigallic acid part of the tannin
molecule imparts this characteristic property to tannin.

     ^          ^
    | |        | |
     OH         OH
  [Illustration: Digallic Acid becomes...]

     ^          ^ OH
    | |        | |
     OH         OH
  [Illustration: Luteic Acid becomes...]

     ^          ^ OH
    | |        | |
  HO V --O.CO-- V OH
     OH         OH
  [Illustration: Ellagic Acid becomes...]

      COOH     COOH
     ^ _______ ^
    | |       | |
  HO V ---O--- V OH
     OH        OH
  [Illustration: Purpuro Tannin.]

3. Ellagic Acid

Ellagic acid was discovered in 1831 by Braconnot, who named it "acide
ellagique." Its presence in the vegetable kingdom was not quite
comprehended for some time, and Nierenstein [Footnote: _Chem. Ztg._,
1909, 87.] was the first to prepare this substance from algarobilla,
dividivi, oak bark, pomegranate, myrabolarms, and valonea. The acid is
obtained by precipitating it with water from a hot alcoholic extraction
of the plants referred to, and recrystallising the precipitate from hot
alcohol. Another method of preparation consists in boiling the
disintegrated plants with dilute hydrochloric acid, washing the residue,
and extracting with hot alcohol, from which the acid will then
crystallise. According to Lowe, [Footnote: _Zeits. f. analyt. Chem._,
1875, 35.] it may be obtained from dividivi, an aqueous extract of
which is heated to 110° C. in a tube closed at both ends, when
crystalline ellagic acid is deposited. Heinemann [Footnote: Ger. Pat.,
137,033 and 137,934.] obtained ellagic acid by simply boiling
repeatedly aqueous tannin solutions.

Lowe [Footnote: _Jour. f. prakt. Chem._, 1868, 103, 464.] first
synthesised ellagic acid by heating gallic acid with arsenic acid or
silver oxide. Herzig [Footnote: _Monatshefte fur Chemie_, 1908, 29,
263.] states that ellagic acid is deposited when air is conducted
through a mixture of the ethyl or methyl ester of gallic acid and
ammonia. Perkin [Footnote: _Proc. Chem. Soc._, 1905, 21, 212.] obtained
a substance very similar to ellagic acid by electrolysis of gallic acid
in sulphuric acid solution; on oxidising gallic acid in concentrated
sulphuric acid solution, Perkin and Nierenstein [Footnote: _Ibid._,
1905, 21, 185.] obtained flavellagic acid. Ellagic acid is also
obtained by heating luteic acid in a 10 per cent. soda solution.

Ellagic acid thus prepared crystallises with 2 molecules of water as
yellow micro-crystalline rhombic prisms or prismatic needles. The
crystals lose this water when heated to 100° C., and it is possible that
it is water of constitution, in which case the substance would be
hexoxydiphenylcarboxylic acid, and the substance left after drying at
100° C., the dilactone.[Footnote: _Arch. d. Pharm_., 1907, 244, 575.]
Ellagic acid is slightly soluble in water, alcohol, and ether, but is
easily soluble in caustic potash. With concentrated nitric acid the
product assumes a red colour, which appears to be due to the presence of
impurities; ellagic acid is commercially known as "alizarin yellow."

The constitution of ellagic acid was uncertain for a long time, and
different structural formulae were proposed which more or less
corresponded to its properties. The most satisfactory structural formula
was proposed by Graebe--[Footnote: _Chem. Ztg_., 1903, 129.]

     ^ -------- ^ OH
    | |        | |
  HO V --O.CO--  V OH

This would represent a tetroxydiphenylmethylolide.

The probability of the correctness of this formula is supported
by the possibility of the following derivatives: monomethylellagic
acid, C'14H'6O'7(O.CH'3); dimethylellagic acid,
C'14H'4O'6(O.CH'3)'2; tetramethylellagic acid, C'14H'2O'4(O.CH'3)'4;
phenylhydrazinellagic acid, C'14H'6O'8.N'2H'3C'6H'5.

By the electrolytic reduction of ellagic acid, hexoxydiphenyl,
(OH)'3C'6H'2-C'6H'2(OH)'3, is obtained; the ordinary
methods of reduction yield leucoellagic acid, C'14H'10O'8, which
crystallises in small sharp needles, melting with decomposition
at 294°-295° C. Leucoellagic acid is soluble in ethyl and methyl
alcohols, and in glacial acetic acid, insoluble in chloroform,
benzene, toluene, carbon tetrachloride, and petrol ether; it
gives a bluish-green colour with ferric chloride which quickly
turns black. Leucoellagic acid is soluble in alkalies, the
solution assuming a deep-red coloration; it reduces silver
nitrate in the cold, but is not adsorbed by mordanted cotton
cloth, in which respect it differs from ellagic acid.[Footnote: Liebig's
_Ann_., 1912, 394, 249.

ELLAGITANNIC ACID, C'26H'28'O'10-3H'2O, is closely related to ellagic
acid; the former consists of faintly yellow needles, M.P. 329°-336°C.
It is soluble in water, precipitates gelatine, and is adsorbed by hide
powder. It occurs with gallic acid, tannin, and ellagic acid in
dividivi, myrabolams, algarobilla, and chestnut wood extracts.

Other bodies of this class include:--

METELLAGIC ACID, Cl_4H_6O_5, derived from methoxybenzoic acid, and
recrystallised from acetic acid, forms small crystalline needles,
M.P. 273°-276° C., and yields fluorene on distillation with zinc dust.

   ^ ---------- ^
  | |          | |
   V ---O.CO--- V

FLAVELLAGIC ACID, C_14H_6O_9, is obtained by the oxidation of gallic
acid with concentrated sulphuric acid and potassium persulphate. It
crystallises from pyridine in prismatic needles melting above 360°
C. Distillation with zinc dust yields fluorene (see above)--

     ^ ---------- ^ OH
    | |          | |
  HO V ---O.CO--- V OH
     OH           OH

By heating ellagic acid for three-quarters of an hour at 185° C. with
concentrated sulphuric acid, ceruleo-ellagic acid (dioxyellagic acid),
C_14H_6O_10, is formed as yellowish needles, M.P. 360° C., which are but
little soluble in the usual solvents. The acid is slightly soluble in
strong caustic soda solution, the colour of the solution, on diluting,
changing to green and blue.

LUTEIC ACID (Luteo Saure, pentoxybiphenylmethylolide carboxylic
acid),C_14H_8O_9, occurs, in addition to ellagic acid, in myrabolams--
[Footnote: _Ber_., 1909, 42, 353.]

     ^ ---------- ^ OH
    | |          | |
     OH           OH

It is obtained by extracting myrabolams for one hour and a half, under
reflux condenser, with pyridine, filtering and adding twice the volume
of water to the filtrate and boiling till complete solution is
obtained. After about thirty hours a reddish powder deposits, from which
ellagic acid may be extracted with pyridine; the mother-liquor on being
concentrated yields luteic acid. It is also obtained by oxidising tannin
with hydrogen peroxide, the other oxidation product being ellagic acid,
and the two may then be separated as indicated above. Luteic acid forms
reddish needles which are decomposed, with evolution of gas, at
338°-341° C. Heated with 10 per cent. caustic soda solution it yields
ellagic acid. In pyridine solution the carboxyl group maybe eliminated
by hydrogen iodide, whereby pentoxybiphenylmethylolide is formed as long
silky needles, which do not melt below 300° C. The same substance may
also be obtained when ellagic acid is boiled with concentrated caustic
potash solution. When luteic acid is treated with diazomethane, it
yields the methyl ester of pentamethoxybiphenylmethylolidcarboxylic


The most common decomposition products of the natural tannoids are
hydroxybenzoic acids, notably gallic and proto-catechuic acids;
furthermore, other aromatic and aliphatic hydroxy compounds frequently
occur. So far, however, attempts at explaining the constitution of the
complex decomposition products obtained by hydrolysing high molecular
tannoids have not been successful. On the other hand, the constitution
of the simpler natural tannoids is known to a greater or less extent; of
these, lecanoric acid (Lecanorsäure) is the best known, being an ester
anhydride of orsellic acid (a dihydroxytoluylic acid). It combines with
erythrite, forming another tannoid, erythrine. The fact that
hydroxybenzoic acids are constantly encountered together with the
products obtained on hydrolysis of the tannins, seems to point toward
the conclusion that anhydrides of hydroxybenzoic acids are frequent
constituents of the natural tannoid molecules.

The assumption that, for instance, in tannin at least part of the gallic
acid radicals are combined with one another is highly probable, and is
supported by the formation of tri- and dimethylgallic acid from
methylotannin, [Footnote: Herzig, _Monatshefte f. Chemie_, 1909, 30,
343.] and by the formation of ellagic acid when tannin is oxidised.
[Footnote: Nierenstein, _Ber_., 1908, 41, 3015.] Further proof is
brought forward by the existence of the pentacetyl-tannin, [Footnote:
Schiff, _Ann. d. Chem_., 1873, 170, 73.] and by the results of
hydrolysis which has yielded up to 104 per cent. anhydrous gallic acid
fiom tannin [Footnote: Sisley, _Bull. Soc. Chim_. 1909, 5, 727.]

Of the three classes of isomeric anhydrides which can be formed from
hydroxybenzoic acids, the chemistry of the natural tannins is only
concerned with the class comprising the ester anhydrides. If the
carboxyl of the first molecule combines with a hydroxyl of the second
molecule (ester formation), then a substance possessing character
similar to that of a hydroxybenzoic acid is formed, which is capable of
combining up with a further molecule in the same way. It is natural to
assume that this ester form is much more prevalent in Nature than a
combination of two carboxyls by the elimination of water. From the point
of view of the chemistry of the tannins, therefore, the starting-point
would naturally be that of synthesising the ester anhydrides of
hydroxybenzoic acids. Amongst the small number of synthetically prepared
ester anhydrides of hydroxybenzoic acids, a few occur exhibiting the
properties of the natural tannoids.

In order to simplify the terminology of these substances, Fischer
[Footnote: Liebig's _Ann_., 1910, 372, 35.] proposed the name
"Depsides" from [Greek: depheiv] = to tan. In analogy with peptides
and saccharides, the names di-, tri-, and polydepsides of hydroxybenzoic
acids would be suitable for these substances.

The principles underlying the synthesis of depsides are the
following:--If the chlorides of carbomethoxy (or carbethoxy)
hydroxybenzoic acids are coupled with the sodium salts of hydroxybenzoic
acids, esters are formed, _e.g._,

          CH_3CO O.O.C_6H_4.CO.Cl + NaO.C_6H_4.COO.Na = NaCl
           + CH_3.COO.O.C_6H_4.CO.O.C_6H_4.COO.Na

On gently saponifying the esters, these are converted into the
corresponding hydroxy derivatives--


According to Fischer and Freudenberg, [Footnote: Liebig's _Ann._, 1909,
372, 32.] this method possesses the following advantages:--

1. The synthesis takes place at low temperatures, so that any
intramolecular rearrangements are improbable.

2. The composition of the substances is controlled by the intermediary
compounds, the carboalkyloxy derivatives.

3. The synthesis permits of more definite evidence as regards the
structure of the resulting compounds.

4. The substances obtained are easily purified.

Depsides produced in this manner are by no means new, and were obtained
by Klepl by simply heating _p_-hydroxy-benzoic acid (_cf._ Introduction,
p. 4). This simple procedure, however, is not applicable to most other
hydroxybenzoic acids which are decomposed at the high temperature
necessary to induce reaction. Lowe and Schiff (_loc. cit._) have
obtained products similar to tannins, the latter investigator by
removing the elements of water from gallic acid, protocatechuic acid,
salicylic acid, _m_-hydroxybenzoic acid, cresotinic acid, phloretinic
acid, and pyrogallolcarboxylic acid. These depsides, however, are
amorphous substances, and it is hence difficult to substantiate their

Carbomethoxylation of Hydroxybenzoic Acids

Amongst other compounds chlorphydroxybenzoic acid is used in the
preparation of the materials employed in the synthesis of depsides; the
free phenolic group, however, exerts a disturbing influence when
aromatic acids are acted upon by phosphorus chloride, and another group,
which can subsequently be easily removed, must therefore be introduced
to cover the disturbing influence referred to. For this purpose, Fischer
[Footnote: _Ber_., 1908, 41, 2860.] chose the carbomethoxy group, and
this investigator succeeded, by the action of chlorocarbonic alkyl
ester and alkali upon hydroxybenzoic acid in cold aqueous solution, in
obtaining substances with the properties required. [Footnote: _Ber._,
1908, 41, 2875.] In such substances (_e.g._, salicylic acid) where the
hydroxyl occupies the ortho-position to the carboxyl, complete
carbomethoxylation does not take place, whereas the _m_- or _p_-
positions offer no hindrance. In the case of the _o_-position, however,
the action of chlorocarbonic alkyl ester is successfully assisted by the
presence of dimethylaniline in an inert solvent, _e.g._,
benzene.[Footnote: U.S. Pat, 1,639,174, 12, xii., 1899.] The difficulty
encountered by the _o_-position is eliminated when the carboxyl is not
directly linked to the benzene nucleus, _e.g._, _o_-cumaric acid. Many
hydroxybenzoic acids require an excess of chlorocarbonic methyl ester,
which then also, to some extent, attacks the carboxyl group; but on
dissolving the product in acetone and treating it with bicarbonate the
carboxyl group as such is again restored without splitting off the
carbomethoxy group.[Footnote: _Ber._, 1913, 46, 2400.] In this way all
hydroxybenzoic acids may be carbomethoxylated. [Footnote: _Ibid._,
1908, 41, 2877, 2881, 2882; 1909, 42, 226, 218, 223, 225; Liebig's
_Ann._, 1912, 391, 357, 366; _Ber._, 1913, 46, 1145, 2390, 2400.] The
carbomethoxy group is easily removed by excess of aqueous alkali in the
cold, and is also partially removed when insufficient alkali is present;
the latter fact is of importance in the synthesis of didepsides.

Chlorides of Carbomethoxyhydroxybenzoic Acids

The chlorides of these compounds are obtained when phosphorus
pentachloride is allowed to act upon the acids, and are as a rule
crystalline. For the purpose of synthesis they may be employed as

1. They readily form esters with alcohols, which on subsequent
saponification with alkali are converted into the esters of the free
hydroxybenzoic acids.

2. The chlorides interact energetically with esters of amino-acids, and
may be coupled with amino-acids in aqueous alkaline solution. On
subsequently removing the carbo-methoxy group derivatives of
hydroxybenzoic acids are obtained, _e.g._,

    CH_3.CO_2.O.C_6H_4.CO.Cl + 2NH_2CH_2.CO.C_2H_5
  = NH_2.CH_2.CO_2.C_2H_5 + HCl + CH_3.CO_2.O.C_6H_4 CO.NH.CH_2CO_2C_2H_5.
    CH_3.CO_2.O.C_6H_4.CO.NH.CH_2.CO_2.C_2H_5 + 3NaOH
  = Na_2CO_3 + C_2H_5OH + CH_3OH + HO.C_6H_4.CO.NH.CH_2.COONa.

3. In the presence of AlCl_3 the chlorides easily combine with benzene,
and on removing the carbomethoxy group unsymmetrical hydroxy derivatives
of benzophenone are formed:--

  CH_3.CO_2.O.C_6H_4.CO.Cl + C_6H_6 = CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + HCl
    CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + 3NaOH
  = NaO.C_6H_4.CO.C_6H_5 + Na_3CO_3 + CH_3OH + H_2O

4. The chlorides may be coupled with free hydroxybenzoic acids, and on
removing the carbomethoxy group didepsides are obtained. Repetition of
these operations yields tri- and tetradepsides.

Preparation of Didepsides

A simple application of these syntheses is offered by _p_-hydroxybenzoic
acid. When the chloride of its carbomethoxy derivative is allowed to
interact with _p_-hydroxybenzoic acid in aqueous alkaline solution, in
the cold, the alkali salt of carbomethoxy-_p_-hydroxybenzoic acid is
formed:--[Footnote 1: _Ber._, 1909, 42, 216.]

  CH_3.CO_2.O.C_6H_4.CO.Cl + NaO.C_6H_4.COONa
  = CH_3.CO_2.O.C_6H_4.CO_2.C_6H_4.CO_2.Na + NaCl.

Being sparingly soluble, the salt in this case is readily deposited as
crystals, but is readily converted into the free acid by hydrochloric
acid. In most other cases, however, the alkali salts are easily soluble
and the aqueous solution is then directly acidified with a mineral
acid. The chlorides, being for the most part solids, the mode of
procedure is as follows:--the hydroxybenzoic acid required for coupling
is dissolved in normal or double-normal alkali (the volume calculated
per molecule acid), a little acetone added, and the mixture well cooled;
a further molecule of 2N caustic soda and the chloride (I molecule)
dissolved in dry acetone are added in small portions, whilst stirring,
to the mixture. In spite of the low temperature the coupling proceeds
quickly and the sparingly soluble product can in most cases be
precipitated from the solution by acidifying and diluting with water. In
case of more easily soluble coupling products the acetone is driven off
under reduced pressure or the liquid acidified and diluted, and the
substance extracted with ether. Instead of alkali, dimethylaniline may
be employed, with the exclusion of water as a solvent for the purpose of

Another suitable method of obtaining _o_-didepsides is that of treating
_o_-hydroxybenzoic acids with phosphorus trichloride and dimethylaniline
(_e.g_., synthesis of disalicylic acid, Boehringer & Sons).[Footnote:
Ger. Pat., 211,403.]

The carbomethoxy derivatives of the depsides are as a rule crystalline
substances of distinct acidic character, and decompose alkaline

The elimination of the carbomethoxy group may be brought about by dilute
alkaline solutions in the cold, or by aqueous ammonia. If the depside
formed is so stable as to resist the action of alkali for several hours,
the use of the latter is very convenient for the purpose required. The
substance is dissolved directly in sufficient normal alkali to
neutralise the carboxyl group and a further 2 molecules of caustic soda
for each carbomethoxy group to be eliminated are added. The temperature
should be about 20° C., when the reaction as a rule is completed after
one-half to three-quarters of an hour. It is usual, however, to use an
aqueous ammonia solution in considerable excess, whereby the temperature
should again be about 20° C., and the solution of ammonia normal or half

The didepsides so far investigated are crystalline bodies, sparingly
soluble in cold water; they--as a rule--decompose when fused, possess
acid reaction, and are dissolved by bicarbonates. On account of the
presence of a free phenolic group they give a coloration with ferric
chloride; if the phenolic group occupies the _o_-position to carboxyl,
the coloration with ferric chloride is red or bluish-violet Excess of
dilute alkali resolved all didepsides into their components at ordinary
temperatures. The didepsides of gallic, proto-catechuic, gentisinic, and
[Greek: b]-resorcylic acids precipitate gelatine and quinine acetate,
and in this respect approach the natural tannins.

The following summary gives an account of depsides which have been
prepared synthetically or which occur naturally:--[Footnote 1: _Ber._,
1908, 41, 2888; 1909, 42, 217; 1913, 45, 2718; 1913, 46, 1130, 2396,
1141, 1143; Liebig's _Ann._, 384, 230, 233, 238; 391, 356, 362.]

  Di-_p_-hydroxybenzoic acid.
  Di-_m_-hydroxybenzoic acid.
  Disalicylic acid.
  Diprotocatechuic acid.
  Digentisinic acid.
  Di-[Greek: b]-resorcylic acid.
  _p_-Diorsellic acid.
  _o_-Diorsellic acid.
  _m_-Digallic acid.
  Disyringic acid.
  Di-_o_-cumaric acid.
  Diferulic acid.
  Di-[Greek: b]-hydroxynaphthoic acid.
  _p_-Hydroxybenzoyl-_m_-hydroxybenzoic acid.
  _m_-Hydroxybenzoyl-_p_-hydroxybenzoic acid.
  Salicyl-_p_-hydroxybenzoic acid,
  Vanilloyl-_p_-hydroxybenzoic acid.
  Feruloyl-_p_-hydroxybenzoic acid.
  [Greek: a]-Hydroxynaphthoyl-_p_-hydroxybenzoic acid.
  Orsellinoyl-_p_-hydroxybenzoic acid.
  Protocatechuyl-_p_-hydroxybenzoic acid.
  Galloyl-_p_-hydroxybenzoic acid.
  Pyrogallolcarboy _p_-hydroxybenzoic acid.
  Syringoyl-_p_-hydroxybenzoic acid.
  _p_-Hydroxybenzoyl-syringic acid.
  Pentamethyl-_m_-digallic acid.
  Pentamethyl-_p_-digallic acid.
  Vanilloyl vanillin.

Preparation of Tridepsides

Monohydroxybenzoic acids allow theoretically of tri-depsides of the type
HO.C_6H_4COO.C_6H_4.COO.C_6H_4.COOH only; if, on the other hand, di- or
trihydroxybenzoic acids are dealt with, two formulae are possible,

               } C_6H_3.COOH

Of the former type, two compounds are known, _i.e._,
acid and vanilloyl-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--

      } C_6H_3.COO.C_6H_4.COO.C_6H_4.COOH

The first named of these two compounds was obtained by Klepl, in
addition to the didepside, by heating _p_-hydroxybenzoic acid. Fischer
and Freudenberg obtained a beautifully crystalline form in the following
way: carbethoxyhydroxy-benzoyl chloride was coupled with
_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution, the
compound dissolved in a mixture of pyridine and acetone, and ammonia
added for the purpose of removing the carbethoxy group. The tridepside
was then obtained as long needles by re-dissolving in acetone.

Both tridepsides melt well above 200° C., are practically insoluble in
water, and are but sparingly soluble in practically all organic
solvents. In alcoholic solution they give colour reaction with ferric
chloride similar to those given by _p_-hydroxybenzoic acids.

Preparation of Tetradepsides
[Footnote: Fischer and Freudenberg, Liebig's _Ann._, 1910, 372, 32.]

Here, again, two forms are known, _e.g._,
tri-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--

  HO.C_5H_4.COO.C_6H_4.COO.C_6H_4COO.C_6H_4 COOH

and vanilloyl-di-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--

      } C_6H_3.COO.C_6H_4.COO.C_6H_4.COO.C_6H_4.COOH

The former has been prepared from
carbethoxyhydroxy-benzoyl-_p_-hydroxybenzoyl chloride and
_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution; the
second tetradepside was prepared from
carbomethoxyvanilloyl-_p_-hydroxybenzoyl chloride and
_p_-hydroxybenzoyl-_p_-hydroxy-benzoic acid.

The preparation of these compounds is rendered difficult by the slight
solubility of the substances and their slight affinities for entering
into reaction. Both tetradepsides were obtained in crystalline form, and
are but very little soluble in most organic solvents. They decompose on
being fused.

Tannoid Substances of the Tannin Type

The preparation of pentagalloyl glucose has proved this compound to be
nearly identical with tannin obtained from galls (_tannin_); a few other
natural tannins belong to this type which Fischer terms acyl compounds
of sugar with hydroxybenzoic acids. The method of preparation employed
in the synthesis of pentagalloyl glucose may be easily applied to other
hydroxybenzoic acids, _e.g._ penta[_p_-hydroxybenzoyl] glucose
[Footnote: Fischer and Freudenberg, _Ber._, 1912, 45, 933.] was
prepared in this way. Similar characteristics are exhibited by
pentasalicylo glucose. Mention must also be made of the corresponding
derivative of pyruvic acid and the compound with pyrogallolcarboxylic
acid, penta-[pyrogallolcarboyl]glucose. [Footnote: Fischer and
Rapoport, _Ber._, 1913, 46, 2397.] The latter is isomeric with
pentagalloyl glucose and possesses similar properties; there is,
however, a vast difference in the solubility of the two. Whereas the
galloyl compound is easily soluble in cold water, its isomer is hardly
soluble in hot, and completely insoluble in cold water. Considering the
very similar structure of these two tannins, such differences appear
surprising, but an analogy may be readily found in the existence of
colloidal solutions of tannin and the (nearly) identical pentagalloyl
glucose. These properties clearly show how dependent is the colloidal
state on small differences in the structure of two substances. On the
other hand, the formation of hydrosols is of the greatest importance
relatively to the part played by these substances in Nature as well as
relating to their chemical characteristics; thus it is extremely
difficult to make a solution of penta-[pyrogallolcarboyl]-glucose, at
the same time ascertaining its astringent taste and its property of
precipitating gelatine.

The experience gained by the methyl glucosides makes it exceedingly
probable that the simpler polyhydric alcohols also are suitable
substances to employ in these syntheses; as a matter of fact, glycerol
has been condensed with gallic acid. [Footnote: Fischer and Freudenberg,
_Ber., 1912, 45, 935.]

One of the chief characteristics of synthetic tannins is their high
molecular weight; for instance, the molecular weight of
penta-[tricarbomethoxygalloyl]-glucose is 1,810, that of
penta-[pentamethyl-_m_-digalloyl]-glucose 2,051. Employing gallic acid
derivatives, especially the tribenzoyl compounds, coupled with glucose,
_e.g._, mannite, yielded a neutral ester of molecular weight 2,967.

The determination of the elementary composition of compounds of high
molecular weight is greatly facilitated by employing their halogen
derivatives; so, for instance, is _p_ iodophenyl maltosazone very
suitable. Coupling the latter with tribenzoylgalloyl chloride yielded
hepta-[tribenzoyl-galloyl]-_p_-iodophenyl maltosazone, the structure of
which is represented by--

  CH.O.R               R = CO.C_6H_2(O.CO.C_6H_6)_2
  CH.O.R    R  R     R  R
  |         O  O     O  O
  |         |  |     |  |
          |       |

The molecular weight of this substance is 4,021, and probably represents
the highest molecular organic body obtained in any chemical synthesis.

From a physiological standpoint the recognition of tannins as esters of
glucose and hydroxybenzoic acids, possessing characteristics similar to
those of tannin, is of great importance. Especially interesting appears
the fact of plants utilising sugars for the esterification of acids,
just as glycerol or monohydric alcohols may be employed for the same
purpose. Free acids, as a rule, are only tolerated in certain parts of
the organism, the latter usually striving to neutralise acidic groups
which may be brought about by salt formation; formation of amino
compounds (proteins) or esterification (fats); and, lastly,
esterformation by means of sugars.

Why Nature should always build up substances of very complex
constitution can only be explained by biochemical investigations, but it
may, at any rate, be assumed that by this means any substance poisonous
to the living organism is rendered inactive. The function of the tannins
present in plants may thus be explained; if, for instance, phenols are
formed by the oxidation of corresponding sugars, [Footnote: Mielke,
"Ueber die Stellung der Gerbstoffe im Stoffwechsel der Pflanzen"
(Hamburg, 1893).] the poisonous character of the former would be
lessened by the introduction of the carbonic acid esters and subsequent
coupling of the substances (depside formation). The depsides thus formed
would serve as vehicle of the sugars and transport the migrating
tannins, [Footnote: Kraus, "Grundlinien zu einer Physiologie der
Gerbstoffe" (1889).] and, after subsequent deposition of the sugars,
would then be eliminated from the plant organism, either by oxidation
into ellagic acid and phlobaphenes or by condensation with the formation
of cork.

Diagrammatically, the following would represent the physiology of the
tannins:--[Footnote: Nierenstein, "Chemie der Gerbstoffe" (Stuttgart,

  Sugar-->Phenol-->Hydroxybenzoic Acid-->Depside-->

  -->Migrating Depside-->Glucoside-->Free Depside-->-{Ellagic Acid

[Illustration: Chart Showing the Decomposition of Products of Tannin.]




In organic chemistry distinction is made between sulphonic acids of the
aliphatic and the aromatic series, the characteristic group of these
acids being the so-called _sulphonic acid group_, HSO_3.

When sulphides or mercaptans in glacial acetic acid solution are heated
with permanganate, the resulting sulphonic acid compounds exhibit great
similarity to compounds containing free carboxyl groups. The sulphonic
acid group may also be directly introduced either by concentrated, or by
fuming sulphuric acid, or by elimination of halogen by the action of
sodium or silver sulphite on the halogen derivatives of the aliphatic
compounds. Saturated hydrocarbons do not react with sulphur trioxide,
but unsaturated hydrocarbons are readily attacked by SO_3. Similarly,
halogenated compounds and alcohols react with concentrated or fuming
sulphuric acid forming sulphonic and hydrosulphonic acids respectively.
The aromatic compounds form, as a rule, sulphonic acids with much
greater facility. Benzene, for instance, is easily converted into the
_m_-disulphonic acid by gently heating with fuming sulphuric acid;
stronger heating converts the _m_- into the _p_-disulphonic acid, and at
190° C. the trisulphonic acid is formed. Toluene treated with fuming
sulphuric acid first yields _o_- and _p_-sulphonic acids, finally _o_-
and _p_-disulphonic acids, ethylbenzene at the boiling point
_p_-ethylbenzene-sulphonic acid. Of the three isomeric xylenes _o_- and
_m_-xylene dissolve in concentrated, _p_-xylene in fuming sulphuric acid

The action of sulphuric acid on naphthalene is stronger even than on
benzene. Equal parts of naphthalene and sulphuric acid heated to 100°
C. yield 80 per cent. [Greek: a] and
20 per cent. [Greek: b]-monosulphonic acid. At 160°-170°C. 25 per cent
[Greek: a]- and 75 per cent. [Greek: b]-sulphonic acid is formed, and at
higher temperatures [Greek: b]-monosulphonic acid only. If, on the other
hand, 8 parts of naphthalene are heated with 3 parts of concentrated
sulphuric acid to 180° C., two different naphthyldisulphonic acids are

Complete solution of the substance in sulphuric acid is, generally
speaking, a criterion of complete sulphonation. A completely sulphonated
compound should remain clear on dilution with water, or, in case
precipitation occurs, the precipitate should be completely soluble in
alkali or ammonia. It is necessary to submit the product to this test,
since many organic substances are soluble in concentrated sulphuric acid
without undergoing any alteration in composition.

Phosphoruspentoxide or potassium sulphate considerably increase the
sulphonating property exhibited by fuming sulphuric acid.

The separation of the sulphonic acids from sulphuric acid is effected by
salting out the former with common salt, or by removing the sulphuric
acid with calcium, barium, or lead salts, provided that the sulphonic
acid salts of these metals are soluble in water.

The sulphonic acid, in its chemically pure state, is best obtained from
its crystalline barium salts, which are decomposed with the equivalent
of sulphuric acid; another way is to decompose the calcium salts of the
sulphonic acids with oxalic acid. The sulphonic acids are frequently
hygroscopic and are easily soluble in water; the majority of their
barium and lead salts are also soluble in water. The sulphonic acids are
insoluble in ether. The halogens do not easily react with sulphonic
acids, but when they do they usually replace the sulphonic acid
group. In order to prepare the halogen substitution products, therefore,
use is made of sulphonic chlorides. The latter are obtained by the
action of chlorosulphonic acid on aromatic hydrocarbons; a simpler
method, however, is to treat the dry alkali sulphonates with phosphorus

  C_6H_5SO_3Na + PCl_5 = C_6H_5SO_2.Cl + NaCl + POCl_3

Derivatives of sulphonic chlorides are sulphonamides, which are easily
prepared from the former by grinding with ammonium carbonate--

  C_6H_5SO_2.Cl + (NH_4)_2CO_3 = C_6H_5.SO_2.NH_2 + NH_4Cl + CO_2 + H_2O

Sulphonic chlorides react with alkaline sulphides to form
thiosulphonic acids--

  C_6H_5SO_2.Cl + K_2S = C_6H_5SO_2.SK + KCl

Sulphonic chlorides, dissolved in ether, yield sulphinic acids on
reduction with zinc dust or metallic sodium--

  C_6H_5SO_2.Cl + H_2 = C_6H_5SO_2.H + HCl

In the sulphonic acid compounds it is assumed that the sulphur is
hexavalent, and it is hence possible to consider the sulphones to be
esters of sulphinic acid.


The sulphones are mostly solid bodies, which soften prior to melting
when heated. They are very stable towards chemical reagents; for
instance, saponification of a mono-sulphone very rarely yields sulphinic

If a hydroxyl is substituted for a hydrogen atom in the aromatic
hydrocarbons, the action of sulphuric acid is greatly facilitated; thus,
by merely mixing phenol with sulphuric acid, the sulphonic acid is at
once formed, whereby, in the cold, _o_-phenolsulphonic acid prevails
which on heating for some time to 100°-110° C. is completely converted
into _p_-phenolsulphonic acid. In the absence of free sulphuric acid the
conversion of _o_- into _p_-phenolsulphonic acid is brought about by
heating the aqueous solution. Phenol-2,4-disulphonic acid is prepared
from _o_- or _p_-phenolsulphonic acid, whereas phenol-2,4,6-trisulphonic
acid is prepared directly from phenol by heating with concentrated
sulphuric acid in presence of phosphorus pentoxide. Phenolsulphonic
acids are also obtained by fusing benzenedisulphonic acid with alkali.

Cresol is not so easily sulphonated as is phenol; _o_-cresol when heated
eight to ten hours at 90° C. with one and one-half times its weight
of concentrated sulphuric acid, yields _o_-cresol-_p_-sulphonic acid.

The phenolsulphonic acids are strong, rather stable acids; their
alcoholic hydroxyl-hydrogen atom may, similarly to that of the phenols,
be substituted by a metal or an alkyl radical.

From [Greek: a]- and [Greek: b]-naphthol a number of sulphonic acids may
easily be prepared; viz., mono-, di-, and trisulphonic acids. Nearly
all these acids are important as basic materials in the dyestuff
industry, especially 2,6-[Greek: b]-naphtholmonosulphonic acid (S-acid),
2,3,6-[Greek: b]-naphtholdisulphonic acid (R-acid) and 2,6,8-[Greek:
b]-naphtholdisulphonic acid (G-acid).

2. Condensation of Phenols

Phenolsulphonic acids exhibit pronounced tendencies to condensation, for
which purpose A. v. Baeyer (1872) employed aldehydes. The reaction is
rather violent, and yields, in addition to well-defined crystalline
substances, amorphous bodies resembling rosins. In addition to
formaldehyde, paraformaldehyde, trioxymethylene, methylal,
hexamethylene-tetramine, and other substances containing a reactive
methylene group, as well as acetaldehyde, benzaldehyde and other
aldehydes may be employed to induce reaction.

A number of these condensation products are derivatives of diphenylamine
or hydroxybenzyl alcohols. When the latter are heated, either by
themselves or in presence of acids, anhydrides and polymerisation
products are formed producing hard, brittle, fusible substances,
insoluble in water but fairly soluble in organic solvents. The same
substances are formed when phenols are condensed with formaldehyde,
especially in the presence of acid contact substances and excess of
phenol by sufficiently long heating at certain temperatures. The
substances referred to are termed "Novolak": similar to these are the
so-called "Resols," insoluble and non-fusible substances, very resistant
to chemical and physical action. Another member of the series is the
so-called "Bakelite" or "Resitol," which does not fuse but softens when
heated and swells in organic solvents. The ultimate product of this
class of substances is "Resit" which is obtained when concentrated
hydrochloric acid is allowed to act upon a mixture of phenol and
formaldehyde; the temperature rises spontaneously, and a hard, porous,
insoluble mass of great resistance is obtained. By heating resols,
resitols are formed which, on further heating, are finally converted
into resits. [Footnote: _Ber.,_ 1892, 25, 3213.]

Of all these products, bakelite (resitol) has found the greatest
industrial application; in its purest form, this substance is a nearly
colourless or light yellow body of sp. gr. 1.25 and, being a poor
conductor of heat and electricity, constitutes an excellent insulating
material; it is exceedingly resistant towards most chemical reagents
even in concentrated forms of the latter. Its pronounced refractivity,
and the ease with which it may be worked, makes bakelite a favourite
substitute for amber (Ger. Pat, 286, 568). Similarly, the resols which
can be easily moulded are used either as such or mixed with sand,
pulverised cork, asbestos or wood, and the moulded substances then
converted into the more highly resistant bakelite by heating.

The constitution of these bodies no doubt depends largely on their
method of preparation; Baekeland [Footnote: _Chem. Ztg.,_ 1913, 73,
733.] considers resit a polymerised hydroxybenzylmethylene glycol
anhydride; Raschig, a diphenylmethane derivative (e.g.,
dihydroxydiphenylmethane alcohol); Wohl [Footnote: _Ber.,_ 1912, 45,
2046.] considers them polymerisation products of methylene derivatives
of tautomeric phenol.

  H_2C:C{       }CO
  [Note: Lower Right CH has double bond to CO]

This group possesses the characteristic property of being capable of
converting animal hide into leather when suitably dissolved. The author
has dissolved a number of these water-insoluble condensation products in
alkali and alcohol and was able to demonstrate their tanning effects on
pelt; bakelite is easily soluble in alkali; a faintly alkaline solution
partially precipitates gelatine, and completely so when the alkali is
neutralised. This latter solution gives a dirty brown precipitate with
iron salts.

These condensation products gained extraordinary importance for the
tanning trade when Stiasny [Footnote: Ger. Pat, 262,558; Austr. Pat,
58,405.] succeeded in preparing them in water-soluble form when they are
enabled to directly exert their tannoid properties. This may be done by
acting upon two molecules of concentrated phenolsulphonic acid with one
molecule of formaldehyde, the temperature thereby not exceeding 35°C. By
condensation, however, considerable heat is liberated, and hence the
rise in temperature can only be limited by adding the diluted
formaldehyde drop by drop, whilst stirring and cooling, to the
phenolsulphonic acid. The original letters patent is worded as follows:
10 kilos each of crude phenol and sulphuric acid (66° Bé.) are heated
with stirring for two hours at 105°-106°C., cooled to about 35°C., and
463 kilos 30 per cent. formaldehyde added during three hours, the
temperature thereby not exceeding 35°C.; the stirring is continued for a
couple of hours after the final addition of formaldehyde. This yields
about 24 kilos of the crude condensation product. On a commercial scale,
however, cresol (cresylic acid) is substituted for phenol. There are
three isomers of cresol, viz., _o_-, _m_-, and _p_-cresol, and it was
naturally of interest to investigate whether one or the other of the
isomers exerted any particular influence on the properties of the final
product. It was found, however, that condensation products from the
three isomers were distinguishable from one another neither in physical
nor in tannoid properties. It is hence possible to employ crude cresol,
which contains varying quantities of the _o_-, _m_-, and _p_-compounds,
in the manufacture of these tanning matters. [Footnote: Gen Pat,

The tar obtained from the Rochling coal-gas generator contains
considerable quantities of phenols (B.P.=200°-250°C.), and the author
has protected the use of these for the production of synthetic tannins
by Ger. Pat, 262,558. A deep brown viscous mass is obtained which, when
partly neutralised, yields similar results to those given by the product
above referred to.

It may be anticipated that by analogy from the chemical reactions taking
place in the condensation of phenols on the one hand and cresolsulphonic
acid on the other, that all other homologues of phenol, its polyvalent
derivatives, substitution products and acids, would yield similar
condensation products.

The particular position occupied by the aromatic hydroxy compounds in
the chemistry of substance possessing tannoid character is not only
evidenced by the natural classification of the tannins, tannin
derivatives, and decomposition products so far isolated and
investigated, but also by other chemical behaviour shown by these
substances. Meunier and Seyewetz [Footnote:_Collegium_, 1908, 315,
195.], for example, were able to show that phenol, _p_-aminophenol,
chlorophenol, trinitrophenol, catechol, resorcinol, hydroquinone,
monochlorohydroquinone, orcinol, pyrogallol, and gallotannic acid
precipitate gelatine from its aqueous solution, that is, to a certain
extent possess tanning properties.

The author has extended this series somewhat and obtained the following

                       Relative Behaviour Towards
  Substances           Gelatine.     Hide Powder.    Pelt.
  Tribromophenol       Slight ppte.    Tans         Surface tannage
    [Footnote: In alcoholic solution]
  _o_-Nitrophenol        No ppte.       "              "
  Br-_o_-Nitrophenol   Slight ppte.     "              "
  Tribromopyrogallic      Ppte.         "              "
  Bromophloroglucinol       "           "           No tannage
  Galloflavine         Slight ppte.     "              "
  Bromosalicylic acid       "           "              "
  Bromo-[Greek: b]          "           "             Tans
    [Footnote: In alcoholic solution]
  Rosolic acid              "           "              "
    [Footnote: In alcoholic solution]
  Gallic acid            No ppte.    No tannage     No tannage

By the condensation of their sulphonic acids, it may be demonstrated
experimentally how the tannoid properties of nearly every member of the
series are intensified. Investigattion in this direction, however, has
not been systematically undertaken, for which reason the author
determined to examine this subject; but the enormous number of samples
required, obtainable only with great difficulty during the war, made it
impossible to conclude completely the researches in this field. What
little has so far been done relatively to this subject should, when
collected, indicate the way to be pursued in this wide field of
investigation. What follows will hence comprise the conversion of a few
of the most important members of this series of substances into their
methylene-condensation products with a brief discussion of the
qualitative and tannoid reactions of the latter.

The didepside of phenolsulphonic acid is obtained by condensing
carbomethoxyphenolsulphonic chloride with sodium phenolsulphonate in the
presence of the calculated amount of caustic soda. A product of the


is first obtained, which on saponification with soda yields the
pure didepside--


By acidifying the concentrated solution the didepside is obtained as a
white crystalline substance; a solution of which precipitates gelatine
without, however, exhibiting any tanning effect upon animal hide. If, on
the other hand, the above ester is converted into the chloride


by treatment with PCl_5, and the chloride thus obtained further
condensed with sodium phenolsulphonate, saponified, and the solution
acidified, the pure tridepside


is precipitated as white crystalline needles which not only precipitate
gelatine, but are capable of converting animal hide into
leather.[Footnote: _Chem. Ztg._, 1919, 43, 318.]

Of the class of hydroxy-cymenes _thymol_,


was converted into the water-soluble sulphonic acid by warming with
concentrated sulphuric acid at 50° C., the sulphonic acid being
subsequently easily condensed with formaldehyde by slightly heating the
mixture. The condensation product thus obtained is a viscous brown mass
which is easily soluble in water, precipitates gelatine completely,
gives a bluish-black coloration with iron salts, and gives a precipitate
with aniline hydrochloride. To investigate its tannoid properties, the
mixture was brought to the acidity 1 gm = 10 c.c. N/10 NaOH and a piece
of bated calf skin was then introduced into a solution measuring about
2° Bé. After eighteen hours the pelt was nearly tanned through, and a
further twenty-four hours completed the tanning process, after which a
light fat-liquor was given.  The dried leather was brownish-grey in
colour, possessed soft and full feel and good tensile strength.

On account of their importance, the three dihydroxybenzenes were
examined with a view to test their suitability for conversion into
tannoid substances.

_o_-Dihydroxybenzene, catechol, yields a sulphonic acid easily soluble
in water, which on the careful addition of formaldehyde assumes a blue
colour. The compound thus obtained may be heated to 100° C., without
depositing insolubles. A further addition of formaldehyde, however,
results in the formation of a considerable quantity of insolubles whilst
the liquid assumes a brown coloration. If, on the other hand, the
sulphonic acid is diluted with twice its volume of water, formaldehyde
added and the mixture heated on the water bath, the liquid immediately
turns brown, the formaldehyde is completely fixed, and a condensation
product soluble in water results. The latter gives a brownish-black
coloration with ferric chloride, completely precipitates gelatine, but
gives no opalescence with aniline hydrochloride. Tanning experiments
with the partly neutralised (1 gm.= 10 c.c. N/10 NaOH) substance
resulted in both grain and flesh being tanned with a black colour,
whereas the interior of the pelt was pickled (white colour). After a
further forty-eight hours, however, the black colour penetrated the
pelt, and tannage was complete. The washed and lightly fat-liquored
leather was soft, of full feel and good tensile strength, and was
greyish coloured throughout.

With regard to the black colour possessed by leathers tanned with
synthetic tannins the following should be noted. When sulphonating and
especially when condensing substances, black dyestuffs or very finely
divided carbon in the colloidal state are often formed. Such a substance
does not deposit the black particles, even when filtered through kaolin,
and hence convert pelt into leather possessing black colour on the
surface. The hide in this case acts as a perfect filtration medium,
whereby the surface layers retaining the coloured particles assume their
colour; thus only the pure tanning matter enters into the interior,
which then, according to the composition of the former, imparts a colour
varying from white to light brown to the inner layers.

_m_-Dihydroxybenzene, resorcinol, is also easily sulphonated by
concentrated sulphuric acid, the brownish-coloured sulphonic acid being
easily soluble in water. If the sulphonic acid is diluted with three
times its volume of water, cooled down, a few drops of formaldehyde
added and the mixture heated on the water bath to completely fix the
formaldehyde, and this process repeated till no more formaldehyde is
taken up, a brown water-soluble condensation product results, the
aqueous solution of which precipitates gelatine completely, aniline
hydrochloride only partly and which gives a deep blue colour with ferric

A piece of calf skin immersed in a solution of the partly neutralised
(as above) product was tanned through in twenty-four hours; when lightly
fat-liquored, the resulting leather possessed a yellowish-green colour
and good tensile strength, and was soft and full.

_p_-Dihydroxybenzene, hydroquinone, was converted into the water-soluble
sulphonic acid by heating it with concentrated sulphuric acid at 100°
C.; the sulphonic acid, mixed with formaldehyde at ordinary temperature,
immediately solidifies to a white mass, which is soluble in water and
which had completely fixed the formaldehyde. If, however, this mass is
heated for some time to 100°C, it assumes a light brown coloration and
its solubility in water is diminished. A slight excess of formaldehyde
and the application of heat result in dark violet insoluble condensation
products. The aqueous solution precipitates gelatine, gives a deep blue
colour with ferric chloride, but gives no precipitate with aniline
hydrochloride; on the other hand, addition of potassium nitrite produces
the yellow colour characteristic of hydroquinone.

The product effects a slower tannage (seven days) than the former
product, when a brown, soft, but rather empty leather of good tensile
strength is obtained.

Of the _trihydroxybenzenes_ pyrogallol and phloroglucinol only were
included in these investigations.

When pyrogallol is sulphonated with concentrated sulphuric acid a
violet-coloured sulphonic acid, soluble in water, is obtained, which,
when treated with formaldehyde first in the cold and then when heated,
yields a solid deep red-coloured mass, which precipitates gelatine but
not aniline hydrochloride, and gives a blackish-brown colour with ferric
chloride. The partly neutralised substance in aqueous solution tans pelt
in twenty-four hours with black colour on the surface only, the
intermediary layer being pickled (white colour) only, but the
black-coloured tanning matter ultimately penetrates the pelt, which
tanned through in seven days. The resultant leather is coloured black
throughout, is full, soft, and possesses good tensile strength.

Sulphonation of phloroglucinol succeeds at higher temperatures only, the
sulphonic acid being a solid which is scarcely soluble in water, the
latter then assuming a wine-red colour. The condensation
product--prepared as described for resorcinol, but requiring higher
temperature--is a brick-red powder, insoluble in water.

The same end-product also seems to be obtained by simply heating the
sulphonic acid at a higher temperature; this also induces condensation
with the formation of a reddish-brown mass insoluble in water. It is, of
course, impossible to attempt any tanning experiments with this product
in aqueous solution; attempts at dissolving the condensation product in
alcohol proved barren of result, since only traces of impurities
accompanying the substance dissolved, imparting a light reddish-brown
colour to the solution. In highly concentrated alcohol, however, the
condensation product is somewhat soluble, yielding a reddish-brown
solution. A piece of pelt introduced into the alcoholic solution was
surface tanned only after forty-eight hours, leaving the remainder of
the pelt pickled; extending the experiment over a further four days
produced no change in the pelt. The latter was therefore rinsed with
water, lightly fat-liquored and dried, when a soft but empty leather of
grey colour and good tensile strength was obtained. It appears,
therefore, to be a case of pseudo-tannage, where an infinitesimal amount
of synthetic tannin produces a tanning effect without, however, a true
tannage being effected.

The Elberfelder Farbenfabriken have protected the use of the
condensation products of di- and polyhydroxybenzenes by Ger. Pat.,
282,313; owing to the high cost of the latter substances, however, it is
doubtful whether synthetic tannins prepared from these materials would
not be too expensive for any other than pharmaceutical purposes.

Before leaving the phenols, mention must be made of the quinones, the
use of which for tanning purposes was first protected by Ger. Pat.,
206,957 (30th April 1907). According to this patent, only 400 gm. of
quinone are required for the conversion into leather of 400 kilos pelt,
drum tannage being preferable. During the process the leather first
assumes a reddish colour, changing through violet to brown; its
resistance to water, acids, and alkalies is said to be considerably
greater than that exhibited by all other kinds of leather.

The chemistry of the quinone tannage has been investigated, and an
explanation given by Thuau [Footnote: _Collegium_, 1909, 363, 211.]
assumes a reaction between the quinone and the amino groups of the hide
protein with the formation of hydroquinone--

                  +-O        OH
                  | |        |
  2R.NH_2 + 2C_8H_4 | = C_6H_4 + C_6H_4(O.NH.R)_2
                  | |        |
                  +-O        OH
    (Pelt.)                      (Leather.)

Fahrion has shown that, during the tanning process, the quinone loses
its active oxygen, and this can only be brought about by the amino group
of the hide protein, the amino group only being capable of effecting
reduction of the quinone. An analogy is here offered by
dianilinoquinone. A spent quinone liquor contains considerable amounts
of hydroquinone. The tannage may also be effected by exposing pelt
saturated with hydroquinone to oxidation by the air. The pelt, which is
unaltered by the hydroquinone bath, on being removed from the latter,
and in the presence of alkali, assumes a red colour at first, which
changes into violet, blue, and finally brown, the pelt being thereby
converted into a quinone-tanned leather.

It may be noted that quinone only effects pseudo-tannage; quinone mixed
with water deposits, in time, a black amorphous substance practically
insoluble in water. This substance is easily adsorbed by hide powder,
but is not capable of converting the latter into that insoluble form
into which it is converted by the natural tannins.

Amongst polyhydric alcohols, the behaviour of the methyl ester of
catechol, _guaiacol_ was investigated. The sulphonic acid was prepared
by heating guaiacol with concentrated sulphuric acid, the resulting
water-soluble product possessing a light, brownish-green colour. On
condensing the sulphonic acid with formaldehyde, the same precautions
were observed as in the case of resorcinol, but complete fixation of the
formaldehyde could only be obtained by finally heating the product for a
short time over a free flame, at about 105° C. Condensation was
indicated by the brownish appearance of the liquid. No insoluble
products were formed. The condensation product easily dissolves in
water, the solution assuming a rich brown colour and exhibiting the
following reactions: gelatine is completely precipitated, aniline
hydrochloride produces opalescence, and ferric chloride a deep brown

Tannage, with the partly neutralised product, was rapid, the pelt being
nearly tanned through in twenty-four hours, excepting a small white
streak in the middle; after a further twenty-four hours this streak had
vanished, and the completely tanned, dark grey-coloured leather, after
washing, fat-liquoring, and drying, was soft, full, and of good tensile
strength, very similar to the leather yielded by the
catechol-condensation product.

Of the nitro-compounds, trinitrophenol, C_6H_2(NO_2)_3OH (picric acid),
was investigated. If a concentrated solution of picric acid is brought
into contact with pelt it will penetrate the latter completely in a few
days; it is, however, difficult to fat-liquor the resultant leather,
since the fat is absorbed only with difficulty. If a pelt treated in
this way be dried, a soft but rather flat leather results, the colour of
which easily rubs off, the leather also tasting intensely bitter. These
disagreeable qualities prevent a general use of this material for
tanning purposes; in spite of them, however, picric acid, in admixture
with boracic acid, salicylic acid, and glycerol, is used in the
production of the so-called transparent leather. The latter is very
flexible and possesses great tensile strength, but loses the latter
quality when exposed to heat, and, when stored, also loses its
flexibility. By simply washing with water, the leather is reconverted
into pelt.

When picric acid is treated with hot sulphuric acid and formaldehyde
gradually added, a dark coloured water-soluble condensation product is
formed which strongly precipitates gelatine. Exposed to the action of
bromine, the condensation product yields a mass which is insoluble in

Experience has taught that the amino bodies--the basic N-derivatives of
the phenols--do not yield substances possessing tannoid properties on
condensation. On account of their importance, however, a few have been
included in this series of investigations.

Aminobenzene, C_6H_5NH_2, aniline, treated with sulphuric acid, yields
the water-soluble aniline sulphate, which, on cautious addition of
formaldehyde, yields a reddish-coloured gel, insoluble in water, in
addition to a small volume of a reddish-yellow liquid. The latter
precipitates gelatine, but is not capable of converting pelt into
leather. The insoluble gel is likewise insoluble in alcohol, so that
tanning experiments with this substance are excluded.

Dimethylaniline, C_6H_5N(CH_3)_2, when treated with sulphuric acid, yields a
product soluble in water which neither reacts with nor fixes
formaldehyde. Hence the substance does not precipitate gelatine.

If, on the other hand, nitrosodimethylaniline,


is sulphonated, and the water-soluble sulphonation product heated with
formaldehyde for some time, the product remains soluble in water and
precipitates gelatine. No tanning effect could, however, be detected.

Arylsulphaminoarylsulphonic acids and arylsulphoxyarylsulphonic acids
precipitate gelatine but are devoid of tannoid character. The latter is
acquired by compounds belonging to this class containing two or more
sulphamino groups, or when they, in addition to one sulphamino group,
contain a sulphoxy group and another sulphonic group. According to
Ger. Pat., 297,187 (Society oc Chemical Industry, Basle), such compounds
are obtained when, for instance, sodium sulphanilide in alkaline
solution acts upon nitrotoluenesulphochloride, and the resulting
nitrotoluenesulphamino compound is subsequently reduced with acetic acid
and iron. The resulting aminotoluenesulphaminobenzenesulphonic acid is
finally treated with p-toluenesulphonic chloride till the latter
disappears. A compound of the composition

   ^                    ^          ^
  | |                  | |        | |
  | |                  | |---NH---| |
   V                    V          V
  SO_2Na               CH_2

is thereby obtained, which, when acidified, is readily capable
of being used for tanning purposes.

The intermediary product of the aminotoluenesulphaminobenzenesulphonic
acid obtained by this process may again be employed for the purpose of
reacting with one-half molecule soda and 1 molecule
nitrotoluenesulphonic chloride. The following compound is obtained--

    ---NH---SO_2---                   ---NH---SO_2---
   ^               ^                 ^ CH_3          ^
  | |             | |               | |             | |
  | |             | |               | |             | |
   v               v ---NH---SO_2--- v               v
   SO_3Na          CH_3                              CH_3

If _p_-toluenesulphaminobenzenesulphonic chloride is condensed
with sodium sulphanilide, a compound,

    ---SO_2---NH---                  NaSO_3
   ^               ^                 ^
  | |             | |               | |
  | |             | |               | |
   v               v ---SO_2---NH--- v

is obtained which, when acidified, exhibits tannoid properties.

On condensing sodium phenolsulphonate with nitrotoluenesulphonic
chloride, reducing the condensation product and condensing the latter
with _p_-toluenesulphonic chloride, a compound similar to the above is

   ^              ^                 ^ CH_3
  | |            | |               | |
  | |            | |               | |
   v              v ---NH---SO_2--- v
   NaSO_3         CH_3

Again, a similar product is obtained when
_p_-toluenesulphaminobenzenesulphonic chloride or its homologues or
isomers are condensed with sodium-_o_-cresylsulphonate--

    ---SO_2---NH---                 SO_3Na
   ^               ^                ^
  | |             | |              | |
  | |             | |          CH_3| |
   v               v ---SO_2---O--- v

The chloride of this compound may again be condensed, for instance, with
sodium aminotoluenesulphaminobenzene-sulphonate, and yields the

   ^               ^                 ^ ---NH---SO_2--- ^
  | |             | |               | |               | |
  | |             | |               | |               | |
   v               v ---NH---SO_2--- v                 v CH
   NaSO_3          CH_3

The three latter compounds, when dissolved in water and the solution
acidified, exert tanning action.

It is also possible to employ mixtures of arylsulphaminobenzylsulphonic
acids in acidified aqueous solution for tanning purposes. According to
Ger. Pat., 297,188, such mixtures are obtained by nitrating
benzylchloride and heating with an equimolecular amount of sodium
sulphite; the sodium nitrobenzylsulphonate thus obtained is reduced to
aminobenzylsulphonic acid with iron and acetic acid, and finally
condensed with the calculated amount of _p_-toluenesulphonic chloride. A
mixture _o_- and _p_-toluenesulphaminobenzylsulphonic acid [Footnote 1:
Cf. also Ger. Pat, 319,713 and 320,613.] thus results.

Amongst _aromatic alcohols_ the dihydric alcohols show characteristic
behaviour; the latter combine with sulphonic acids with the elimination
of water, condensation taking place without formaldehyde, and the
resulting products being soluble in water and possessing tannoid
properties. [Footnote 2: Ger. Pat., 300,567, of 20th September 1917.]
In addition to phenolic mono- and disulphonic acids (and higher
sulphonation compounds), the homologues, cresols, xylenols, and
naphthols enter into reaction. The two components condense with great
ease, liberating heat; dilute solutions (of the components) are heated
to about 100° C., the process being complete in a few minutes. The
products obtained are exceedingly pure and are easily
crystallisable. Employing 1, respectively 2, molecules of sulphonic
acid, the reactions take place according to:--

  OH                    CH_2.OH         OH                     OH
    }C_6H_4 + HO.C_6H_3{       = H_2O +    }C_6H_3-CH_2-C_6H_3{
HSO_3                   CH_2.OH        HSO_3                   CH_2.OH

  OH                    CH_2.OH                      CH_2.C_6H_3{
    }C_6H_4 + HO.C_6H_3{       = 2(H_2O) + HO.C_6H_3{            HSO_3
HSO_3                   CH_2.OH                      |           OH

  OH                         CH_2.OH          OH                        OH
    }C_6H_3.CH_3 + HO.C_6H_3{       = H_2O +    }C_6H_2.CH_3.CH_2.C_6H_3{
HSO_3                        CH_2.OH        HSO_3                    CH_2.OH

  OH                         CH_2.OH                     CH_2.C_6H_2.CH_3{
    }C_6H_3.CH_3 + HO.C_6H_3{       =2(H_2O) + HO.C_6H_3{              HSO_3
HSO_3                        CH_2.OH                     |                OH

  OH                       CH_2.OH         OH                       OH
    }(C_10)H_6 + HO.C_6H_3{      = H_2O +    }(C_10)H_5.CH_2.C_6H_3{
HSO_3                      CH_2.OH       HSO_3                      CH_2.OH

  OH                       CH_2.OH                     CH_2.(C_10)H_5{
    }(C_10)H_6 + HO.C_6H_3{       =2(H_2O) + HO.C_6H_3{               HSO_3
HSO_3                      CH_2.OH                     |              OH

The condensation products above enumerated were tested with regard to
their tanning power, both non-neutralised and partly neutralised (1:10,
1:20, and 1:30 c.c. N/10 NaOH) samples being examined. In all cases
rapid tannage was observed yielding firm and soft leathers of light
brown colour and varying degrees of swollenness.

Relatively to their reactions, all the products strongly precipitate
gelatine, whereas only the condensation products of phenol, cresol, and
xylenol derivatives give a characteristic coloration with iron salts.

The tannin contents of the non-neutralised condensation products lie
between 72-80 per cent.--figures which clearly indicate the purity and
efficiency of these substances.

Notable amongst _aromatic acids_ is salicylic acid, C_6H_4.OH.COOH,
which at higher temperatures is easily sulphonated with concentrated
sulphuric acid; the sulphonation product represents a white solid, which
easily dissolves in water forming a clear liquid. The sulphonic acid,
when mixed with about one-third of its weight of water and heated to
about 120° C., is easily condensed with formaldehyde. Towards the end
of the reaction, considerable frothing sets in, but in spite of the high
temperature required by this reaction no insoluble bakelites are
formed. A reddish-brown fluid is obtained easily soluble in water, to
which it imparts a brown colour. An aqueous solution of the product
completely precipitates gelatine, gives a strong opalescence with
aniline hydrochloride and a deep violet coloration with ferric chloride.
Neutralised as usual, the product, in a 3° Bé solution, converts pelt
within three days into a white, full leather of good tensile strength.

This process has been patented by the Deutsch-Koloniale Gerb und
Farbstoff Gesellschaft (German-Colonial Tanning and Colour Extracts
Ltd.) in Karlsruhe, the letters patent also including the ring
homologues of salicylic acid. Similar results are obtained when
cresotinic acid (hydroxy-toluic acid), OH.C_6H_3.CH_3.COOH, is employed
as base.

If the phenyl ester of salicylic acid, _Salol_,


is sulphonated, a product is obtained which is easily soluble in water,
but which is identified as a mixture of the sulphonation products of
salicylic acid and phenol, the salol being dissociated on
sulphonation. The temperature must not exceed 80° C. by condensation
with formaldehyde, or insoluble bakelite will be formed from the phenol;
the aldehyde must also be added gradually. An aqueous solution of the
partly neutralised condensation product has a pronounced tanning effect
on pelt, and converts the latter into leather in one to two days; the
leather being very similar to that produced by the salicylic acid
condensation product. The qualitative reactions of the product in
aqueous solution are the same as those given by the salicylic acid
condensation product.

Salicylic acid may, however, also be condensed with formaldehyde without
first being sulphonated; in this case, a little hydrochloric acid should
be present. A product slightly soluble in water is obtained, which may
be looked upon as being methylenedisalicylic acid. In alkaline solution
it is easily soluble,

[Footnote 1: Its solubility in alcohol and alkalies renders this product
an effective and cheap substitute for shellac.--_Transl._]

the liquid possessing an intensely bitter taste. The sodium salt gives a
deep violet coloration with ferric chloride, a slight precipitate with
gelatine, and slight opalescence with aniline hydrochloride. In contact
with pelt, however, it exhibits no tanning effect, but when dissolved in
alcohol, a pickling effect may be observed.

[Footnote 2: A similar reaction is observable in the case of the sodium
salts of METHYLENEDISALICYLIC acid brommated or iodised, which form a
clear solution varying from red to reddish-brown.]

The attempt at preparing a condensation product from
sodium-_m_-hydroxybenzoate by means of formaldehyde and bisulphite is
worthy of attention. A dark brown, viscous liquid is obtained which is
perfectly soluble in water, and the aqueous solution of which gives
opalescence with gelatine, a precipitate with aniline hydrochloride, and
a bluish-black coloration with ferric chloride. Its behaviour towards
pelt is very similar to that of phenolsulphonic acid, and it yields a
similar leather.

A very similar condensation product was obtained by condensing
sodium-_p_-hydroxybenzoate with formaldehyde and subsequent sulphonation
with sulphuric acid. From a practical standpoint, however, these
substances cannot be employed, since their tanning action is only
effective in acid solutions of such concentration of acid as would
gelatinise the pelt.3

If, on the other hand, non-condensed methane derivatives of phenol,
_e.g._, hydroxyphenylmethanesulphonic acid, are partly neutralised and a
solution of the product thus obtained used for tanning experiments, no
tanning action is observable. The acidified solution does not
precipitate gelatine, and gives a dark brown coloration only with ferric

GALLIC ACID, C_6H_2(OH)_3COOH, when heated with sulphuric acid, is
easily converted into the insoluble rufigallic acid, which is also
insoluble in alcohol. If, however, gallic acid is heated with an excess
of sulphuric acid, the product cooled and treated with formaldehyde, a
deep brown condensation product is obtained which is soluble in alcohol,
and in this state is capable of converting pelt into a substance similar
to leather which, though rather hard, possesses good tensile
strength. This water-insoluble condensation product is also soluble in
alkalies, the solution exhibiting properties similar to that described
above. Gallic acid, therefore, is not a suitable base for the production
of synthetic tannins soluble in water.

Phthalic acid also is difficult to sulphonate: the sulphonated compound
treated with formaldehyde gives only water-insoluble condensation

3. Condensation Of Naphthalene Derivatives

The simplest method of condensing [Greek: b]-naphthalene-sulphonic acid
is to heat it at 135° C. at a pressure of 20 mm. for several
hours.[Footnote: Austr. Pat., 61,061, of 10th September 1913.] The
resulting product is a cheesy mass which reacts strongly acid. By
reducing the acidity of the substance to 1 gm. = 10 c.c. N/1O NaOH, a
grey, cheesy mass results, which easily dissolves in water, the solution
being coloured a light yellow-brown and precipitating gelatine aniline
hydrochloride; no coloration, however, appears on adding ferric

The condensation of [Greek: b]-naphthalenesulphonic acid, however,
proceeds with much greater energy in the presence of formaldehyde. In
practice, for instance, 10 kilos of naphthalene is heated with the same
weight of concentrated sulphuric acid (66° Bé), when the mixture is
converted into [Greek: b]-naphthalenesulphonic acid by heating for
several hours at 150°-160° C; the sulphonation completed, the sulphonic
acid is cooled to about 85° C., and 4 kilos of formaldehyde (30 per
cent, by weight) slowly added; finally, the product is stirred at the
temperature mentioned till all formaldehyde has combined.[Footnote:
Austr. Pat., 69,194, of 25th June 1915; Ger. Pat, 290,965.]

Tanning experiments with this product yielded, in a short time, a nearly
white coloured leather (see later).

In addition to formaldehyde, there are other substances which induce
condensation of naphthalenesulphonic acid; if, for instance, sulphur
chloride is allowed to act upon [Greek: b]-naphthalenesulphonic acid, a
light brown solid of pronounced acidic character is obtained; if the
latter is partly neutralised with caustic soda, a greyish-brown solid
results, which dissolves in water with a light brown colour, the
solution precipitating gelatine and aniline hydrochloride, but giving no
coloration with ferric chloride.[Footnote: Austr. Pat., 96,194.]

Tanning experiments with this product in aqueous solution gave a light
brown, rather soft leather, and this, in addition to the qualitative
reactions of the substance, prove that this method of condensation
hardly alters the character of the product from a tanning point of
view. The brown coloration imparted to the leather tanned with this
condensation product owes its existence to coloured intermediary

Attempts at condensing chloronaphthalenesulphonic acid and
nitronaphthalenesulphonic acid resulted in soluble condensation products
which gave some of the reactions given by the tannins (precipitation of
gelatine and aniline hydrochloride), but which were incapable of tanning
pelt, a light tannage being effected on the surface only.

[Greek: a]-Naphthol dissolved in hot concentrated sulphuric acid and
heated for some time on the water bath, yields the light brown,
water-soluble [Greek: a]-naphtholsulphonic acid. A dilute solution of
the latter, when treated with formaldehyde in the cold, undergoes no
change; on heating the mixture on the water bath a brown precipitate is
thrown down. If gelatine solution is added to the opaque liquid, a
yellow flocculent precipitate separates. If caustic soda is added to the
opaque liquid containing the condensation product described above, a
clear solution results from which no deposit separates on the addition
of acetic acid. Gelatine is precipitated by this solution.

The concentrated hot a-naphtholsulphonic acid, upon addition of
sufficient formaldehyde, effervesces strongly and yields a dark brown
condensation product insoluble in water, but soluble in caustic soda. If
acetic acid is added in excess to the alkaline solution, the resultant
solution strongly precipitates gelatine.

A suspension in water of the insoluble condensation product does not
precipitate gelatine.

b-Naphthol, dissolved in hot concentrated sulphuric acid and heated for
some time, yields the light brown, viscous b-naphtholsulphonic acid. A
dilute solution of the latter, mixed with formaldehyde, remains clear;
when heated on the water bath, however, it assumes a dark,
reddish-yellow colour, and remains soluble in water and precipitates
gelatine strongly. This condensation product, on adding excess of
caustic soda, assumes a deep blue coloration, the alkaline solution
giving no precipitate with gelatine; on adding acetic acid the solution
turns brown, remains clear, and now precipitates gelatine.

The concentrated b-naphtholsulphonic acid heated with formaldehyde on
the water bath yields as condensation product a dark, reddish-yellow
mass, soluble in water, which precipitates gelatine. A dilute solution,
when allowed to act upon pelt, gave in a few days a light brown leather,
the properties of which are very similar to those possessed by vegetable
tanned leathers.

The use of naphtholsulphonic and aminonaphtholsulphonic acids for the
manufacture of synthetic tannins is protected by Ger. Pats., 293,640,
293,693, 293,042, and 303,640. [Footnote: _Cf._ Austr. Pat., 70,162.]

It is a remarkable fact that non-condensed methane derivatives of
naphthol, _e.g._, b-naphthol-a-methanesulphonic acid, dissolved in water
and partly neutralised, are devoid of tanning character when allowed to
act upon pelt. Neither does this substance precipitate gelatine, but it
does give a deep blue coloration with ferric chloride.

The condensation product of b-naphthol above referred to precipitates
gelatine and aniline hydrochloride and gives a brown coloration with
ferric chloride.

Thionaphtholsulphonic acid, when acted upon by formaldehyde, yields a
condensation product of the following constitution:--

  HSO_4 ^ ^ SH          SH ^ ^ HSO_4
       | | |              | | |
       | | |_____CH_2_____| | |
        v v                v v

This is a light yellow powder which, dissolved in water, yields an
opaque solution; the latter only exhibits any tanning properties when it
is not neutralised and even slightly acidified and then precipitates
gelatine, aniline hydrochloride and barium chloride; dissolved in
alkali, it forms a clear, yellow solution devoid of tannoid
properties. Leather tanned with the acidified solution is very similar
to those tanned with the phenolsulphonic acid condensation products; its
colour, however, is more pronouncedly yellow.

b-Naphthol condensed with hydrochloric acid and formaldehyde yields a
methylenedinaphthol, which is insoluble in water; the sodium salt,
however, easily dissolves. The same condensation, however, takes place
in alkaline solution with direct formation of the sodium salt. The
condensation product gives a slight precipitate with gelatine, and a
bluish-grey precipitate with ferric chloride; acids re-precipitate the
insoluble methylene compound. Towards pelt it exhibits tanning
properties, whereby the insoluble product referred to above is
deposited, and soft, full, and white leather is obtained, possessing,
however, but little tensile strength.

4. Condensation of the Anthracene Group

Anthracene heated with excess sulphuric acid yields the water-soluble
anthracenesulphonic acid; the latter, when heated with formaldehyde,
yields water-soluble, reddish-brown condensation products, which remain
soluble on prolonged heating with formaldehyde. The aqueous solution of
the condensation product shows no particular reactions; it gives a
flocculent precipitate with gelatine and a green precipitate with copper
sulphate, soluble with blue colour in excess of the reagent.

The partly neutralised solution tans pelt--to which it imparts a brown
colour--in eight days, but on the surface only; the inner layers are
merely pseudo-tanned (white colour). When dried, pelt thus treated
yields a full and soft leather with brown grain and flesh possessing but
little tensile strength. Hence, this condensation product exerts a
pickling rather than a tanning effect.

Anthraquinone heated with sulphuric acid and treated with formaldehyde
in the usual manner, yields a substance which, when mixed with water,
forms an opaque, milky solution. This is not altered by excess of
caustic soda. The aqueous solution precipitates gelatine and aniline
hydrochloride; all other tannin reagents give no reaction.

The partly neutralised solution of the condensation product exerts, in
the main, a pickling action on pelt; only the surface of which is
tanned, with brown colour, the remainder being merely pickled (white
colour). During "tannage," bakelite is formed in the liquid, and
practically all solubles originally present are deposited. The tannage
completed, a light brown, fairly soft and full leather, possessing
little tensile strength, results; this leather can be washed only with
great difficulty and approaches more the character of a pickled pelt.

1-Hydroxyanthraquinone, 1,5-dichloroanthraquinone,
l,5-diaminoanthraquinone, 1-methylaminoanthraquinone,
1-benzoylamino, 6-chloranthraquinone, 1-_m_-toluidoanthraquinone, when
treated with sulphuric acid and formaldehyde, all yield condensation
products which are but little soluble in water, and which do not at all
precipitate gelatine. Tanning experiments with these condensation
products in alcoholic solution yielded empty leathers of pronounced
pickle character.

If, however, 1-methylamino-4-bromanthraquinone is condensed with
sulphuric acid and formaldehyde, a condensation product is obtained
which is but slightly soluble in water, but which precipitates gelatine.

When phenanthrequinone is heated with excess of sulphuric acid for some
time, a water-soluble, reddish-yellow coloured condensation product
results. The latter, when treated with formaldehyde in the cold and then
finally heated, gradually fixes the formaldehyde and forms a substance
soluble in water. If the heating, however, is prolonged, insoluble
bakelites are formed, which are neither soluble in alkali nor in

An aqueous solution of these condensation products gives no reactions
with the usual tannin reagents, though it completely precipitates
gelatine. When acting upon pelt, the partly neutralised dilute solution
of the condensation product pickles the former, and after a few days the
pelt is converted into a light brown, full, and rather soft leather
possessing good tensile strength.

When the condensation product is acted upon by bromine in hot aqueous
solution, an additive compound is formed and the resulting product is
soluble in water. The aqueous solution of the brominated product gives
no special reactions with the usual tannin reagents, but precipitates
gelatine completely. Its tanning action upon pelt is much slower than
that of the original condensation product; the surface of the pelt only
is tanned with brown colour, the inner pelt being only pickled (light
brown colour). When dried, a hard and empty leather of good tensile
strength is obtained, possessing mainly the properties of a pickled

               CO OH
              ^ ^ ^
QUINIZARENE, | | | | , treated with sulphuric acid
             | | | |
              v v v
               CO OH

and formaldehyde, yields a condensation product which is but little
soluble in water and which does not precipitate gelatine.

QUINOLINE, when sulphonated and condensed with formaldehyde, yields a
dark coloured condensation product, completely soluble in water; the
solution does not precipitate gelatine.

OXYQUINOLINE exhibits similar behaviour.

On the other hand, the use of _retene_ (methylisopropylphenanthrene),

  CH_3 ^ ___________ ^
      | |           | |
      | |___CH:CH___| |
C_3H_7 v             v

for the production of synthetic tannins, is protected by
Ger. Pat., 290,965 [Footnote 1: _Cf_ Austr. Pat., 69,194]

5. Di- and Triphenylmethane Groups

If DIPHENYLMETHANE, (C_6H_5)_2CH_2, is heated with excess sulphuric
acid, a dark blue mass, easily soluble in water, is obtained. The
product gently heated with formaldehyde yields a brown, water-soluble
condensation product; once condensation is complete, the product will
stand stronger heat. If, on the other hand, more formaldehyde is added,
brown, water-insoluble bakelites are formed. The water-soluble
condensation product precipitates gelatine, but not aniline
hydrochloride. Dissolved in water, it possesses tannoid properties: the
pelt is, however, tanned on the surface only, the intermediary layers
being merely pickled; after four days in the solution, the pelt after
drying was found to be converted into a greyish-brown, badly coloured
leather, which was empty, hard, and possessed but little tensile

CARBAZOLE (dibenzopyrrole),

       ^ _____ ^
      | |     | |
      | |__ __| |
       v   v   v

on the other hand, was found a suitable base for the commercial
production of synthetic tannins; its use is protected by Ger. Pat,

TRIPHENYLMETHANE, (C_6H_5)_3CH, heated with excess sulphuric acid,
yields a nearly black mass which, when condensed with formaldehyde in
the cold, and subsequently heated, yields a mass which is soluble in
water. With gelatine and aniline hydrochloride it exhibits reactions
similar to those given by the diphenylmethane condensation products; its
tanning properties also are similar to those of the latter. The
resultant leather is black, but is soft and full and possesses good
tensile strength.

Baeyer's observation, [Footnote: _Ber_., 1872, 5, 280, 1096.] that
pyrogallol on condensation with formaldehyde yields an amorphous body
soluble in water, which precipitates gelatine and is very similar to
tannin, was confirmed by Caro [Footnote: _Ibid_., 1892, 25, 947.] and
Kahl. [Footnote: _Ibid_., 1898, 31, 114.] These investigators found
that by the condensation of phenols and hydroxybenzoic acids with
formaldehyde, diphenylmethane derivatives were formed; pyrogallol yields


Nierenstein [Footnote: _Collegium_, 1905, 221.] repeated these
experiments, and found that in addition to the insoluble
diphenylmethanes, water-soluble bodies were formed, which latter
precipitate gelatine. The condensation product yielded by gallic acid
was identified as hexahydroxyaurinecarboxylic acid--

  | }C_6H(OH)_2COOH

which is formed in addition to hexahydroxydiphenylmethane-dicarboxylic


Baeyer's experiment with pyrogallol probaly also yields, according to
Nierenstein, another compound of the following constitution--


Nierenstein considers these bodies confirmation of his hypothesis of the
existence of a "tannophor,"--CO--, in the tannins.

This supposition was adopted by Stiasny [Footnote: _Gerber_, 1905, 233.]
and Kauschke [Footnote: _Collegium_, 1906, 362.] and the latter points
out that these easily soluable substances exhibit tanning
properties. Nierenstein [Footnote: _Ibid_., 1906 424.] was further able
to show that by all processes of condensation between phenols (or
hydroxybenzoic acids) and formaldehyde, compounds of the character of
hydroxyaurine (or hydroxyaurinecarboxylic acid) were formed in addition
to the insoluble hydroxydiphenylmethanes (or
hydroxydiphenylmethanecarboxylic acids), the former possessing the
characteristic tannophor group and hence precipitating gelatine, _i.e._,
exerting tanning action. If the formation of leather is viewed in the
light of Schiff's base, [Footnote: _Ibid_., 1905, 159.] one may
consider the constitution of a hexahydroxyaurinecarboxylic acid leather
as follows:--


In the preparation of these and similar condensation products,
Nierenstein and Webster [Footnote: _Ber_., 1908, 41, 80.] observed a
peculiar steric effect of the carboxyl group. Each 2.5 gm. of the phenol
or the acid in question were dissolved in 30 c.c. of water, the solution
brought to boil and 5 c.c. formaldehyde (20 per cent.) and 2.5
c.c. hydrochloric acid added drop by drop; the precipitate formed was
filtered off after twenty-four hours, dried at 110° C. to constant
weight, extracted (in a Gooch crucible) freely with water, and the
residue again dried at 110° C. till constant. The following values were

                               Total       Insol. Aq.    Sol. Aq. Oxy-
                            Precipitate  Diphenylmethane aurinecarboxylic
                             in Grammes.  Derivatives      Acid.

                                          Per Cent.      Per Cent.
Phloroglucinol                  2.4002       100            ...
Hydroquinone                    2.3716       100            ...
  "                             2.0542       100            ...
Pyrogallol                      2.5150       100            ...
  "                             2.7940       100            ...
Pyrocatechol                    2.9805       100            ...
  "                             2.9574       100            ...
Resorcinol                      2.9954       100            ...
  "                             2.9725       100            ...
Gallic acid                     2.0706        78.84        21.16
  "                             1.2240        83.18        16.82
  "                             1.1405        59.94        41.06
[Greek: b]-Resorcylic acid      2.1040        51.08        48.92
  "       "                     2.2008        47.12        52.88
Protocatechuic acid              ...           ...          ...
  "       "                      ...           ...          ...
Vanillic acid                    ...           ...          ...
Tannin                          2.0599         ...     Nearly all sol.
Digallic acid                   2.1042        80.16        19.84
Leucodigallic acid              2.0041        1.94         98.06

With the introduction of the carboxylic group the tendency of
condensation to diphenylmethane derivatives is lessened; by
protocatechuic acid the tendency is nil. Nierenstein considers this
reaction analogous to the formation of cork, to the genetic relation of
which with the diphenylmethane formation Drabble and Nierenstein have
referred in an earlier publication. [Footnote: _Biochemical Jour._.,
1907, 2, 96.] It is hence possible that the plants may employ
formaldehyde as a methylation medium, and produce these insoluble
condensation products for the purpose of ridding themselves of the
poisonous phenols and aromatic hydroxy acids (and tannins), in addition
to oxidising processes whereby
phlobaphenes, ellagic acid, etc., are formed.

The reaction between phenols and aldehydes has been further studied by
Michael, [Footnote: _Amer.Jour_., 5, 338; 9, 130.] who prepared a
condensation product from phenol and resorcinol with benzaldehyde, and
Russanow, [Footnote: _Ber_.9 1889, 22, 1944.] who also employed
benzaldehyde and phenol. Lipp [Footnote: Diss., Bern., 1905.]
investigated the action of benzaldehyde and piperonal on phenols,
anisoles, cresols, cresylic ether, resorcinol, and the ether of the
latter and phenol, and showed that when free phenols are condensed with
benzaldehyde the hydroxyls occupy the same position as by the
interaction between benzaldehyde and the corresponding phenolic
ethers. The resulting dihydroxytriphenylmethane derivatives form
beautiful crystals, which on oxidation are converted into benzaurines,
the constitution of the latter probably being--

    O= ^=_____ ^ OH
      | |     | |
      | |== __| |
      =v   v   v

In alkalies, the hydroxylated triphenylmethanes dissolve without
imparting any colour to the solution; by concentrated sulphuric acid
they are taken up with intense coloration.

If the hydroxyls occupy the ortho-position to methyl, they may form
xanthenes by splitting off water--

       ^ ^ ^
      | | | |
      | | | |
  CH_3 v v v CH_3

In the benzene series this reaction is difficult to establish, and has
to be induced by distilling the particular dihydroxy-diphenylmethane at
ordinary pressure. In the naphthalene series, on the other hand, the
ring closes up by, for instance, the condensation of [Greek: b]-naphthol
with benzaldehyde or paraldehyde, and yields the following compounds:--

            C_10H_6              C_10H_6
  C_6H_5-CH{       }O    CH_3-CH{       }O
            C_10H_6              C_10H_6

These xanthenes are white, silk-glossy needles, which are soluble in
water and in alkalies. In concentrated sulphuric acid, they are taken up
with beautiful fluorescence.

6. Summary

From the qualitative reactions of the different condensation products
described it may be seen that their tannoid properties are not dependent
on whether they precipitate gelatine or are adsorbed by hide powder or
not. Hydroxynaphthylmethanesulphonic acid, for instance, precipitates
gelatine but does convert pelt into leather; on the other hand, sodium
dicresylmethanesulphonate does not precipitate gelatine, and neither
does it tan pelt; nevertheless it is adsorbed by hide powder as "tanning
matter". The author discovered that _o_-nitrophenol does not precipitate
gelatine, but has some tanning action on both hide powder and pelt.

Relatively to the possibilities of forming condensation products
possessing tannoid properties, the following may be stated:--

All mono- and polyhydric phenols may be converted into true tanning
matters by either condensing them as such, or after their conversion
into the corresponding sulphonic acids, by substances capable of
eliminating the elements of water. It makes no difference to the final
product whether the condensation is the first step followed by
sulphonation and consequent solubilisation of the intermediary insoluble
product, or whether, vice versa, the sulphonic acid is subjected to
condensation. Alkaline solution of phenols may also be condensed, the
reaction products, when condensed, constituting tanning matters soluble
in water.

Among the substitution products of the phenols, the thio-, chloro-,
bromo-, nitro-, and aminophenols as a rule yield tanning matters similar
in character.

The quinones are as such--_i.e._, without being condensed--substances
possessing tannoid properties.

The aromatic dihydric alcohols are easily condensed with the different
sulphonic acids and yield valuable tanning matters.

Of aromatic acids all those which yield water-soluble sulphonation
products seem suitable for the industrial production of tanning
matters. If the acids themselves do not yield water-soluble sulphonation
products, the alkali salts of the latter may be condensed with
formaldehyde, and the resulting products then constitute tanning matters
provided their solutions can be neutralised or faintly acidified without
the solute being thrown out of solution in insoluble form.

The diphenyl derivatives of the above groups often possess tannoid

The same holds good of those compounds with condensed nuclei
(naphthalene, anthracene, etc.), and all their derivatives which satisfy
the above conditions.

The choice of condensing agent is, as a rule, of little
significance. Elimination of the elements of water by the mere
application of heat succeeds in few cases only, since the high
temperature required to induce reaction in many cases causes
decomposition of the substances. This difficulty is overcome by heating
_in vacuo_. Condensation with formaldehyde always succeeds, with
acetaldehyde and benzaldehyde only partly.

The action on hide powder, pelt, and gelatine by these characteristic
substances is tabulated below:--
                                   Relative Behaviour towards
Substance.                        Gelatine.   Hide Powder   Pelt
Formaldehyde                        ...          ...        Tanning
Phenol                             Ppte.         ...         ...
Chlorophenol                        "            ...         ...
Tribromophenol                   Slight ppte  Tanning       tanning
_o_ Nitrophenol                   No ppte        "            "
Bromonitrophenol                 Slight ppte     "            "
Trinitrophenol                      Ppte         "          Tanning
Bromotrinitrophenol              Slight ppte     "            "
_p_ Aminophenol                     Ppte         ...          ...
_m_ Dihydroxybenzene                 "           ...          ...
Orcinol                              "           ...          ...
_p_ Dihydroxybenzene                 "         Tanning      Tanning
Monochloro _p_
dihydroxybenzene                     "           ...          ...
_o_ Dihydroxybenzene                 "           ...          ...
Pyrogallic acid                      "           ...          ...
Tribromopyrogallic acid              "         Tanning      Tanning
Gallic acid                        No ppte    Not tanning  Not tanning
Bromophloroglucinol                 Ppte       Tanning        "
Gallotannic acid                     "            "         Tanning
Galloflavine                     Slight ppte      "        Not tanning
Quinone                              "            "         Tanning
Bromosalicylic acid                  "            "        Not tanning
Dinaphthylmethanedisulphonic acid   Ppte          "         Tanning
Diphenylmethanedisulphonic acid      "            "           "
Dicresylmethanedisulphonic acid      "            "           "
dicresylmethanedisulphonate acid   No ppte        "        Not tanning
Dixylylmethanedisulphonic acid      Ppte          "         Tanning
Naphtholdisulphonic acid             "        Not tanning  Not tanning
Methylenedinaphthol                  "         Tanning      Tanning
Hydroxyphenylmethanesulphonic        "            "           "
acid                                                       Not tanning
Hydroxynaphthylmethanesulphonic   Slight ppte     "           "
Diaminonaphthylmethanedisulphonic   Ppte       Tanning     Not tanning
acid                                  "           "           "
acid                                  "           "        Surface tanning
acid                                  "           "           "
acid                                  "           "         Tanning
Bromo _[Greek: b]_ naphthol [1]   Slight ppte     "            "
Rosolic acid_ [1]                   Ppte          "            "

[Footnote 1: In alcoholic solution.]



1. Mixture of Phenolsulphonic Acid and Formaldehyde

The most important invention relatively to the search for new tanning
materials was that of Weinschenk,[Footnote: Ger. Pat., 184,449.] who
first showed that pelt may be converted into leather by the action upon
it of mixtures of naphthols and formaldehyde. This process consists of
two steps: the pelt is first immersed in a 0.25-0.50 per cent,
formaldehyde solution, and secondly in an aqueous solution of -[Greek:
a] or -[Greek: b] naphthol; this order may be reversed. If, on the other
hand, a pasty mixture is made of formaldehyde and naphthol, and this is
allowed to act upon the pelt, the latter is rapidly converted into
leather, but the mixture must be administered very gradually or
otherwise the insoluble methylenedinaphthol is formed outside the pelt
and hinders any tanning effect.

Leather obtained through the action of [Greek: a]-naphthol is, when
freshly tanned, pure white and sufficiently soft and firm, but quickly
assumes a brown colour on storing; if, however, [Greek: b]-naphthol is
employed, a cream-coloured leather results, the colour of which turns
only slightly more yellowish even when exposed to the direct rays of the

A similar process has recently (25, xii., 1915) been protected by
Ger. Pat, 305,516, granted to the Deutsch-Koloniale Gerb--und Farbstofif
Gesellschaft, in Karlsruhe.  According to this patent, pelt is treated
in separate solutions, one of which is formaldehyde, the other being
that of such aromatic compounds or their salts which yield water-soluble
condensation products with formaldehyde; for example, pelt is immersed
in 2-5 per cent, solution of formaldehyde for a few days, and is
subsequently treated with 1-2 per cent neutral or faintly acidified
solutions of [Greek: a]-naphthylamine hydrochloride, resorcinol or
sodium phenate or cresylate, for several days. The resultant leather is
claimed to be soft and full and to possess good tensile strength.

The tanning properties of mixtures of phenolsulphonic acid and
formaldehyde have been examined by the author with the following

                                         I.    II.     III.

Grammes formaldehyde                     10    20      40
    "   phenolsulphonic acid             20    50      100
    "   caustic soda (sol, 40 per cent.) 10    20      40
    "   water                            500   500     500

The above solutions were made up and allowed to act upon pelt pieces
weighing 15 gm.; whereas Solution I. remained clear throughout the
experiment, Solution II. became somewhat clouded, and Solution
III. assumed a milky appearance. The pelts were tanned through in seven
days and yielded leathers which, after drying and finishing, possessed
yellow colour, long fibre, and good tensile strength, but a rather empty

To prevent separation of insoluble matter during tannage, another
experiment was carried out, in which the pelts were first submitted to
the action of formaldehyde (10, 20, and 40 gm. in 500 c.c. water) for
three days, being subsequently removed to fresh solutions of partly
neutralised phenolsulphonic acid (_cf_. above). Similar results were
obtained, but the leather felt even more empty than those obtained by
the former experiment.

Attempts at converting pelt into leather by first immersing the pelt in
a partly neutralised solution of phenolsulphonic acid, and subsequently
transferring it to fresh solutions of formaldehyde, gave merely negative
results; the phenolsulphonic acid effected pickling action upon the
pelt, but was subsequently quickly replaced by the formaldehyde, before
the latter had penetrated the pelt in sufficient quantity to induce
condensation, thereby exerting tanning action.

To explain the tanning effects of these mixtures, the author analysed
the leathers resulting from the effects of the latter, and was able to
show, that in these cases also, condensation of phenolsulphonic acid and
formaldehyde takes place _inside_ the pelt, since on the one hand the
analyses left no doubt but that true tannage had been effected, and on
the other hand an ammoniacal extract of the leathers gave the typical
reaction for condensation products of phenolsulphonic acid, with aniline
hydrochloride. [Footnote: _Collegium_ 1913, 516, 142.]

The leather analyses gave the following figures:--

             Moisture  -    -    -   18.30 per cent.
             Fats -    -    -    -    0.47    "
           _ Ash  -    -    -    -    0.98    "
Leather    { Tannin    -    -    -   26.37    "
substance  { Hide substance -    -   53.88    "

A characteristic feature is the low value of tannin, which is
considerably higher [Footnote: _Ibid_., 1913, 521, 478.] where
condensation products of phenolsulphonic acids are used as tanning
agents; the action effected by the separate constituents, therefore, is
more that of pickling.

2. Mixture of Phenolsulphonic Acid and Natural Tannins

A piece of pelt was immersed in a half-neutralised solution, measuring
6° Bé., of phenolsulphonic acid, and left sixteen hours in the solution,
which completely penetrated the pelt during this time; it was then
transferred to a 12° Bé. solution of a mixture of quebracho and
chestnut, which in two days converted the pelt into a light coloured
leather possessing good tensile strength.

By using a bath composed of half-neutralised phenolsulphonic acid and
quebracho extract in 7° Bé. solution, another piece of pelt was
completely tanned in two days. The same result was obtained by first
half neutralising the phenolsulphonic acid and then adding sulphited
quebracho extract till a 5° Bé. solution was obtained.

A piece of pelt received a 2º Bé. liquor composed of 3 parts of
phenolsulphonic acid and 1 part of formaldehyde for sixteen hours, and
was then completely penetrated; it was subsequently transferred to a 10º
Bé. liquor composed of chestnut and quebracho, being completely tanned
in two days. The same result was obtained on adding sufficient sodium
sulphate to the above mixture of phenolsulphonic acid and formaldehyde
to raise the density from 2º-3º Bé.

Sixty grammes of phenolsulphonic acid were partly neutralised with 100
c.c. of a 10 per cent solution of caustic soda, and 10 c.c. formaldehyde
added to 400 c.c. of the mixture (2º Bé.): a piece of pelt was
completely penetrated by the solution in sixteen hours, and was
subsequently tanned in two days, using an extract of 10º Bé. Similarly,
by treating a pelt with 400 c.c. of a half-neutralised solution of
phenolsulphonic acid (3º Bé.) plus 8 c.c. formaldehyde, and adding after
eighteen hours sulphited quebracho extract to the same bath,
strengthening the latter to 6º Bé., the pelt was converted into leather
in two days; in this case, however, much of the tannin was precipitated
by the formaldehyde present in the solution. If, on the other hand, a
mixture of 80 gm. dilute phenolsulphonic acid (1:1 aq) and 14 gm. of
formaldehyde were cooled for several hours and subsequently strengthened
with sulphited quebracho extract to 7º Bé., no tannin was precipitated
in the liquor, and a piece of pelt immersed in the latter was completely
tanned in sixteen hours.

To prevent the precipitation of tannin caused by the formaldehyde,
sulphite cellulose extract (wood pulp) was substituted for sulphited
quebracho extract, and the following experiments carried out:--

To 200 c.c. of a 6º Bé. sulphite cellulose extract plus 200 c.c. of
half-neutralised phenolsulphonic acid solution was added 15
c.c. formaldehyde, and this solution tanned pelt in four days; the
resultant leather was light brown, firm, and possessed good tensile
strength and long fibre.

Another piece of pelt was immersed in a solution of 400
c.c. phenolsulphonic acid of 3ºBé. plus 15 c.c. formaldehyde for
eighteen hours, and was then tanned in a 6º Bé. solution of sulphite
cellulose extract. The resultant leather was extremely light coloured,
and possessed qualities similar to those described in the former
experiment. Finally, pelt was immersed in a 6° Bé. solution composed of
140 gm. of a 15° Bé. sulphite cellulose extract, 10 gm. of formaldehyde,
400 gm. water, 15 gm. phenolsulphonic acid, and 30 gm. of a 10 per cent
caustic soda solution, and was tanned in four days. This leather also
was coloured light brown, of good tensile strength, and rather firm.

These experiments prove that when pelt is treated with formaldehyde,
phenolsulphonic acid, and vegetable tannins, the two former components
effect, more or less, actual tannage; it is admittedly a matter of some
difficulty to establish whether the effect is one of pickling or
pseudo-tannage, or whether the tannage may be considered a true one. The
final effect, however, is nearly always that of a true tannage, _i.e_.,
by varying the composition of the tanning solutions leather is obtained
with properties identical with those tanned with true tannins of
vegetable origin. The only difficulty encountered in these combinations
is the property of formaldehyde, of precipitating the natural tannins,
and it is hence essential, for practical purposes, to so arrange the
combination that their value is not reduced by the property referred to.
The fact that not only compounds already existing may convert pelt into
leather, but that a similar effect is obtained _inside the pelt_, by
their components, is indeed of theoretical interest.

3. Tanning Effects of Different Natural Substances

In addition to the vegetable tannins, Nature has also provided other
substances of vegetable origin, which, admittedly, do not effect tannage
in their original state, but which may, by suitable treatment, acquire
this property. The oldest information on this point is supplied by
Resch, [Footnote: _Scherer's Jour_., 1801, 6, 495.] who carried out
tanning experiments, using three parts of peat and one part of oak bark.

By the action of nitric acid on substances of vegetable and animal
origin, Hatchett, [Footnote: _Gehlen's Jour_., 1805, 1, 545.] Chevreul,
[Footnote: _Ann. Chim_., 1810, 73, 36.] and Vogel [Footnote:
_Jour. Chem. Phys_., 1812, 6, 101.] claim to have obtained tanning
materials, whilst later, Buff [Footnote: _Ibid_., 1827, 51, 38.]
obtained a material suitable for tanning purposes from indigo.

By subsequent treatment with lime and soot, or tar, Ashmore [Footnote:
_Dingier's Jour_., 1833, 48, 67.] claims to have converted pelt into

By treating peat with nitric acid, Jennings [Footnote:
_Jahresber. d. Chem_., 1858, 666.] and Payne [Footnote: _
Chem. Centralbl_., 1908, ii. 554; Ger. Pat., 200, 539.] have produced
artificial tanning materials.

Skey [Footnote: _Chem. News_, 1866, 206; _Zeits. f. Chem_., 1866, 753.]
obtained a dark brown extract, soluble in water and precipitating
gelatine, by treating bituminous coal or lignite with nitric acid; by
extracting coal with alkalies, Reinsch [Footnote: _Pharm. Centralh_.,
1887, 141.] isolated a substance (pyrofuscine) which, when partly
neutralised with carbon dioxide, was capable of converting pelt into

In addition to these tanning materials the recovery of a substance
possessing tanning properties from the so-called acid rosins has been
made the subject of a patent; [Footnote: _Ger. Pat_., 36,019.] this
rosin is formed when crude oil is treated with concentrated sulphuric
acid in the oil refineries. The greasy substance is partly neutralised
with alkali and is claimed to produce a very springy leather.

The waste liquors obtained in the manufacture of cellulose, the
so-called sulphite and sodium cellulose waste, have, however, been the
subject of numerous investigations, and several hundred publications
have appeared and a great number of patents [Footnote: "Literatur
überiSulfitablauge" 1910-13. (Reprint from
_WocheWochenblPapiePapierfabrikation_)] taken out, the first one being
that of Mitscherlich [Footnote: _Jahresber. d. Chem_., 1893, 890;
Ger. Pat., 72,161.] and Hönig [Footnote: _Chem. Centralbl_., 1902,
ii. 174; Ger. Pat., 132,224.]

The waste liquors contain large quantities of acids and lime, and in
order to utilise the liquors for tanning purposes, the excessive
sulphuric and sulphurous acids as well as the lime must be removed. The
active tannin is no doubt the ligninsulphonic acid, and those cellulose
extracts containing the largest amounts of free ligninsulphonic acid may
also be considered the most efficient.

According to the author,[Footnote: _Technikum_, 1912, 20, 156.] such
sulphitecellulose extracts precipitate gelatine, aniline hydrochloride,
ammoniacal zinc acetate, and basic coal-tar dyes, and give a
greenish-black coloration with ferric chloride. These reactions indicate
the presence of tanning matters in cellulose extracts.

The official shake method of analysis gives the following
results:--[Footnote: _Ibid_.]

Tanning matters              23.0 per cent.
Non-tannins                  30.3    "
Insoluble matters             0.7    "
Water                        46.0    "
                            100.0 per cent.

Ash                           4.3    "
Sulphurous acid               0.6    "

Many other substances have been used for tanning experiments, a number
of them precipitating gelatine. Zacharias [Footnote:
_Zeits. f. Ang. Chem_., 1907, 1645.] obtained leather by the action of
many coal-tar dyes on pelt, similarly Herzog and Adler, by using
Prussian blue, Neufuchsin, patent blue V, crystal violet, and colloidal

Most inorganic substances possess tanning properties when in the
colloidal state, _e.g_., sulphur, halogens, chromium salts, iron salts,
silver oxide, and the salts of mercury, copper, bismuth, zinc, lead,
platinum, cesium, vanadium, and the rare earths (salts of cerium,
lanthanum, didymium, neodymium, thorium, and zerconium).

For practical purposes, however, only sulphur, chrome, and alum salts
are used, the latter two being of the greatest importance.



Whereas the evaluation of vegetable tanning matters necessitates
determinations of their practical applicability in addition to
qualitative and quantitative analyses, the latter two determinations are
of practically no value when dealing with synthetic tannins. The way in
which tanning matters obtained by chemical means exert their action, in
addition to the intensity with which they convert pelt into leather, is
the only criterion of their quality for practical (tanning) purposes;
both may be demonstrated by experimental tests.

When dealing with the natural tanning materials it is desirable to know
their contents of actual tanning matter, from which their special
qualities as tanning agents may be deduced. Where the vegetable tanning
materials have already been converted into extracts, it is essential to
establish the identity of the original material used by the qualitative
reactions of the extract in addition to the quantitative estimation of
actual tannin contents. It is frequently necessary to examine whether
the extract in question has been actually prepared from the material
giving the extract its name, or whether the extract has suffered the
addition of other extracts of tanning materials of but low quality. Such
determinations may be undertaken by microscopical observations and by
means of qualitative and quantitative reactions; for this purpose many
colour reactions and precipitation methods are available in addition to
the determination of the molybdenum figure (Lauffmann),[Footnote:
Collegium, 1913, 10.] the alcohol and ethyl acetate figures and
microscopical examination (Grasser).[Footnote: Ibid., 1911, 349.] Of
other adulterants tending to reduce the quality of extracts may be
mentioned sugars, mineral salts, and coal-tar dyes; [Footnote: Grasser,
_Collegium_, 1910, 379.] for the determination of these, the special
literature should be consulted. [Footnote: Grasser, "Handbuch
f. gerbereichem. Laboratorien" (Leipzig, 1914);
Procter-Paessler, "Gerbereichem. Untersuchungen" (Berlin, 1901).]

Two methods are devised for the purpose of quantitatively determining
the tannin contents, both of which employ hide powder, and which are
known as the "shake method" and the "filter bell method" respectively:
the former is adopted as the official method of the "International
Association of Leather Trades' Chemists" (I.A.L.T.C.). [Footnote: And
also by the Society of Leather Trades' Chemists.-_Transl._]

The original method, [Footnote: _Leather Manufacturer_, 1894, No. 9
J.S.C.I.,1894, 494.] worked out in the laboratory of the Yorkshire
College (now the University of Leeds), essentially consists in
introducing 6-9 gm. of hide powder in a shaker, washing it at least
twice with distilled water and carefully squeezing out the powder in a
linen cloth between each washing. 100 c.c. of the solution to be
examined, which may not contain more than 1 per cent, total solids, are
introduced into the shaking bottle which is then weighed. About
one-third of the washed hide powder is then added, and the bottle shaken
ten to fifteen minutes; another third is then added and, after shaking,
the third portion. The bottle plus contents is now weighed, and the
amount of hide powder introduced ascertained by difference of the two
weighings. The liquid is then filtered through filter paper, 50 c.c. of
the clear filtrate evaporated in a basin, dried and weighed. The residue
in the original solution is then obtained by multiplying the former by
100 (plus weight of water added with hide powder), and dividing by 100.

This method was closely investigated by a large number of leather
trades' chemists, was considerably improved, and in its final form
presented a method of the highest degree of accuracy; the method was
therefore adopted as _The Official Method of Tanning Analysis_ by the
I.A.L.T.C., which body, at the same time, gave precise instructions as
to the details of the method. The latest instructions, which are
reprinted below, permit of any method of analysis which observes the
following conditions:--

1. The solution for analysis must contain between 3.5 and 4.5 gm. of
tanning matter per litre, and solid materials must be extracted so that
the greater part of the tannin is removed at a temperature not exceeding
50° C.

2. The total solubles must be determined by the evaporation of a
measured quantity of the solution previously filtered till optically
clear, both by reflected and transmitted light. This is obtained when a
bright object such as an electric light filament is distinctly visible
through at least 5 cm thickness, and a layer of 1 cm. deep in a beaker
placed on a black glass or black glazed paper appears dark and free from
opalescence when viewed from above. Any necessary mode of filtration may
be employed, but if such filtration causes appreciable loss when applied
to a clear solution, a correction must be determined and applied as
described in paragraph 6.

Filtration shall take place between the temperatures of 15° C. and 20°
C. Evaporation to dryness shall take place between 98.5° C. and 100°
C. in shallow, flat-bottomed basins, which shall afterwards be dried
until constant at the same temperature, and cooled before weighing for
not less than twenty minutes in air-tight desiccators over dry calcium

3. The total solids must be determined by drying a weighed portion of
the material, or a measured portion of its uniform turbid solution, at a
temperature between 98.5° C. and 100° C. in shallow, flat-bottomed
basins, which shall afterwards be dried until constant weight at the
same temperature, and cooled before weighing for not less than twenty
minutes in air-tight desiccators over dry calcium chloride.

"Moisture" is the difference between 100 and the percentage of total
solids, and "insoluble" the difference between "total solids" and "total

4. _Non-Tannins._--The solution must be detannised by shaking with
chromed hide powder till no turbidity or opalescence can be produced in
the clear solution by salt-gelatine solution. The chromed powder must be
added in one quantity equal to 6.0-6.5 gm. of dry hide powder per 100
c.c. of the tanning solution, and must contain not less than 0.2 per
cent. and not more than 1 per cent. of chromium calculated on the dry
weight, and must be so washed that in a blank experiment with distilled
water, not more than 5 mg. of solid residue shall be left on evaporation
of 100 c.c. All water contained in the powder should be determined and
allowed for as water of dilution.

5. _Preparation of Infusion_.--Such a quantity of material shall be
employed as to give a solution containing as nearly as possible 4 gm. of
tanning matter per litre, and not less than 3.5 or more than 4.5
gm. Liquid extracts shall be weighed in a basin or beaker and washed
with boiling water into a litre flask, filled up to the mark with
boiling water, and well mixed and rapidly cooled to a temperature of
17.5° C., after which it shall be accurately made up to the mark, again
well mixed, and filtration at once proceeded with. Sumac and myrabolam
extracts should be dissolved at a lower temperature.

Solid extracts shall be dissolved by stirring in a beaker with
successive quantities of boiling water, the dissolved portions being
poured into a litre flask, and the undissolved being allowed to settle
and treated with further portions of boiling water. After the whole of
the soluble matter is dissolved, the solution is treated similarly to
that of a liquid extract.

Solid tanning materials, previously ground till they will pass through a
sieve of sixteen meshes per square centimetre, are extracted in Koch's
or Procter's extractor with 500 c.c. of water at a temperature not
exceeding 50° C.; the extraction is then continued with boiling water
till the filtrate amounts to 1 litre. It is desirable to allow the
material to soak for some hours before commencing the percolation, which
should occupy not less than three hours, so as to extract the maximum of
tannin. Any remaining solubles in the material must be neglected or
reported separately as "difficultly soluble" substances.

The volume of liquid in the flask must, after cooling, be accurately
made up to 1 litre.

6. _Filtration_.--The infusion shall be filtered till optically clear
(_vide_ 2). No correction for absorption is needed for the Berkefeld
candle, or for S. and S. 590 paper [Footnote: Schleicher and Schüll,
Düren (Rheinland), Germany.] if a sufficient quantity (250-300 c.c.) is
rejected before measuring the quantity for evaporation, and the solution
may be passed through repeatedly to obtain a clear filtrate.

If other methods of filtration are employed, the average correction
necessary must be determined in the following manner:--About 500 c.c. of
the same or a similar tanning solution is filtered perfectly clear, and
after thorough mixing 50 c.c. is evaporated to determine "Total Soluble
A." A further portion is now filtered in the exact method for which the
correction is required (time of contact and volume rejected being kept
as constant as possible), and 50 c.c. is evaporated to determine "Total
Soluble B." The difference between "A" and "B" is the correction sought,
which must be added to the weight of the total solubles found in
analysis. An alternative method of determining correction, which is
equally accurate and often more convenient, is to filter a portion of
the tanning solution through the Berkefeld candle till optically clear,
which can be generally accomplished by rejecting 300 or 400 c.c., and
returning the remaining filtrate repeatedly; and at the same time to
evaporate 50 c.c. of the clear filtrate obtained by the method for which
correction is required, when the difference between the residues will be
the correction sought. An average correction must be obtained from at
least five determinations. It will be found that this is approximately
constant for all materials, and amounts in the case of S. and S. 605,
150 c.c. being rejected, to about 0.005 gm., and where 2 gm. of kaolin
are employed in addition to 0.0075 gm. The kaolin must be previously
washed with 75 c.c. of the same liquor, which is allowed to stand
fifteen minutes and then poured off. Paper 605 has a special absorption
for a yellow colouring matter often contained in sulphited extracts.

7. Hide powder shall be of a woolly texture, thoroughly delimed,
preferably with hydrochloric acid. It shall not require more than 5
c.c. or less than 2.5 c.c. of decinormal NaOH or KOH to produce a
permanent pink colour with phenolphthalein on 6.5 gm. of the dry powder
suspended in water. If the acidity does not fall within these limits it
must be corrected by soaking the powder before chroming for twenty
minutes in ten to twelve times its weight of water, to which the
requisite calculated quantity of standard alkali or acid has been
added. The hide powder must not swell in chroming to such an extent as
to render difficult the necessary squeezing to 70-75 per cent. of water,
and must be sufficiently free from soluble organic matter to render it
possible in the ordinary washing to reduce the total solubles in a blank
experiment with distilled water below 0.005 gm per 100 c.c. The powder,
when sent out from the maker, shall not contain more than 12 per
cent. of moisture, and shall be sent out in air-tight tins.

The detannisation shall be carried out in the following manner:--

The moisture in the air-dried powder is determined, and the quantity
equal to 6.5 gm. actual dry powder is calculated, which will be
practically constant if the powder be kept in an air-tight vessel. Any
multiple of this quantity is taken according to the number of analyses
to be made, and wet back with approximately ten times its weight of
distilled water. Two grammes per 100 of dry powder of crystallised
chromic chloride, CrCl_3.6aq., is now dissolved in water and made basic
with 0.6 gm. of Na_2CO_3 by the gradual addition of 11.25 c.c. of normal
Na_2CO_3, thus making the salt correspond to the formula
Cr_2Cl_3(OH)_3. In laboratories where analyses are continually being
made, it is more convenient to employ a 10 per cent stock solution, made
by dissolving 100 gm. of Cr_2Cl_6.6aq. in a little distilled water in a
litre flask and very slowly adding a solution containing 30 gm. of
anhydrous sodium carbonate, with constant stirring, finally making up to
the mark with distilled water and well mixing. Of this solution 20
c.c. per 100 gm., or 1.3 c.c. per 6.5 gm. of dry powder, should be
used. This solution is added to the powder, and the whole churned for
one hour. At the end of the one hour the powder is squeezed in linen to
free it as far as possible from the residual liquor, and washed and
squeezed repeatedly with distilled water, until, on adding to 50 c.c. of
the filtrate one drop of 10 per cent. K_2CrO_4 and four drops of
decinormal silver nitrate, a brick-red colour appears. Four or five
squeezings are usually sufficient. Such a filtrate cannot contain more
than 0.001 gm. of NaCl in 50 c.c.

The powder is then squeezed to contain 70-75 per cent, of water, and the
whole weighed. The quantity Q containing 6.5 gm. dry hide is thus found,
weighed out, and added immediately to 100 c.c. of the unfiltered tannin
infusion along with (26.5-Q) of distilled water. The whole is corked up
and agitated for fifteen minutes in a rotating bottle at not less than
60 revs. per minute. It is then squeezed through linen, the fitrate
stirred and filtered through a folded filter of sufficient size to hold
the entire filtrate, returning till clear. Sixty c.c. of the filtrate
is then evaporated and calculated as 50 c.c., or the residue of 50
c.c. multiplied by 6/5. The non-tannin filtrate must give no turbidity
with a drop of a solution of 1 per cent, gelatine and 10 per cent,
common salt. [Footnote: It is convenient for technical purposes to
employ the commercially obtainable chromed hide powder as prepared, for
instance, by the German Experimental Station at Freiberg, Saxony.]

One gramme of kaolin, freed from all soluble matter, may be added to the
filtrate, or it may be used by mixing it with the hide powder in the
shaking bottle.

The analysis of used liquors and spent tans shall be made by the same
methods as are employed for fresh tanning materials; the liquors being
diluted, are concentrated by boiling _in vacuo_, or in a vessel so
closed as to restrict access of air, until the tanning matter is if
possible between 3.5 and 4.5 gm. per litre, but in no case beyond a
concentration of 10 gm. per litre of total solids, and the weight of
hide powder used shall not be varied from 6.5 gm.

The results shall be reported as shown by the direct estimation, but it
is desirable that in addition efforts shall be made, by determination of
acids in the original solution and in the non-tannin residue, to
ascertain the amount of lactic and other non-volatile acids absorbed by
the hide powder, and hence returned as "tanning matters."

In the case of tanning materials it must be clearly stated in the report
whether the calculation is on the sample with moisture as received, or
upon some arbitrarily assumed percentage of water; and in that of
liquors whether the percentage given refers to weight or to grammes per
100 c.c., and in both cases the specific gravity shall be reported.

All analyses reported must be the average result of duplicate
determinations, which must agree in the case of liquid extracts within
0.6 per cent, and of solid extracts within 1.5 per cent, or the analysis
shall be repeated until such agreement is obtained.

All reports shall be marked: Analysed in accordance with the rules of
the S.L.T.C. (I.A.L.T.C.)--when the analyses have been carried out
according to the method described above.

As has been repeatedly emphasised in this treatise, the synthetic
tannins form a special class of substances, and the results obtained by
either of the two hide-powder methods do not give figures which are
always comparable to those of the natural tannins. An example of the
inapplicability of the methods where synthetic tannins are concerned is
illustrated by the behaviour towards hide powder of them when partly
neutralised to varying degrees: commercial Neradol D of acidity 1 gm.=
10 c.c. N/10 NaOH contains 33 per cent. tanning matters, completely
neutralised Neradol D, which exerts no true tanning action on pelt,
still contains 20 per cent tanning matter when analysed according to the
Official Method; a difference hence exists regarding the adsorption by
hide powder of a tannin and the adsorption of the latter by hide. As,
however, we are unable to make a distinction between these two different
properties by using hide powder only, we are also unable to draw the
factor into account.

Another source of error is the swelling influence on hide powder by
acids; for instance, an acid extract of vegetable tannins would show
higher tannin contents in the analysis than would the same extract when
less acid. The free sulphonic acid, however, is the active principle in
synthetic tannins, and since the latter always contain other acids (of
organic and inorganic origin) devoid of tannoid character, a source of
error is thus introduced, which we cannot eliminate by the present
method of analysis.

Of other methods of estimating the quality of a tanning material or
tanning extract the _determination of solubility_, _ash_, _colour_, and
_weight-giving properties_ in addition to the _firmness imparted to the
leather_ by the particular material are of importance. As regards the
synthetic tannins they are as a rule very soluble and it will generally
be found sufficient to subject them to the ordinary qualitative
examination. The ash determination in synthetic tannins, on the other
hand, is not of such value as in the case of natural tanning
extracts. From their composition we know that synthetic tannins contain
considerable quantities of mineral salts, the presence of some of which
on the one hand emphasises their pickling effect, and that on the other
hand the property of dissolving phlobaphenes exhibited by the synthetic
tannins is closely connected with their salt contents.

A colour determination of synthetic tannins is not of much importance,
since synthetic tannins nearly always impart a white or light brown
colour to the hide. In those cases only where coloured decomposition
products appear as a result of intermediary reactions, may the former
impart greyish or dirty colorations of little beauty to the hide. This
is easily ascertained by lightly tanning a pelt.

The determination of the weight and solidity-giving properties is
important both for leathers tanned with vegetable tanning extracts and
for those treated with synthetic tannins, but the results obtained when
using animalised cotton are not directly convertible into figures
required for practical purposes. Comparative figures are better
obtained by actually tanning pieces of pelt on as practical a scale as
is possible, and testing the weights and tensile strengths of the pieces
as against those of the original pelts, whereby in the former case the
yield (pelt --> leather) is obtained.

Its capability as a tanning agent may be ascertained by submitting the
synthetic tannin to an actual test tannage. The latter is carried out
by introducing the dilute extract into open glass jars, holding about
400 c.c. at a width of about 8 cm. [Footnote: Accumulator jars are
excellent for the purpose.--_Transl_.] The concentration of the
solution is chosen according to acidity and salt contents of the
synthetic tannin, the most suitable being 1.5°-2.5° Bé. A piece of bated
pelt is suspended in the liquor in such a way that the pelt is
completely surrounded by liquor, without, however, being creased or
touching the bottom. If the pelt were creased during tannage, the
wrinkles would become fixed and would show in the finished leather. Thus
an unfair judgment of the extract would be delivered, since similar
results are produced by liquors which are either too concentrated or are
not properly composed, and naturally this property of an extract would
be greatly to its disadvantage.

The various stages of tannage may be judged from various standpoints
when examining the pelt as tannage proceeds. On the one hand, the
surface of the but slightly porous pelt is altered so as to present a
more porous appearance, which is now rendered more capable of absorbing
liquids. On the other hand, a similar alteration takes place _within_
the pelt, to the extent to which the tanning matter has penetrated it.
How far the penetration has proceeded is easily determined by utilising
the different adsorption of coal-tar dyes by untanned and tanned pelt
(see p. 121). An indicator for those synthetic tannins, which are
derived from the phenols, is ferric chloride, which only colours those
parts of the pelt which have been penetrated by the synthetic tannins;
clearer and better results are, however, obtained when the dyestuffs
referred to above are employed.

As soon as the tanning matter has completely penetrated the pelt, the
total time of tannage is noted, and the velocity with which the tanning
matter converts the pelt into leather at that particular concentration
is thus obtained. The tannage completed, the leather must be well washed
in running water to remove excess of synthetic tannin and then dried. On
examining the dry leathers, the colour may then be observed, and a cut
will give an idea of the tensile strength and the length of fibre of the
leather. The tensile strength is, however, not of much value in such a
barely tanned leather and cannot be compared with that obtained in
leathers tanned on a practical scale. The length of fibre is, however,
of some importance, since a special feature of finished leathers tanned
with synthetic tannins is the beautifully long fibre--a property which
manifests itself when the leather is torn and in which an expression of
the quality of the synthetic tannin may be found.

Similarly, tanning experiments combining synthetic and natural tannins
may be carried out, the most interesting features of these being the
different proportions in which the two products are mixed. Such
experiments may be done, for instance, by preparing 2° Bé. solutions of
each extract and then mixing them in proportions of, say, 10:90, 20:80,
30:70, etc. Here it is again possible to infer the _tanning intensity_
of the synthetic tannin from the concentration and the time used for

A further determination of the quality of a synthetic tannin is the
capability of the latter of dissolving or precipitating the natural
tannins. As is well known, synthetic tannins frequently possess the
practically important property of rendering natural tannins easily
soluble in water. In some cases, however, synthetic tannins appear to
solubilise natural tannins in concentrated solutions; when, however, the
latter are diluted, the natural tannin is precipitated with varying
completeness, the reason of which is often the presence of excessive
acid or the presence of such salts as have no phlobaphene-solubilising

For practical purposes this determination may be carried out by mixing,
in different proportions, concentrated tannin solutions and the
synthetic tannin; heating the mixture on the water bath for a short
time, cooling and finally diluting 10, 20, and 30 gm. of the mixture to
100 c.c., which are then left in measuring cylinders for twelve to
twenty-four hours; the amount deposited will then be an indication of
the solubilising or precipitating effect exhibited by the synthetic

Other properties of the synthetic tannins connected with their practical
application will be discussed in Part II. of this treatise.



With regard to their _industrial production_, but few synthetic tannins
are, to-day, of practical and commercial interest. In addition to
simplicity in the method of manufacture a certain degree of purity of
the raw materials constitutes the criterion of their suitability. The
methods of manufacture, of which nearly all are the property of the
B.A.S.F., have been so worked out that the production of synthetic
tannins presents no difficulties on a practical scale. Cresols,
naphthalenes, and higher hydrocarbons are used as starting materials in
the production of synthetic tannins; the former substances or their
oxidation products are sulphonated by means of concentrated sulphuric
acid, and the tanning matter produced by condensing the sulphonic acids
with formaldehyde. The crude synthetic tannin thus obtained has yet to
be diluted and partly neutralised before it can be applied in practice,
and this is carried out by mixing the crude product with strong caustic
lye. By these means the high acidity is reduced to a suitable degree
learned from experience on the one hand; on the other hand, the salts of
the sulphonic acids form valuable components of the commercial synthetic

The first product placed on the market was named _Neradol D_; this
represents the condensation product of cresolsulphonic acid. The second
synthetic tannin was _Neradol N_, which represents the condensation
product of naphthalenesulphonic acid; when diluted and neutralised to
the same extent as is done in the case of Neradol D, the product is
named _Neradol N D_. The latest synthetic tannin has been called
_Ordoval G_, the starting material of which is a still higher

The tannoid-chemical properties of these synthetic tannins have been
exhaustively determined by the author, who employed Neradol D, which is
most suitable for such a purpose, and the investigations relating to it
will now be treated fully in the following chapters. The two other
synthetic tannins exhibit very similar properties, but their few
characteristics shall be shortly dealt with.

The condensation product obtained by the method described on p. 55 forms
a viscous, dark coloured mass, the analysis of which by the shake method
gives the following figures:-

Tanning matters     62.6 per cent.
Non tannins          6.4    "
Insolubles           0.0    "
Water               31.0    "
                   100.0 per cent.

Acidity: 1 gm. = 40 c.c. N/10 NaOH.

According to its chemical constitution, this product may
be considered to be dinaphthylmethanedisulphonic acid.

Samples of this crude, strongly acid material were partly
neutralised, and the following figures obtained on analysis:--

Acidity.                      Tanning   Soluble    Water.
                              Matters.  Non-tans.

                              Per Cent. Per Cent. Per Cent.
1 gm. = 35 c.c. N/10 NaOH        61.8     7.0       31.2
1 "   = 30      "     "          58.9     7.1       34.0
1 "   = 25      "     "          50.1     7.9       42.0
1 "   = 20      "     "          42.2     8.9       48.9
1 "   = 15      "     "          37.4    10.4       52.2
1 "   = 10      "     "          31.6    13.6       54.8
1 "   = 5       "     "          26.3    16.6       57.1

Experimental tanning tests which were carried out with the various
partly neutralised samples yielded leathers which, on an average, were
nearly white, but which in comparison with a leather tanned with Neradol
D appeared rather more greyish and were much harder.

A solution of the half-neutralised substance (1gm. = 20 c.c.
N/10 NaOH) gives the following reactions:---

Gelatine--Precipitate, partly soluble in excess tannin solution.
Ferric chloride-----No coloration.
Barium chloride-----Precipitate, insoluble HNO_3.
Bromine water-----No reaction.
Silver nitrate-----No reaction.
Aniline hydrochloride----Precipitate, dissolves when solution
is heated.

This condensation product is very soluble in water, but insoluble in
most solvents, excepting methyl and ethyl alcohols. The above reactions
show the similarity of this dinaphthyl derivative to the dicresyl
derivative, and the absence in the former of characteristic reactions
with iron salts is mainly accounted for by its lack of phenolic groups.
The absence of this reaction does not, of course, influence the tannoid
character of dinaphthylmethanedisulphonic acid in the least, and is of
no importance in practice, since the various stages of tannage may be
demonstrated by means of a solution of indigotine.

From a technical point of view the absence of this reaction is
advantageous to this extent, that it eliminates the exceedingly great
care to avoid the contact of tan liquors and tanned pelt with iron
particles which has to be observed when tannins of phenolic character
are employed.

In a chemical and technological evaluation of this tanning matter, all
those details apply which will be described when discussing Neradol
D. The most important advantage possessed by this tanning matter, from a
commercial point of

view, is the lower price which it owes to the greater ease with which
naphthalene may be obtained.

By treating the non-condensed crude product with barium chloride, a
product completely devoid of sulphuric acid is easily obtained; the
contents of sulphuric acid calculated as BaSO_4 is about 9.5 per
cent. This value is higher than that found by Neradol D, and may be
explained by the fact that a slight excess of sulphuric acid is
necessary for the preparation of [Greek: b]-naphthalenesulphonic acid.

Comparative tanning tests using products containing sulphuric acid and
products free from sulphuric acid (neutralised to the same degree of
acidity) yielded leathers which were very similar; the liquor containing
no sulphates yielded slightly softer leather than that obtained from a
liquor containing sulphates.

An experiment was also carried out, using a liquor containing the tannin
completely neutralised with caustic soda and subsequently acidified with
acetic acid till the acidity of 1 gm. = 10 c.c N/10 NaOH; here, again,
no essential difference could be detected in the leather as compared
with that from a liquor containing sulphates.

One of the most striking properties of this tanning matter is its
solubilising effect on natural tannins and the phlobaphenes; this
property may mainly be compared to the similar one of other condensed
sulphonic acids in their behaviour towards natural tannins.

If, therefore, natural tannins are mixed with this product and the
solution used for tanning purposes, the resultant leather will possess a
dark colour owing to the presence of solubilised phlobaphenes; if, on
the other hand, a dark coloured leather, which has been tanned with
natural tannins, is washed over with a 5° Bé solution of this synthetic
tannin, or immersed for some time in the solution, the leather assumes a
lighter colour owing to the phlobaphenes being dissolved and removed
from the leather by the synthetic tannin.

The presence of Neradol ND in leathers is detected by methods to be
described under Neradol D (_cf_. p. 108). The oxyazo reaction only
succeeds when the solution has been boiled with a few drops of
hypochlorite solution, quickly cooled and excess of ammonia added. When
applying the indophenol reaction, the solution must be treated as
follows: 3-4 drops of hypochlorite solution is added, and the solution
heated for a short time; or 5-6 drops hypochlorite solution may be
added, and the solution left for some time, in which case the heating
may be omitted. The solution is then made distinctly ammoniacal, 1-2
drops of dimethyl-_p_-phenylenediamine solution and a layer of alcohol
poured on the top. In most cases a blue coloration will appear; the
addition of 1-2 drops of potassium ferricyanide solution with formation
of a blue coloration indicates the presence of Neradol ND without fail.

The fact that a product possessing tanning properties may be obtained by
condensing [Greek: b]-naphthalenesulphonic acid makes it interesting to
investigate the behaviour of a non-condensed [Greek:
b]-naphthalenesulphonic acid towards pelt. The following solutions were
allowed to act upon pelt for twelve days:--

(1) Concentrated solution of [Greek: a]-naphthalenesulphonic acid (10° Bé).
(2)       "         "        [Greek: b]-  "      "      (6° Be.)
(3)       "         "        2,7-            "      "      (18° Bé.).

Solution 1 swells the pelt to a considerable extent without, however,
solubilising it. Solution 2 produces a similar effect. Solution 3
dissolves the pelt appreciably on the first day; after six days,
solubilisation is complete. The reason of this different behaviour of
the mono- and disulphonic acids is mainly to be sought in their
difference of solubility; the monosulphonic acids are not very soluble,
and are only capable of giving solutions measuring 10° and 6° Bé,
respectively, whereas the disulphonic acid yields an 18° Bé solution, in
addition to which the much higher acidity of the latter quickly
gelatinises the pelt.

As regards the capability of the naphthalenesulphonic acids of
dissolving phlobaphenes, the following results were obtained:--solid
Argentine quebracho extract was mixed with--

5 percent, [Greek: a]-naphthalenesulphonic acid: opaque sol.,
                                               large quantity of insolubles.
10   "                  "            "             lesser  "   "
20   "                  "            "             no insolubles.
30   "                  "            "                 "
5    "  [Greek: b]-naphthalenesulphonic acid: opaque sol.,
                                               lesser quantity of insolubles.
10   "                  "            "                 "
20   "                  "            "         clear solution, no insolubles
30   "                  "            "                 "
5    "  2,7-naphthalenedisulphonic acid: opaque sol.,
                                               large quantity of insolubles.
10   "  2,7             "            "         as above.
20   "  2,7             "            "  slightly opaque, some insolubles.
30   "  2,7             "            "  nearly clear solution, no insolubles.

It is hence clear that the [Greek: b]-sulphonic acid possesses
phlobaphene-solubilising qualities greater than those of the [Greek:
a]-sulphonic acid or the disulphonic acid; the Greek: b]-sulphonic acid
was therefore made the subject of Ger. Pat., 181,288 (8th February

The synthetic tannin, _Ordoval G_, is the formaldehyde condensation
product of higher hydrocarbons (mainly _retenes_), and is a partly
neutralised product containing no sulphuric acid. The author's analysis
gave the following figures:--

  Tanning matters             10.7 per cent.
  Soluble non-tannins         16.4     "
  Insolubles                   0.0     "
  Water                       73.0     "

     Acidity: 1 gm. = 4 c.c. N/10 NaOH.
     Density: 23° Be.

Ordoval G is completely soluble in water and glacial acetic acid. Only
its organic constituents are soluble in alcohol, ethyl acetate, and
acetone, whereby a dark coloured crystalline mass separates. Ordoval G
is insoluble in benzene.

The aqueous solution of Ordoval G gives the following reactions:--

Gelatine                         Moderate flocculent precipitate.
Ferric chloride                  Darkish coloration.
Potassium dichromate             No reaction.
Aniline hydrochloride            Dark brown precipitate.
Formaldehyde hydrochloric acid   No precipitate.
Bromine water                    No reaction.
Zinc acetate                     Very slight opalescence.
Barium chloride                  Slight opalescence.

Its capability of solubilising and consequent saving of natural tannins
is shown by the fact that 100 kilos of vegetable tanning material may be
substituted by 40 kilos of Ordoval G and the material in question in
order to obtain the entire tanning intensity of the latter.

In one respect--that of its salts--Ordoval G differs from the Neradols;
whereas the chromium and aluminium salts of the latter possess no such
tannoid properties as will make the resultant leather exhibit any of the
characteristics of either tannage, it is possible to carry out combined
tannage with a mixture of Ordoval G and metallic salts. Tanning
experiments carried out with the chromium, iron, aluminium, and calcium
salts of Ordoval G yielded leathers which possessed proportionate
characteristics of either kind of tannage to the extent to which either
material was present. This combination tannage seems to be assured of a
great future; especially may a combination tannage of iron salts and
Ordoval G eventually entirely replace chrome tannage.

The detection of Ordoval G in leather is carried out as follows: 10
gm. of leather are boiled with 150 c.c. of acetic acid, a solution of 25
gm. of CrO_8 in 25 c.c. of a 50 per cent, solution of acetic acid
gradually added, and the mixture boiled for three hours, till the
leather is decomposed and the solution has assumed a brown instead of
the original light yellow colour. The solution is then evaporated, the
residue dissolved in 600 c.c. hot water, and the chromium precipitated
with a 40° Bé. solution of caustic soda. The solution is filtered and
cooled, and a little hydrosulphite is added to 20 c.c. of the cold
alkaline filtrate; in the presence of Ordoval G, a red colour will
appear (oxanthranolsulphonic acid).

Brief mention must be made of the so-called _Corinal_ [Footnote: Swiss
Pat, 78,282, 78,797, 79,39.] a synthetic tannin placed upon the market
by Chem. Fabrik Worms A.-G., in Worms-on-the-Rhine. It is a viscous,
brown fluid, containing the aluminium salts of the tannoid acids. The
latter are formaldehyde-condensation products of sulphonated tar oils,
or the hydroxylated derivatives of the latter. The density being 33° Bé,
it contains 28.1 per cent. tanning matters, 13 per cent. soluble
non-tannins, and 10.8 per cent. inorganic matter (3.2 per cent. Al_2O_3
and 7.6 per cent. Na_2SO_4.

A similar product, containing chrome salts as base, is the so-called
ESCO-EXTRACT, [Footnote: Schorlemmer, _Collegium_, 1917, 124]
manufactured by the Chem. Fabrik Jucker & Co. in Haltingen
(Baden). This product is a dark, reddish-brown fluid, possessing acid
reaction, which strongly precipitates gelatine. Analysed by the filter
method it contains 12-15 per cent. tanning matters, 17-20 per cent.
soluble non-tannins, and 18 per cent. ash, of which 3 per cent. is
Cr2O_3. This synthetic tannin may be employed alone or in conjunction
with other tannins, and yields a leather similar to that obtained by
chrome tannage.

A. Condensation of Free Phenolsulphonic Acid

In practice, the results of condensing phenolsulphonic acid with
formaldehyde are manifold, according to whether these materials are used
in their concentrated or dilute state; whether they interact in the cold
or when heated; or whether their interaction is gradual or rapid.

1. If a moderately dilute solution of phenolsulphonic acid (1:1) is
mixed with one-sixth of its volume of a dilute formaldehyde solution (1
part 30 per cent. HCHO solution plus 3 parts of water) in the cold, with
continuous stirring, the solution remains clear and assumes a brown
colour. When left several hours, a light, white flocculent precipitate
deposits, which increases in quantity on diluting with water. The
solution precipitates gelatine; the flocculent precipitate is easily
soluble in hot caustic soda solution, and, when subsequently neutralised
with acetic acid, precipitates gelatine.

If equal parts of dilute phenolsulphonic acid and dilute formaldehyde
(concentrations as above) are gradually mixed in the cold, whilst
stirring, the mixture soon becomes opalescent, and a flocculent deposit
separates after eighteen to twenty-four hours.

These experiments carried out on the water bath immediately yield
opalescent liquids, from which an insoluble, brown, gluey, and very
sticky mass separates after twenty-four hours; the latter is sparingly
soluble in alkalies, partly so in organic solvents.

2. If a moderately dilute solution of phenolsulphonic acid (1:1) is
gradually mixed with one-sixth of its volume of a concentrated (30 per
cent.) formaldehyde solution in the cold, whilst stirring, slight
opalescence immediately results, and a flocculent deposit separates
after about twenty minutes, which gradually increases in quantity during
the next few hours. If the volume of formaldehyde is increased to the
same as that of phenolsulphonic acid solution, the flocculent deposit
immediately separates, and after twenty-four hours a brown, gluey, and
very sticky mass--of the same solubility as that described in the
previous experiment--is to be found at the bottom of the vessel used.

It should be noted that in both these experiments with concentrated
formaldehyde solution a slight increase in temperature occurs
concurrently with the process of condensation. If the experiments are
carried out on the water bath, a gelatinous mass is instantly formed,
which assumes the colours of grey, dirty light violet and dark violet,
in the order named, and which, whilst left several hours--or when heated
on the water bath--is suddenly converted into the insoluble, brown,
gluey mass above referred to.

3. If, for the purpose of condensation, phenolsulphonic acid to which 10
per cent, of water has been added, is employed, the reaction proceeds
very quickly and energetically. If one-sixth of its volume of
formaldehyde (1:3 of the 30 per cent. solution) is added drop by drop to
a cold solution of phenolsulphonic acid, a reddish, milky solution
results, which assumes a slightly lighter colour on addition of more
formaldehyde and deposits an insoluble flocculent precipitate. If the
solution is kept below 45° C., by artificial cooling, the light colour
is maintained, but a gelatinous precipitate is soon formed, the
viscosity of which increases on stirring, and finally is converted into
an insoluble, tough, gummy mass. If, on the other hand, the mass is
heated at the beginning of the reaction, or if the amount of
formaldehyde is increased and the mass cooled during reaction,
effervescence occurs, and a cheesy, dirty-coloured mass results, which,
on cooling, rapidly becomes solid and yields a very firm, elastic,
rubbery mass, which is absolutely insoluble in water.

4. The condensation proceeds exceedingly violently when concentrated
phenolsulphonic acid is acted upon by one-sixth of its volume of
formaldehyde. If the latter is firstly added drop by drop to the
phenolsulphonic acid, a gel immediately results, the temperature of
which quickly increases on further addition of formaldehyde and suddenly
boils over, yielding a reaction product which, when cooled, forms a
dirty violet, firm, elastic, and rubbery mass, insoluble in alkalies and
hardly affected by organic solvents.

Finally, if the amounts of concentrated phenolsulphonic acid and
formaldehyde stated above are mixed, strong effervescence occurs and
heat is evolved, and a dirty blackish-violet mass is instantly formed
which, on cooling, yields a rather brittle, hard product insoluble in

5. Totally different end-products are, however, obtained when the
addition of formaldehyde (30 per cent.) in the proportion of one-sixth
of the volume of dilute phenolsulphonic acid (1 plus 9 aq.) to the
latter is extended over several hours. In this case a slightly
opalescent liquid is obtained which, when left twelve hours, is
transformed into a brown mass soluble in water, which strongly
precipitates gelatine and possesses tanning properties. Hence direct
tannoid substances are obtained by this method of condensation.

Whereas no direct tanning experiment can be carried out with the
insoluble compact mass obtained in the preparations described above on
account of their absolute insolubility, it is still possible to carry
out tanning experiments with opalescent colloidal solutions in the
following ways:--

(a) If a bated pelt is immersed in a liquid containing a condensation
product obtained by gradually mixing a moderately dilute solution of
phenolsulphonic acid and a dilute solution of formaldehyde, the pelt is
rapidly tanned on the surface. Complete penetration of the substance
does not occur even after several days, since the strong acidity of the
solution causes a strong swelling of the pelt.

(b) If a pelt is shaken for six hours in a shaking apparatus containing
the liquid mentioned under (a), tannage again only takes place on the
surface, penetration being impeded by the strong swelling effect of the
liquid. Repetition of the latter two experiments, with the addition of
15 per cent, common salt, increases the tanning effect to some extent;
the pelt, however, is not tanned through, but the non-tanned layers may
be clearly seen to be pickled.

The tanning effects described above are only exhibited when the
colloidal tan-liquor is present in great excess over the pelt, since the
former obviously only contains small amounts of tanning matter, and even
the presence of common salt does not bring about complete tannage of the

In order to prove the presence of "tanning matters" in the liquid
described above, several freshly prepared samples of the latter were
analysed by the shake method of analysis without being first filtered
and the following figures obtained:--

                          1.          2.          3.          4.
                       Per Cent.   Per Cent.   Per Cent.   Per Cent.
Tanning matters           6.4         7.7        8.2          9.1

Soluble non-tannins      15.2         17.4       14.5         11.8

These condensation products suspended in water all precipitate gelatine
strongly and leave behind a perfectly clear liquid. In all cases, an
intense blue colour was obtained on adding ferric chloride, a slight
precipitate only was obtained with aniline hydrochloride, and bromine
was rapidly absorbed with the separation of an insoluble white deposit.

The condensation products obtained by the interaction of dilute
solutions of phenolsulphonic acid and formaldehyde at moderately high
temperature, which form slimy masses and are insoluble in water, are
soluble in alcohol. An alcoholic solution of such a product was used in
a tanning experiment, and a piece of pelt immersed in the solution was
tanned through in a few days; the resultant leather being rather firm,
springy, and slightly hard, and the colour was a light brownish-grey.

All those condensation products which are easily or partly soluble in
alcohol dissolve in caustic soda, sodium carbonate, in some cases also
in borax and sodium sulphite. They are rendered soluble with greater
ease when the _freshly prepared_ solution is heated on the water bath
with the alkali; the alkaline solution, neutralised as far as is
possible with acetic acid, yields light brown coloured solutions, the
tanning effects of which have proved very satisfactory. Leathers tanned
in such solutions, however, are rather empty and hard, possess but
little resilience and an uneven, dirty greyish-brown colour.

A sample of such a product, as nearly as possible neutralised with
acetic acid, contained 14.8 per cent. tanning matters, by the shake
method of analysis.

B. Condensation of Partly Neutralised Phenolsulphonic Acid

Attempts were made at condensing partly neutralised phenolsulphonic
acid; the latter was obtained by mixing equal quantities of
phenolsulphonic acid and sodium phenolsulphonate (prepared by exactly
neutralising phenolsulphonic acid with a concentrated solution of
caustic soda).

The consequent dilution and decrease in acidity, however, considerably
diminished the velocity of the reaction. Hence, if the half-neutralised
Solution A1 (_cf_. p. 98) is diluted with water, taking equal volumes,
and one-sixth of the volume of dilute formaldehyde (1:3) gradually added
in the cold, condensation is not induced. When heated several hours an
opalescent liquid results from which, however, no flocculent deposits
separate when left for some time. Using a concentrated solution of
formaldehyde (Experiment A2, p. 98) in the cold produces no reaction,
but after heating for a time an opalescent liquid is obtained. Both
liquids give only slight precipitates with gelatine. Excess formaldehyde
does not influence the reaction.

A repetition of Experiment A3 (_cf_. p. 99), using the above
half-neutralised phenolsulphonic acid, similarly required heat to induce
condensation, when a milky liquid of light reddish colour resulted.

Whereas the addition of formaldehyde to non-neutralised concentrated
phenolsulphonic acid caused violent reaction, this proceeded very slowly
in the case of half-neutralised phenolsulphonic acid, resulting in the
formation of a semi-solid mass, which on heating became more viscous,
and finally, when left twenty-four hours, became a solid, compact,
insoluble mass possessing a dirty light violet colour.

Tanning experiments with these opalescent solutions proved them to exert
a rapid penetration on the surface, complete tannage, however, taking
place after eight days only, when a flat, greyish-coloured and rather
hard leather resulted.

C. Condensation of Completely Neutralised Phenolsulphonic Acid

If concentrated phenolsulphonic acid is gradually neutralised with
concentrated caustic soda solution till the former is faintly alkaline,
the sodium salt thus obtained is not so easily condensed with
formaldehyde as is the case with the free acid.

1. If formaldehyde is gradually added to the neutralised phenolsulphonic
acid in the cold, opalescence immediately results; on addition of water,
the liquid assumes a milky appearance. On adding gelatine to this
liquid, a slimy precipitate is thrown down, leaving a slightly
opalescent liquid.

2. If formaldehyde is added to neutralised phenolsulphonic acid whilst
it is heated on the water bath, a slimy mass instantly separates, which
on cooling solidifies and forms a greyish-blue brittle mass, insoluble
in water and but sparingly soluble in alcohol; the alcoholic solution is
capable of converting pelt into leather.

The filtrate from the solidified mass strongly precipitates gelatine,
whereas the insoluble condensation product is soluble in caustic soda;
this alkaline solution also precipitates gelatine and the addition of
acetic acid transforms the mixture into the gel state.

If the insoluble condensation product is dissolved in warm concentrated
sulphuric acid, the solution remains clear upon the addition of water,
but does not precipitate gelatine. If, finally, this solution is
neutralised with caustic soda, the solution remains clear and
precipitates gelatine strongly.

D. Condensation of Cresolsulphonic Acid

Experiments were carried out with the object of condensing _o_-, _m_-,
and _p_-cresolsulphonic acids with formaldehyde in various ways; no
essential differences could be detected as regards the mode of reaction
or the properties of the intermediary and end-products as compared to
those of phenolsulphonic acid. Similarly, condensation of different
samples of crude cresol containing varying quantities of _o_-, _m_-, and
_p_-cresol did not yield end-products sufficiently different to justify
describing them in detail.

E. Relative Behaviour of an Alkaline Solution of Bakelite and Natural

Phenolsulphonic acid was condensed with a little formaldehyde, and the
reddish pasty condensation product dissolved in caustic soda. This
alkaline solution of bakelite was exactly neutralised with acetic acid
and mixed with strong solutions of an untreated quebracho extract. It
was observed that the solubility of the quebracho extract was not
increased by this treatment, but the faintly acidic character of the
natural tannin caused the bakelite to be thrown down as an insoluble

Crude phenolsulphonic acid, when added to a solution of the quebracho
extract referred to, does not increase the solubility of the latter,
which even deposits considerable amounts of insoluble tannin particles.

Quite different properties are exhibited by sodium phenolsulphonate,
which completely converts quebracho tannin into a water-soluble
substance, the aqueous solution of which deposits no insolubles. The
partly neutralised condensation product of phenolsulphonic acid and
formaldehyde exhibits similar properties [Footnote: Grasser, _Collegium_,
1913, 521, 478.] (see later).

F. Dicresylmethanedisulphonic Acid (Neradol D) [Footnote: Ger, Pat.,
291, 457; Austr. Pat., 61, 057.]

Neradol D is a viscous liquid, measuring about 33° Bé., which is similar
to extracts of natural tannins. One of its characteristics is its
phenolic odour; it is completely soluble in water, forming a clear,
semi-colloidal solution, but is insoluble in all organic solvents with
the exception of alcohol, glacial acetic acid and ethyl acetate, which
dissolve all but its inorganic constituents. The latter owe their
presence to the neutralisation of the crude Neradol with caustic soda,
and are composed of sodium salts of the sulphonic acid in addition to
Glauber salts.

The aqueous solution of Neradol D shows properties similar to those
exhibited by solutions of natural tannins and reacts as
follows:--[Footnote: Grasser, _Collegium_, 1913, 520, 413.]

Methyl orange                    Acid reaction.
Barium chloride                  White precipitate, insoluble in HNO_3.
Ferric chloride                  Deep blue coloration.
Silver nitrate                   Slight opalescence.
Bromine water                    No precipitate.
Formaldehyde hydrochloric acid   No precipitate.
Gelatine                         Complete precipitation.
Aniline hydrochloride            Strong precipitate.

The reactions with ferric chloride and gelatine should be especially
noted, since they are analogous to those given by natural tannins. On
the other hand, the reactions with BaCl_2, bromine water and formaldehyde
hydrochloric [Footnote: Stiasny carries out the reaction with
formaldehyde-hydrochloric acid as follows:--50 c.c. of the tannin
solution, plus 5 c.c. concentrated hydrochloric acid and 10
c.c. formaldehyde (40 per cent.) are heated under reflux condenser for
ten minutes; most natural tannins are completely precipitated
(_Collegium_, 1906, 435; 1907, 52 _et_ 188).] acid prove the different
chemical composition of Neradol D as compared to that of the natural

The fact that a positive reaction is given with aniline
hydrochloride [Footnote: This reaction is carried out as follows:--5
c.c. of the tannin solution to be examined (about 4 gm. tanning matter
per litre) are shaken violently in a test tube with 0.5 c.c. aniline and
2 c.c. concentrated HCl added. All natural tannins are unaffected by
this treatment, ligninsulphonic and other sulphonic acids cause
opalescence. _Note_.--Employing formic acid in lieu of hydrochloric
acid (Knowles) renders the reaction no more reliable.--_Transl_.] is
very puzzling; none of the natural tannins are precipitated by this
reagent, but only sulphite cellulose on account of its content of
ligninsulphonic acid. One is justified in assuming that there is at
least some connection between the constitution of ligninsulphonic acid
and that of dicresylmethanedisulphonic acid.

Stiasny [Footnote: _Collegium_, 1913, 516, 142.] recommends the
following reaction for the detection of and differentiation between
Neradol D and wood pulp extract:--10 c.c. of a 5 per cent. solution of
the extract to be analysed are violently shaken with 1-2 drops of a 1
per cent. alum solution and about 5 gm. of ammonium acetate. If only
Neradol D is present no precipitate separates even after twenty-four
hours, but if wood pulp be present, a precipitate is thrown down in a
quantity corresponding to the amount of wood pulp present.

The official analysis gives the following figures:
[Footnote: Grasser, _loc. cit._]

    Tanning matters        32.5 per cent.
    Soluble non-tannins    33.0    "
    Insolubles              0.0    "
    Water                  34.5    "
                          100.0 per cent.

    Ash                    17.0    "

        Acidity: 1 gm. = 10 c.c. N/10 NaOH.
        Density: 33º Bé.

A comparison of its quantitative analysis to that of a natural tanning
extract is illustrated by the following figures of a chestnut and a
quebracho extract of same density (26º Bé):--

                           Chestnut    Quebracho
                           Per Cent.   Per Cent.
    Tanning matters          32.0        34.0
    Soluble non-tannins      12.0         8.0
    Insolubles                1.5         2.0
    Water                    54.5        56.0
                            -----       -----
                            100.0       100.0

    Ash                       0.4         2.0

This comparison shows that extracts of natural tannins firstly contain
certain amounts of "insolubles," whereas Neradol is completely soluble
in water, forming a clear solution; secondly, natural tanning extracts
contain smaller quantities of soluble non-tannins, consisting of
colouring matter and sugars, in addition to small quantities of mineral
matters (ash). Neradol D contains considerable amounts of soluble
non-tannins, derived from salts of sulphonic and sulphuric acids, again
offering a satisfactory explanation of the high ash. If, therefore, a
mixture of Neradol D and a natural tanning extract was submitted to a
quantitative analysis, the higher non-tannins and the high ash would
indicate the presence of Neradol D, provided that wood pulp or a highly
sulphited extract were not components of the mixture.

The chemical reactions taking place in the preparation of
Neradol D may be expressed thus:-

       OH            OH          OH            OH
     H ^ H___O_____H ^ H       H ^ H__CH_2__H ^ H
      | |    ||     | |    =    | |          | |  + H_2O
      | |    CH_2   | |	        | |          | |
     H v CH_3   CH_3 v	       H v CH_3  CH_3 v H
       HSO_3         HSO_3       HSO_3        HSO_3

1. Neradol D Reactions

1. The quantitative determination of phenols introduced by Bader,
[Footnote: _Bull. soc. scient., Bucarexi_, 1899, 8, 51.] which consists
in precipitating them as oxyazo compounds, has been modified by Appelius
and Schmidt [FootNote: _Collegium_, 1914, 597.] for the purpose of
detecting Neradol D:--To 50 c.c. of the tannin solution (analytical
strength) 15 c.c. of diazo solution are added, the mixture filtered, if
necessary, and the filtrate made alkaline with caustic soda; in the
presence of Neradol D in sufficient quantity, a blood-red coloration
results. If but little Neradol D be present, the procedure is altered as
follows:--The tannin solution, to which the diazo solution has been
added, is filtered, and the filtrate poured on a piece of filter paper
which is then dried; a solution of caustic soda is spotted on the paper,
when, if Neradol D be present, a red-edged spot will appear.

According to Tschirch and Edner, [Footnote: _Archiv. d. Pharm_., 1907,
150.] the diazo solution is prepared as follows:--5 gm._p_-nitraniline
are introduced into a 500 c.c. measuring flask, 25 c.c. of water and 6
c.c. concentrated sulphuric acid added, the mixture shaken and a
solution of 3 gm. of sodium nitrite in 25 c.c. of water plus 100 c.c. of
water added, and the whole then filled up to 500 c.c. The solution
should be stocked in the dark.

2. A less sensitive reaction for Neradol and wood pulp extract
constitutes that of Appelius and Schmidt employing cinchonine,
[Footnote: _Collegium_ 1914, 597.] while the presence of the substances
in question yields characteristic precipitates.

3. Seel and Sander [Footnote: _Zeits. f. ang. Chem._, 1916, 333.]
recommend the following method of detecting Neradol D:--

_(a) Oxyazo Reaction_.--About 5 c.c. of the tannin solution are rendered
alkaline with caustic soda; after cooling with ice, about half the
volume of alcohol is added. 3-4 drops of diazo solution are then
added. Frequently, this results in the solution assuming a blue
coloration. If not, the solution is acidified with hydrochloric acid,
ether added, and the mixture well shaken. The water is now separated
from the mixture, fresh water added, together with some caustic soda
solution, when, if Neradol D be present, the salt of the colour acid
formed dissolves in the water with a beautiful green or bluish-green
colour. At the place of contact of the water and the ether a
bluish-green ring appears.

The diazo solution is prepared by dissolving _p_-aminophenol or its
hydrochloric in a little dilute hydrochloric acid, cooling in ice and
carefully diazotising in the cold till a slight excess of nitrous acid
is present. It is essential that this solution should be tested before
use, and this is carried out by coupling it with an alkaline phenol
solution; if a dark blue oxyazo colour is formed, the solution may be
used. It must be kept cool by surrounding it with ice.

_(b) Indophenol Reaction_.--To 5 c.c. of the solution to be tested, a
drop of a solution of dimethyl-_p_-phenylenediamine is added, the
solution rendered alkaline with caustic soda and 1-2 drops of a 5 per
cent. solution of potassium ferricyanide added. If Neradol D be present,
a blue colour appears, either immediately or after some time. The
reaction is rendered more sensitive if alcohol is carefully poured on
the solution after it has been rendered alkaline, and potassium
ferricyanide is then added. At the place of contact a blue layer is
formed, which ultimately diffuses into the alcohol.

According to Lauffmann [Footnote: _Collegium_, 1917, 233.] the presence
of natural tannins as well as that of wood pulp diminishes the
sensitiveness of the reactions described above; [Footnote:
_Zeits. f. ang. Chem._, 1916, 333.] this investigator recommends a
modification of these reactions.

2. Electro-Chemical Behaviour of Neradol D

The author's investigations of the electro-osmosis of an aqueous
solution of Neradol D [Footnote: _Collegium_, 1920, 597, 24.] proved
that dicresylmethanedisulphonic acid exhibits anodic migration; hence
this product possesses negative charge and acidic character. The
impurities accompanying the synthetic tannin, _i.e._, salts, free
sulphuric acid, and some phenols, migrated anodic and cathodic
respectively, according to their charges. A Neradol D purified by
electro-osmosis finally yielded a pure solution of
dicresylmethanedisulphonic acid, which precipitated gelatine and
exhibited pronounced tanning effects, but gave a greenish-black
coloration with iron salts. This conclusively proves that the blue
coloration given by Neradol D with iron salts is no characteristic
feature of the _pure_ synthetic tannin, but is caused by the phenolic
impurities accompanying the latter. Especially the first stage of the
electro-osmosis produces a cathodic migration of the phenols, which may
then be detected at a cathode by means of the iron and bromine

It is characteristic of a dicresylmethanedisulphonic acid purified by
electro-osmosis that it does not precipitate aniline hydrochloride. It
appears, therefore, that this reaction--which is characteristic of most
synthetic tannins--is again caused by the presence of impurities.

3. The Influence of Salts and Acid Contents on the Tanning Effect of
Neradol D

Chemical analysis of crude Neradol revealed a natural
dicresylmethanedisulphonic acid (the tanning agent) contents of about 68
per cent, which agrees fairly well with the calculated amount. Like
other "strong" and "weak" acids this sulphonic acid exercises a strongly
swelling influence on pelt. Whereas the effect of acid present in
solutions of Neradol D of medium concentration and its tanning effect
both influence the pelt and are fairly well balanced, this is not the
case as regards highly concentrated and very dilute solutions. If, for
instance, a very dilute solution of crude Neradol (about 0.25° Bé.) is
used, the tanning effect of this solution is exceedingly small and does
not show itself till after several hours. The relatively high
dissociation of the acids at this high degree of dilution causes an
extremely rapid and strong swelling of the pelt, which has therefore
absorbed its maximum amount of water (maximum swelling) before the
tanning effect of the sulphonic acid comes into play and by fixing the
surface of the pelt is enabled to prevent the excessive swelling effect
of the acids.

The addition of neutral salts to the tan liquor diminishes the effect of
the acids on pelt (dehydrates the pelt) and prevents "drawing" of the
grain. If, for instance, common salt be added to a solution of crude
Neradol, the original quantity of sulphonic acid present would remain
constant, but the presence of salt would diminish the degree of
dissociation and consequently the swelling. This effect is still more
pronounced when the absolute amount of free sulphonic acid is
diminished. Hence, if crude Neradol is treated with increasing amounts
of caustic soda, a series of products containing increasing quantities
of salt and decreasing concentrations of sulphonic acid is obtained.

The acidity of the Neradols may be determined by titration with N/10
caustic soda; this procedure hence establishes a means of determining
the (unknown) acidities which may be expressed in terms of c.c. N/10
NaOH. The acidity of crude Neradol was found to be--

1 gm. = 50 c.c. N/10 NaOH

_i.e._, 1 gm. of crude Neradol requires 50 c.c. N/10 NaOH for complete
neutralisation; the decrease in acidity causes a decrease in contents of
tanning matters and the quantities of salts increase. The following
table gives the figures obtained by differently neutralised neradols:--

  Acidity.              Tanning Matters.       Na_2SO_4.

                        Per Cent.              Per Cent.
  1 gm. = 50 c.c.       68                     ...
  1 gm. = 40 c.c.       59                      4
  1 gm. = 30 c.c.       50                      8
  1 gm. = 20 c.c.       41                     12
  1 gm. = 10 c.c.       33                     17
  1 gm. =  5 c.c.       28                     20
  1 gm. =  0 c.c.       20                     ...

Tanning experiments with these different neradols (employing solutions
of 1° Bé. strength) demonstrated that neradols of acidity 50°, 40°, and
30° exerted strong swelling and gave comparatively hard leathers;
neradols of acidity 20°, 10°, and 5° exert no swelling, yield quick
tannage and soft leather. The swelling (hardening) effect of the acid
and the dehydrating (softening) effect of the salts in this case,
therefore, are well balanced, and this fact affords an explanation of
the rapid change from hardening to softening effects exhibited by partly
neutralised Neradol where less acid and a greater quantity of salts
respectively are present.

It may finally be noted that the acidity of Neradol D, 1 gm. = 10
c.c. N/10 NaOH, has been found to be the most suitable one for practical
purposes. The author has, however, successfully employed some neradols
of considerably higher acidities. The acidity above mentioned is
possessed by a Neradol D containing 17 per cent. ash and 30 per cent.
sodium sulphonates and Glauber's salts crystals respectively. This
large quantity of salts present on the one hand effects the rapid pickle
and tanning effect exhibited by Neradol D, on the other hand it also
effects the softness in the leather resulting from its use either alone
or in admixture with natural tannins.

4. Phlobaphene Solubilising Action of Neradols

A special feature of Neradol D is its property of solubilising
phlobaphenes, which may be ascribed to its contents of sulphonic acids
or their salts. In order to demonstrate whether the sulphonic acids and
their salts are capable of solubilising the insoluble or sparingly
soluble anhydrides of the tannins (the phlobaphenes) before and after
condensation, the following experiments were carried out:--

Crude Argentine solid quebracho extract was converted into a highly
viscous liquid by treating it for several hours with water at 100° C.,
and the anhydrides rendered insoluble by diluting the liquid with a
large volume of cold water. The precipitate formed, consisting of
quebracho phlobaphenes, was separated from the liquid by decantation,
and purified by washing it several times with water. Each 10 gm. of this
moist paste were treated in the cold with (_a_) free phenolsulphonic
acid; (_b_) sodium phenolsulphonate; (_c_) crude Neradol and (_d_)
Neradol D, 20 c.c. of water at 45° C. added, and the mixture allowed to
cool slowly; the following solutions resulted:--

  (_a_) Opalescent solution, much deposit,
  (_b_) Clear solution, no deposit.
  (_c_) Nearly clear solution, very little deposit.
  (_d_) Clear solution, no deposit.

This clearly proves that free and condensed phenolsulphonic acids as
such are not capable of completely solubilising phlobaphenes, whereas
the sodium salts of free and condensed phenolsulphonic acids possess
this property. The salt contents of Neradol D, therefore, constitute an
advantage in this respect, that not only may Neradol D be mixed with
solutions of any natural tannin without insolubles being thereby
deposited, but it may also be added in large quantities to a tannin
solution with the result that the sparingly soluble and wholly insoluble
constituents (phlobaphenes) are completely brought into solution.

The practical importance of the solubilising effect of Neradol D
relating to solid Argentine quebracho extract is demonstrated in the
following series of investigations carried out by the author:--
[Footnote: _Collegium_, 1913, 478; Austr. Pat., 68, 796.]

Solid       Neradol   Matters   Tanning      Abs.         Increase
Argentine   D.        Calc.     of Mixture   Increase     per
Quebracho                       Found.       in Tanning   100 gms.
Extract.                                     Matters.     Extract.

Gm.         Gm.       Per Cent.              Per Cent.
100           0       66.0      66.0         ...          ...
  0         100       32.5      32.5         ...          ...
 90          10       62.7      64.7         2.0           2.2
 80          20       56.1      58.7         2.6           3.3
 60          40       52.6      56.9         4.3           7.1
 50          50       49.3      55.2         5.9          11.8
 30          70       42.6      47.3         4.7          15.6
 20          80       39.2      42.3         3.1          15.5

The maximum solubilising effect is exhibited in the mixture of 70 parts
of Neradol and 30 parts of quebracho, with an additional percentage of
tanning matters in the mixture of 15.6 per cent.--a figure which is very
nearly identical with that of the insolubles present in the original
Argentine quebracho extract.

The phlobaphene-solubilising property of Neradol D is closely connected
with the influence of the latter on the colour of leathers tanned with
natural tannins. If, on the one hand, a pelt is tanned with natural
(_i.e._, non-treated) quebracho extract, a rather light coloured leather
results, the fleshy colour of which is characteristic of quebracho. The
dark coloured phlobaphenes present, on account of their insolubility,
will have no influence on the colour of the leather. If, now, the
quebracho extract be treated with sulphite and bisulphite in the usual
way, the phlobaphenes are solubilised, but the reducing effect of the
bisulphite tends to brighten the colour of the otherwise dark coloured
phlobaphenes as well as that of the soluble tannins, and a
reddish-yellow coloured extract results, imparting its own colour to the
pelt. When, on the other hand, the quebracho extract is solubilised by
means of Neradol D, the phlobaphenes are brought into solution without
reduction taking place, and a dark brownish-red extract results, which
imparts a similar colour to the finished leather. This darkening effect
of Neradol D is most conspicuous in the case of mangrove, maletto, and
chestnut, but is absent in the case of algarobilla, dividivi, gambir,
sumac, and valonea. The varying phlobaphene contents of the tannins
easily afford an explanation of the different properties above alluded
to: the mangrove phlobaphenes are dark coloured bodies, those of mimosa,
maletto, and chestnut are of lighter colour, and the last-named tanning
materials enumerated above are either devoid of phlobaphenes or possess
them only as very light coloured bodies. Algarobilla, sumac, gambir,
dividivi, and valonea, on the other hand, are associated with large
amounts of sparingly soluble ellagic acid, known as "bloom" or "mud"
which imparts a light colour to the finished leather, and conveniently
covers the dark colour imparted to the leather by other tanning
materials; for this reason the former are often used in the lay-aways or
in the finishing processes.

Similar effects to those of Neradol D are exhibited by other salts of
sulphonic acids, _e.g._, sodium benzylsulphanilate (Solvenol B.A.S.F., or
solution salt ("Solutionsalz") Hoechst); the author prepared mixtures of
such salts and untreated quebracho extract in order to determine their
solubilising effects, and arrived at the following results:--

30 parts Solvenol plus 70 parts quebracho extract: clear solution,
                                                   no deposit.

25 parts Solvenol plus 25 parts quebracho extract: clear solution,
                                                   very little deposit

20 parts Solvenol plus 80 parts quebracho extract: nearly clear solution,
                                                   very little deposit.

15 parts Solvenol plus 85 parts quebracho extract: slightly opaque solution
                                                   some deposit.

Leathers tanned with these mixtures were more or less dark coloured
according to the amounts used of solvenol and the consequent
solubilisation of the phlobaphenes.

A similar effect, though of opposite nature from a tanning standpoint,
is exhibited by sulphonates on certain colloidal dark coloured
substances. A phenolsulphonic acid, which had been overheated during
sulphonation and subsequently condensed (crude Neradol), imparted a
conspicuous greyish-brown colour to the leather; samples of this crude
product were then partly neutralised with varying amounts of alkali, and
these samples (containing increasing quantities of salts) tested for
tannin and colour effects. It was found that the more highly neutralised
samples imparted a darker colour to the solutions, but these dark
products did not deposit the dark impurities on the pelt. One may
therefore assume that tannoid substances are colloidally suspended, and
when converted into true solutions are incapable of being fixed in
insoluble form by the pelt.

Just as, by adding Neradol D to a tanning extract, the phlobaphenes are
solubilised and a dark coloured extract results, it is also possible to
remove the mechanically deposited phlobaphenes and oxidised tannins from
the finished leather, and, as a consequence, lighten the colour of the
leather. For practical purposes, bleaching with Neradol D is carried out
by brushing over the darkly coloured leather with a 2°-3° Bé. solution
of Neradol D, and then rinsing well with water, in order to remove the
solubilised tannin. A lighter colour may also be obtained by immersing
the leather in a liquor of the strength mentioned above for several
hours, and then rinsing with water, but by this procedure not only the
surface tannin is removed, but also tannin from the leather substance
itself; this method is therefore not suitable for heavy leathers which
are sold by weight.

The advantage of employing Neradol D as a bleach in this way is to be
found in the fact that, on the one hand, the bleaching sulphonic acid
attacks the leather to a much slighter extent than is the case with
inorganic acids usually employed for this purpose; on the other hand,
the method of brushing the sulphonic acid on the leather only introduces
small amounts of sulphonic acid in the leather, thus lessening the
harmful effects of acids upon leather. Furthermore, the common methods
of using alkalies as tannin-solubilising agents with the consequent
running off and waste of alkaline tan liquors are here substituted by a
method leaving liquors rich in tannin and Neradol, and which may be used
in the ordinary procedure of tannage.

Since Neradol D contains neutral sodium sulphate (about 3 per cent.),
and the latter, by precipitating colouring matters present in tan
liquors, may slightly bleach these, it was of interest to determine
whether the sodium sulphate plays any part in the bleaching effected by
Neradol. Mixtures of chestnut and quebracho extracts were prepared, to
which were added:--

(1) 5 per cent. Neradol D.
(2) 5 per cent. Neradol D. free from Na_2SO_4.
(3) ° 15 Per cent. sodium sulphate (corresponding to above
Neradol D).

These mixtures were allowed to act upon pelt alongside of comparison
tests using quebracho and chestnut extracts only, the strength of the
liquors in all cases being 1.5° Bé; the pelt was left in the solution
till tanned through. The following results were obtained:--

(1) Quebracho tanned leather was darker; no difference in
colour by chestnut extract.
(2) Similar to (I).
(3) Same colour as given by the original extracts.

This experiment demonstrates that absence of sodium sulphate in the
mixture is without influence on the colour of the resulting leather, and
that an addition of sodium sulphate to natural extracts does not affect
the colour imparted by them to pelt

5. Effect of Neradol D on Pelt

Being a sulphonic acid derivative, the chemical constitution of Neradol
is obviously considerably different from that of the natural tannins,
and the question has been asked: Will Neradol D, in its concentrated
form, attack the hide substance?[Footnote 1: _Collegium_, 1913, 521,
487.] Bearing in mind that concentrated extracts of vegetable tannins
in some circumstances effect a "dead" tannage (_cf_. case-hardening) and
hence reduce their practical value, and that for this reason it is
impossible to allow either concentrated extracts or concentrated Neradol
D to act upon pelt, the author still decided to carry out some
experiments in this direction. Concentrated Neradol D (33° Bé.) and
strong aqueous solutions of this material in strengths of--

  30°  25°  20°  15°  10°  5°  3°  1° Bé.

were therefore allowed to remain in contact with pelt for a period of
ten days, when the pelts were taken out and washed in running water for
twenty-four hours, and then dried. The resultant leathers possessed the
following properties:--

33° Bé. solution: completely gelatinised.[Footnote 2]
30°       "           "          "       [Footnote 2]
25°       "       two-thirds gelatinised; surface tanned.
20°       "       one third gelatinised; surface "dead" tanned.
15°       "       pelt was glassy throughout.
10°       "       rather cracky leather, but well tanned.
5°        "       normal tannage.
3°        "          "     "
1°        "          "     "
[Footnote 2: Impossible to subject the pieces to a proper washing out.]

The interiors of the leathers obtained from the 25° and 20°
Bé. solutions were completely gelatinised; this may be accounted for by
assuming that the surface was "dead" tanned, and that hence the free
dissociated sulphonic acid diffused into the leather, towards which it
exhibited hydrolysing rather than a tannoid effect with the consequent
result described above. Above 10° Bé. the effect is more that of an acid
with concentrations below 10° Bé.--the only ones of technical
importance--however, no ill-effects may be observed.

For tanning purposes, Neradol D solutions of 2° Bé. are quite
satisfactory, and it has been found [Footnote 1: _Technikum_, 1913, 80,
324.] that solutions of this strength do not dissolve out any protein of
the hide. [Footnote 2: The translator cannot agree with the author on
this point. He has, for instance, found that solutions of analytical
strength dissolve considerable amounts of hide substance, and his
practical experience confirms results arrived at in the laboratory.]

A purely Neradol D tanned leather may be produced by immersing a bated
pelt, free from lime, in a 2° Bé. Neradol D liquor for about four days;
the resultant leather being nearly white and otherwise very similar to a
leather tanned with vegetable tanning materials.

The main application of Neradol D is in admixture with vegetable tanning
materials; especially in the early stages of tannage is this substance
of value, since by its use not only a light coloured leather surface is
obtained, but its presence prevents a subsequent dead tannage when
strong vegetable tan liquors are applied, and it also imparts strength
to the grain layer. It is thus possible to shorten the time consumed by
the tanning process by employing Neradol D in the manner described.

A further explanation as to why the tanning process is considerably
hastened by using Neradol D, either alone or in conjunction with natural
tannins, is afforded by the fact that though Neradol D quickly
penetrates the grain, it is but "loosely" fixed by the latter, _i.e._,
it is not deposited to such an extent that it would prevent penetration
of the vegetable tannins. In the case of a mixture of Neradol D and
vegetable tannins, the former quickly diffuses into the pelt and fixes
the fibres, thus facilitating penetration of the vegetable tannins.
This assumption is justified in view of the speed with which Neradol D
completely penetrates and tans the pelt, since Neradol D containing
acids and salts exhibits effects similar to those of a pickle.

6. Reactions of Neradol D with Iron and Alkalies

A special characteristic of Neradol D tannage is the sensitiveness of
the latter to the action of iron and alkalies. The active principle of
Neradol D being free dicresylmethanedisul-phonic acid, which is easily
neutralised by lime, ammonia, and amino-acids and hence rendered
inactive for tanning purposes, it is essential that the pelt prior to
tannage with Neradol D should be completely delimed, bated, and freed
from all constituents possessing alkaline reaction. It is, however,
possible to regenerate Neradol D liquors contaminated with alkali or
partly neutralised by the addition of small quantities of organic
(formic, acetic, lactic, and butyric) or inorganic (hydrochloric or
sulphuric) acids,_i.e._, the dicresyl-methanedisulphonic acid is again
partly liberated, and this procedure is always preferred where the
tanning process does not allow of a complete deliming of the pelt prior
to introducing the latter into a Neradol D liquor. If, on the other
hand, such liquors are kept properly, and the addition of acid referred
to kept up, they will remain active for weeks and need only
strengthening up with the requisite quantity of Neradol prior to
introducing fresh pack.

The sensitiveness to alkalies of Neradol D is considerably greater than
in the case of natural tannins, and it appears that a vegetable tan
liquor neutralised with lime will not even surface-tan when acting upon
pelt and will neither impart a dark colour to the leather nor remove
from it any appreciable amount of protein. Similarly, a Neradol D liquor
neutralised with lime exerts no tanning action, but in contradistinction
to the vegetable tan liquor similarly treated, will impart a blue or
blackish-blue colour to the pelt, from which it removes larger
quantities of protein. The author examined two such liquors relating to
their contents of tanning matters and protein and obtained the following

            Reaction.   Bark.   Tans.   Non-Tans   Insol.   Proteins

                                Per     Per        Per      Per
                                Cent.   Cent.      Cent.    Cent.

Vegetable   Slightly    12°         0       2.93     0.35       0.01
tan         alkaline

Neradol     "    "      10°         0       4.43     0.17       0.17

These figures do not only show the higher protein contents of the
Neradol D liquor, but do also show higher contents in soluble
non-tannins, which consist mainly of lime (2.12 per cent.) and sodium
salts (1.8 per cent.), thus establishing the fact of the sensitiveness
of Neradol D to alkalies in addition to its lime-solubilising effects.

The sensitiveness towards alkalies is also noticeable on a large scale
where the tanpits have been built of cement; though the pelt may be
quite free from lime, the Neradol D is quickly neutralised by the
cement, with results similar to those enumerated above.

The blue coloured soluble compound of Neradol D and iron salts, to which
frequent reference has been made, is very important from a practical
standpoint. Whereas the catechol tannins (_i.e._, fir, gambir, hemlock,
cutch, mangrove, and quebracho) are coloured black, those of the
pyrogallol class (_i.e._, algarobilla, dividivi, valonea, gallotannic
acid, myrabolams, and sumac) bluish-black, and the "mixed" tannins
(_i.e._, canaigre, oak, and mimosa bark) bluish-purple by iron alum,
Neradol D is coloured a pure blue. How sensitive this reaction is, the
following comparative analyses illustrate: to each litre of tan liquor
containing 4 gm. tanning matter prepared from (_a_) quebracho extract
and (_b_) Neradol D, 10 c.c. of a 10 per cent. iron alum solution were
added, the solutions heated to 100° C., cooled and filtered, and the
colour of the filtrates and the weight of the precipitates determined:--

(_a_) Quebracho solution: light reddish-brown filtrate, 3.22 gm.

(_b_) Neradol solution: deep blue filtrate, 0.02 gm. precipitate.

Hence, on adding a soluble iron salt to a solution of a natural tannin,
most of the tanning matter is precipitated; the colour of the filtrate,
however, is much the same as that of the original solution. A Neradol D
liquor similarly treated gives no precipitate, but is coloured blue
throughout. The filtrates from the above solutions were allowed to act
upon pelt, and the following observations were made:--

(_a_) The light reddish-brown filtrate from the quebracho liquor
exhibited no well-defined tanning effect on pelt, to which it imparted a
light brown colour.

(_b_) On the other hand, the deep blue filtrate from the Neradol D
liquor exhibited well-defined tanning effects, and imparted a deep blue
colour to the pelt.

For practical purposes, the sensitiveness of Neradol D to iron is not
only remarkable because any contact with iron particles will colour the
liquor (and hence the pelt) blue, but also because the slight amount of
iron always present in cement renders the use of cement pits prohibitive
where Neradol D liquors are used.

This intense blue coloration might have made possible a colorimetric
estimation of Neradol D. The author has investigated this possibility,
using different concentrations of Neradol D liquors to which a solution
of iron ammonium alum was added, and found that when, at certain
concentrations, the maximum blue colour had been obtained, it was still
possible to increase the quantity of Neradol without the intensity of
the colour being affected. Addition of a little alkali tends at first to
darken the blue colour, more alkali changes the blue colour to brown and
yellow, successive additions of a weak organic acid (_e.g._, acetic
acid) rapidly lighten the blue colour. Since industrially used Neradol
D liquors always contain varying quantities of acid and may be neutral
or even slightly alkaline, it must be considered impossible to make any
use of such a colorimetric estimation for practical purposes.

7. Reagents Suitable for Demonstrating the Various Stages of Neradol D

The extent to which tannage with Neradol D proceeds on the surface and
within the pelt may be judged from the feel of the skin, but such a
method is totally unsuited to any but a practical tanner. A suitable and
reliable reagent is indigotine (B.A.S.F.), which clearly distinguishes
tanned and untanned layers of the pelt. If, for instance, a 1-2 per
cent, solution of indigotine is brought into contact with a fresh cut on
a pelt, and the latter subsequently washed with warm water, the
indigotine is only retained by the untanned parts; a leather tanned with
Neradol D is therefore only coloured by indigotine to the extent to
which it has combined with the Neradol. [Footnote: According to Seel
and Sander (_Zeits. f. ang. Chem._, 1916, 333), basic dyestuffs are also
very suitable for demonstrating tanned parts of the pelt.]

Another reagent is constituted by iron ammonium sulphate; the extent of
the penetration of Neradol D, which gives an intense blue coloration
with iron salts, into the leather may be determined by washing the pelt
treated with Neradol D, making a cut, again washing and treating the cut
with a few drops of a weak solution of iron ammonium sulphate. Those
parts of the pelt which have been converted into leather then appear
deep blue; on the other hand, those which have been in contact with
Neradol D, but have not yet been converted into leather, are light
blue. Those parts which have not yet been in contact with Neradol D
appear pure white; the results of this reaction are therefore opposite
to those obtained by the use of indigotine.

8. Combination Tannages with Neradol D

Whereas mixtures of Neradol D and vegetable tannins impart properties to
the leather consistent with the proportions in which these materials are
present, it is not possible to combine Neradol D with mineral tanning
agents or fats (_e.g._, fish oils, etc.) in such a way that a leather
characterised by the properties of either material is
obtained. Experiments were carried out using (1) chrome salts plus
Neradol D; (2) aluminium salts plus Neradol D; and (3) oils plus Neradol
D, and the following conclusions were arrived at:--

1. CHROME-NERADOL D liquors, containing comparatively larger amounts of
Neradol D, act too rapidly on the pelt and draw the grain; smaller
amounts of Neradol D seem without influence on the finished leather,
which possesses pronounced characteristics of chrome leather. Another
disagreeable factor is the following: the chrome salts must possess a
certain degree of basicity in order to produce good leather; the Neradol
D must, on the other hand, possess a certain acidity to produce the
optimum results, and it is hence impossible to balance practically the
basicity of the chrome salts and the acidity of the Neradol in order to
justify the presence of both. If one of the two is used separately
before the other, a leather always results possessing the
characteristics of the material first employed, provided the time of
action has been sufficiently extended. If insufficient time has been
allowed, the characteristics imparted by the main tanning agent are not

2. ALUMINIUM SALTS AND NERADOL require practically the same basicity and
acidity respectively, and when combined always yield a leather
possessing mainly the properties of one of the components. In addition
to this fact, leathers tanned with aluminium salts possess great
softness and stretch, those tanned with Neradol D greater firmness and
less stretch, and these opposing qualities completely compensate one
another and render _nil_ the value of such mixtures.

In addition to this, the presence of aluminium salts produces no better
fixation on the leather fibre of basic coal-tar dyes, so that in this
respect also a combination of aluminium salts and Neradol D is of no

3. FAT NERADOL D TANNAGE: Just as aluminium salts impart special
characteristics to leather, this property is exhibited by fatty matters,
especially so as regards stretchiness and softness. Both of the latter
are not apparent to the same extent in an oil tannage into which Neradol
D and oil enter as constituents. It is, however, not excluded that, in
view of the fact that the combination of oils and Neradol D appear to
produce the most promising results of the three from a technical point
of view, such combination would yield products possessing less stretch
and greater softness which, by occupying an intermediary position, might
possess certain advantages and be useful for certain technical purposes.

9. Analysis of Leather Containing Neradol D

Chemical examination of leathers tanned with Neradol D or with mixtures
of natural tannins and Neradol D often involve a determination of the
materials employed in tannage. In most leathers exclusively tanned with
vegetable tanning materials, it is usually possible to determine at
least the nature of the main tanning agent, whereas the attempts at
determining those tannins which are only present in minor quantities
rarely succeed. Since Neradol D usually is employed in comparatively
small quantities, it has been imperative to find a method which also
permits of the detection of smaller quantities of Neradol D. Provided
the presence of not less than 5 per cent. (on the weight of the leather)
of Neradol D, the following method yields reliable results:--20-30
gm. of the leather are ground or sliced as finely as possible and the
powder (or the slices) treated in the cold with a sufficient volume of
dilute ammonia solution (5 c.c. ammonia plus 95 c.c. of water) for
eight to twelve hours. The object of this is to dissolve the tannins,
but no protein should go into solution. The solution is filtered and
the filtrate evaporated on the water bath till it occupies a volume of
about 30 c.c. A few c.c. of aniline hydrochloride are now cautiously
added, when it should be carefully noted if a precipitate is thrown down
which might be either completely or only partly soluble in excess of
aniline hydrochloride. A precipitate is always thrown down when Neradol
D or wood pulp is present; only the Neradol D precipitate is soluble in
excess of aniline hydrochloride. Partial solubility of the precipitate
therefore indicates the presence of both wood pulp and Neradol D.

The quantitative determination of sulphuric acid--the detection and
estimation of which in leather is important--is considerably influenced
by the presence of Neradol D. Practically all methods in vogue dealing
with its determination were based on the estimation of the sulphur
introduced into leather by sulphuric acid. The presence of Neradol D,
the main constituent of which is dicresylmethanedisulphonic acid,
renders it impossible by such methods to determine whether the combined
sulphur owes its origin to sulphuric or sulphonic acid. It remains yet
to be determined whether the sulphonic acid influences the leather
substance to the extent that sulphuric acid does; it must, however, be
borne in mind that Neradol D in addition to free sulphonic acid also
contains sulphonates and sulphates, which may enter into the leather and
thus increase the sulphur contents of the latter. A method must hence be
devised which estimates the free acid only and provides the means of
distinguishing this from all other acids of organic and inorganic
acids. Paessler, [Footnote: _Collegium_, 1914, 527, 126; 531, 509; 532,
567.] by extracting the leather and dialysing the filtrate, has
effected a separation of the acids and the tanning and colouring matters
and quantitatively estimated the sulphuric acid in the dialysate.

Immerheiser [Footnote 1:_Collegium_, 1918, 582, 293.] devised a method,
based upon the property of sulphuric acid of combining with ether, for
the purpose of determining free sulphuric acid in leathers:--10 gm. of
the leather, cut into small pieces, are extracted three times with 200
c.c. distilled water at room temperature, the time of each extraction
being ten to twelve hours, and the combined extracts evaporated to
dryness on the water bath, 5 gm. of quart sand being added. The dry
residue is now powdered, introduced into an Erlenmeyer flask provided
with a glass stopper, and 200 c.c. of anhydrous ether [Footnote 2: To be
tested for water by shaking with anhydrous copper sulphate.] added.
After about two hours, during which the flask is occasionally shaken,
the ether is poured through a filter, the residue washed with a little
ether, and the operation repeated twice with each 40 c.c. anhydrous
ether, using the same filter. To the combined ether extracts (about 200
c.c.) HCl and [Greek: b]aCl_2 are added, the ether distilled off and the
residue evaporated on the water bath, in order to decompose the
ether-sulphuric acid compound. 50 c.c, of hot water acidified with HCl
are now added, the precipitate allowed to settle, filtered, washed,
dried, and weighed. The sulphuric acid thus estimated was present in the
leather as _free sulphuric acid_. That present as sulphates soluble in
water is estimated in the residue on the filter: the residue is
extracted with hot water, the sand filtered off, the filtrate acidified
with HCl, boiled for one quarter hour and filtered if necessary. The
clear filtrate, which may be coloured, is brought to boil and
[Greek: b]aCl_2 is added. The barium sulphate indicates the sulphuric
acid present in the leather as water-soluble sulphates.

Whether the latter be sulphates or bisulphates may be indicated by the
aqueous extract of the above residue, since neutral reaction would
indicate the absence of bisulphates, acid reaction their presence in
addition to possible normal sulphates; the quantitative estimation of
the metals would decide this point definitely.

10. Properties of Leathers Tanned with Neradol D

Whereas the colour of leathers tanned with Neradol D only is nearly a
pure white, those tanned with mixtures of Neradol D and vegetable
tanning materials are more or less light coloured according to the
quantity of Neradol D present, as has been explained when discussing the
phlobaphene-solubilising action of Neradol D. In any case, all leathers
tanned with Neradol D possess fibre of remarkable length, which explains
their increased tensile strength and elasticity. The tensile strength
of a leather tanned with a mixture of Neradol D and vegetable tannins
was 3.7 per cent, as compared to 3 per cent when no Neradol was used;
the extension was 56 per cent, when tanning with Neradol D as against 36
per cent, without the latter.

The sensitiveness to light of leathers tanned with Neradol D may be
mentioned. Exposed to direct sunlight, the surface of the leather
assumes a yellowish colour after two days' exposure, and assumes a pure
yellow colour after a further three days. A further fifteen days'
exposure only darkens the leather slightly, the final colour being very
little different from the one obtaining after five days' exposure.

In passing, it may be remarked that this yellow colour is observed on
the surface only, the grain otherwise possessing that pure white colour
characteristic of Neradol D tanned leather. Further, it may be noted
that leathers tanned--with Neradol D fix basic coal-tar dyes
excellently, whereas acid and substantive dyestuffs are fixed with other
than their natural shades.

The author has analysed a leather exclusively tanned with Neradol D, and
has obtained the following results:--[Footnote: _Collegium_, 1913, 521,

Moisture  -     -     -     -     -     15.53 per cent.
Ash -     -     -     -     -     -      0.93 per cent.
Fats-     -     -     -     -     -      1.26  per cent.
Extraneous matters    -     -     -      0.00  per cent.
Leather Substance |Tanning matters-     36.92 per cent.
Leather Substance |Hide substance -     45.36 per cent.
                                       100.00 per cent.
       [Footnote: Sp. gr., 0.642.]

From these figures, those of "degree of tannage" and "yield"
(pelt-->leather) are calculated as 81.4 and 220 respectively.

These figures correspond closely to those obtained by the analysis of
leathers tanned with vegetable tanning materials, and this proves the
similarity between the Neradol D tannage and a vegetable tannage in
their chemical aspects.

11. Neradol D Free From Sulphuric Acid

In order to prepare phenol and cresulphonic acids, such quantities of
technical sulphuric acid are used as do not allow of the assumption of
complete utilisation of the sulphuric acid; hence it was of theoretical
interest to remove eventual traces of free sulphuric acid from the
product. For this purpose, the author diluted crude Neradol to 20°
Bé. and gradually added small quantities of milk of lime; the
precipitates were freed from the liquid by suction and washing, and a
Neradol free from sulphuric acid resulted, which was then brought to the
acidity of Neradol D with the calculated amount of alkali. From the
calcium sulphate precipitate, the amount of sulphuric acid originally
present was calculated, and was found to be only 4 per cent.

The acid-free sample of Neradol was tested with regard to its
suitability as a tanning agent; leather tanned with this sample differed
from one tanned with an untreated sample (Neradol D) by being harder and
possessing a pronouncedly greyish colour. This difference, however, may
not be due to the absence of sulphuric acid but to the presence of the
slightly soluble calcium sulphate in the sample treated with milk of
lime. To prove this point, another way of preparing Neradol D free from
sulphuric acid was looked out for. Sodium acetate was added to a
solution of crude Neradol until the latter was no longer acid to
congo-red; at this point no free sulphuric acid can be present in the
solution. The product, partly neutralised till the acidity of Neradol D
was reached (part of the acidity then being due to liberated acetic
acid), yielded a leather which neither in colour nor in feel differed
from the usual Neradol D tanned leather. This proves that the grey
colour and the hardness of the leather described in the former
experiment is due to the presence of calcium sulphate.

If the crude Neradol treated with sodium acetate is not partly
neutralised, the analysis gives the following figures:--

Tanning matters                  67.3 per cent.
Soluble non-tannins               8.6   "
Insolubles                        0.0   "
Water                            24.1   "
                                 100.0 per cent.
     Acidity: 1 gm. = 46 c.c. N/10 NaOH.

Compared to the analysis of crude Neradol containing sulphuric acid, the
figures show that, on the one hand, the presence of the comparatively
small quantity of sodium acetate but slightly influences the contents of
non-tannins and water, but, on the other hand, reduces the contents of
tannins and also the acidity. The tanning intensity of this product,
however, is considerably increased, and using a 1° Bé. solution a
leather is obtained in a very short time very similar to that yielded by
ordinary Neradol D, but considerably harder; the latter property is due
to higher acidity and almost complete absence of salts in the product
treated with sodium acetate.

The author finally attempted to partly neutralise crude Neradol with
various hydroxides and carried out tanning tests with samples containing
the different metals. Hardly any difference in the finished leathers
could be observed as regards colour or quality; the tannage could by no
means be described as that of a combination of Neradol D and the
respective metals.

12. Neutral Neradol

Crude Neradol, completely neutralised with caustic soda, yields a
product of the following composition:--

Tanning matters                    19.8 per cent.
Soluble non-tannins                37.9    "
Insolubles                          0.0    "
Water                              42.3    "
                                  100.0 per cent.

The qualitative reactions of this product differ from those of
non-neutralised Neradol to the extent that gelatine is not precipitated
and iron salts are not coloured blue, but dirty brown, by the aqueous
solution of this product.

The completely neutralised product, diluted to various concentrations
(of 1°, 2°, 3°, and 5° Bé.) and tested as to tanning properties,
revealed the surprising fact that the pelts were not even surface
tanned, and were coloured evenly blue throughout by indigotine.

It might have been anticipated that sodium dicresylmethanedisulphonate
would be as devoid of tanning powers as is a neutralised vegetable
tannin, but it is difficult to explain the fact of the sodium salt being
adsorbed by hide powder as "tanning matters" in the Official Method of
Analysis. Brought to a logical conclusion, the figure 19.8 per cent,
should be deducted from 32.5 per cent, obtained in the analysis of a
_partly_ neutralised Neradol D, which comparatively large quantities of
the sodium sulphonate also adsorbed by hide powder, leaving the "tanning
matters" of Neradol D at 13.5 per cent.

This diminished figure, however, does in no way reduce the value as a
tanning agent of Neradol D; it merely shows how inadequate is the hide
powder method of analysis when applied to substances of the composition
of Neradol D. This is further confirmed by the Loewenthal permanganate
method, which yields the following figures:--

Tanning matters            7.2 per cent.
Soluble non-tannins       59.1 [Footnote: Collegium, 1913, 521,487.]

If, on the other hand, completely neutralised Neradol is acidified with
an organic acid, such as acetic acid, till the acidity, (1 gm.= 10
c.c. N/10 NaOH) is reached, the resulting product is in all respects
similar to Neradol D and yields a corresponding leather.

It is permissible to assume that the irregularity exhibited by Neradol D
as regards the analytical estimation of its tannin contents is connected
with the low molecular weight of the tanning principle. Whereas all
tannins so far isolated from the natural tanning materials possess
rather high molecular weights, that of Neradol D deviates considerably
from this rule, as is shown by the following table:--

Neradol D tannin    Cl_5H_16S_2O_8       358
Mangrove    "       C_24H_40O_2l         670
Oak bark    "       C_28H_28O_23         840
Myrabolam   "       C_54H_48O_35        1256
Dividivi    "       C_54H_46O_35        1270
Malletto    "       (C_4lH_50O_20)_2    1724

This low molecular weight may mainly account for the figures obtained by
the incorrect oxymetric estimation with permanganate; the apparent
tannoid property of the tannoid-inactive neutral salt of
dicresylmethanedisulphonic acid may be explained by assuming that though
it is, probably, in the colloidal state, and as such adsorbed by hide
powder, it is still devoid of astringent properties.

G. Different Methods of Condensation as Applied to Phenolsulphonic Acid

In addition to formaldehyde, many other substances may, theoretically,
induce condensation of phenolsulphonic acid; condensation takes place
either with the elimination of water or, in addition to this, with the
introduction of methane group.

So far, the following condensing agents have been investigated:--

   (1) Heating _in vacuo_.
   (2) Sulphur chloride.
   (3) Phosphorus compounds.
   (4) Aldehydes.
   (5) Glycerol.

1. Condensation Induced by Heat

If phenolsulphonic acid is heated _in vacuo_ at 130° C. for twenty
hours, condensation takes place [Footnote: Austr. Pat., 64,479.]
without the addition of any condensing agent, and an anhydride of the

   ^ __O__ ^
  | |     | |
  | |     | |
   v       v
   HSO_3    HSO_3

composition is formed. This product is a viscous liquid, possessing a
very corrosive action. Added to a solution of gelatine, a light, fine
flocculent precipitate is thrown down. Analysed by the shake method of
analysis, the tannin content of the product equals about 46 per
cent. Its strongly acidic and hence swelling character does not express
qualities consistent with the conception of suitability for tanning
purposes: a sample of the product was therefore partly neutralised to
the acidity of Neradol D, when the shake method of analysis yielded the
following figures:--

Tanning matters                21.5 per cent.
Soluble non-tannins            48.3    "
Water                          30.2    "
                              100.0 per cent.

This partly neutralised sulphonic acid represents a white, pasty mass,
which is not particularly easily soluble in water, yielding a solution
of milky appearance. Treated with the usual tannin reagents, it exhibits
the following characteristics:--

Gelatine                 Light Flocculent precipitate.
Bromine water            Compete fixation.
Ferric chloride          Cherry-red coloration.
Lead acetate             Very slight Percipitate, insoluble HNO_3.
Aniline hydrochlonde     Slight percipitate.

Solutions of this product in concentrations from 1°-8° Bé. exerted no
tanning action whatever, whereas more concentrated solutions (15° Bé.)
converted pelt in eight days into a leather very similar to a Neradol D
leather in colour and feel, but considerably harder.

In order to determine its phlobaphene-solubilising effects, samplesof
the product were mixed with concentrated quebracho extract in the
proportions 5,10, 20, and 30 per cent. on the weight of extract, and the
following observations made:--5 and 10 per cent. were without effect,
20 and 30 per cent. showed some solubilising tendency, but on diluting
the mixture with water the quebracho was completely thrown out of
solution. Apparently this anhydride is, in this respect also, quite
different from the partly neutralised diphenylmethanedisulphonic acid.

2. Condensation with Sulphur Chloride

When sulphur chloride is allowed to act upon phenolsulphonic acid whilst
heat is applied, a yellowish-grey mass results, which dissolves in
water, forming a reddish-yellow solution. Neutralised to acidity 10, it
exhibits the following reactions:--

  Ferric chloride---------Deep blue coloration.
  Lead acetate------------White precipitate, insoluble HNO_3.
  Aniline hydrochloride---Precipitate.
  Bromine water-----------No reaction.

The partly neutralised 2° Bé. solution of this product yielded a
reddish-grey coloured leather, the qualities of which were very similar
to that yielded by Neradol D.

3. Condensation with Phosphorus Compounds

Schiff's well-known synthesis, [Footnote: Liebig's _Ann_., 178, 173.]
in which phosphorus oxychloride interacts with phenolsulphonic acid,
yields a product which exhibits some tannin reactions, but which, when
acting on pelt, converts the latter into a leather which, when dried, is
very cracky. If, on the other hand, cresolsulphonic acid is condensed
with phosphorus oxychloride by heating the two together, products
eminently suitable for tanning purposes result. These products are
non-crystalline bodies easily soluble in water, and are coloured
bluish-violet by ferric chloride and precipitate gelatine. Solutions of
the free acids and acidified solutions of the salts convert pelt into
firm and white leathers possessing great softness and
pliability.[Footnote: Austr. Pat, 66,895.]

4. Condensation with Aldehydes

By treating phenolsulphonic acid with acetaldehyde in the usual way, a
viscous brown mass is obtained, which is very soluble in water, the
solution being of a brown colour. When brought to acidity 10, the
following reactions are exhibited by the product:--

Gelatine     -    -     -    Precipitate.
Ferric chloride   -     -    Deep blue coloration.
Aqueous ammonia   -     -    Cherry-red coloration.
Lead acetate -    -     -    Yellowish precipitate, insoluble
Aniline hydrochloride -  -    Yellow precipitate, soluble excess
Bromine water-    -     -    No reaction.

Tanning experiments with this substance yielded, even after extended
tannage, an undertanned leather, the surfaces being coloured brown, the
inner layers, however, white. Further neutralisation reduces the
tanning intensity of the product; the addition of sodium sulphate to the
original partly neutralised product hastened tannage, the leather,
however, possessing dark colour and being undertanned. The following
constitution may be ascribed to this product:--

   OH                  OH
   ^ ---CH_2---CH_2--- ^
  | |                 | |
  | |                 | |
   v                   v
   HSO_3               HSO_3

If benzaldehyde is used in lieu of acetaldehyde for condensing
phenolsulphonic acid, a water-soluble product results, exhibiting
reactions similar to those of the acetaldehyde-condensation product. The
former product is more suitable as a tanning agent and yields a
reddish-brown rather firm and hard leather; it possesses the

   OH    ||    OH
   ^ ____C____ ^
  | |    ^    | |
  | |   | |   | |
   v    | |    v
  HSO_3  v     HSO_3

For the purpose of condensing phenol with formaldehyde, it is not
essential to first convert the phenol into the water-soluble
phenolsulphonic acid, since it is possible to convert the condensation
products of phenol and its derivatives, which are soluble in alkali,
into water-soluble form by either heating the condensation products with
concentrated solutions of formaldehyde and neutral sulphites, or by
dissolving the condensation products in alkali and inducing reaction by
means of formaldehyde bisulphite. [Footnote: _Collegium_, 1913, 518,
324.] Highly concentrated solutions result, which may be concentrated
either as such or after the alkali present has been neutralised. The
sulphurous acid formed prevents oxidation of the product on
evaporation. A special advantage of this method of preparation is the
fact that sulphuric acid, which is but difficultly removed from the
end-product, is not employed at all.

The product thus obtained is a yellowish-white crumbly mass, which is
very soluble in water, forming a clear solution. The latter exhibits
the following reactions:--

  Ferric chloride--------Deep blue coloration.
  Aqueous ammonia--------Cherry-red coloration.
  Lead acetate-----------White precipitate, insoluble in
  Aniline hydrochloride--Precipitate.
  Bromine water----------No reaction.

The product brought to acidity 10, yielded on analysis the
following figures:--

  Tanning matters------------------ 25.2 per cent.
  Soluble non-tannins-------------- 56.3    "
  Insolubles-----------------------  0.0    "
  Water---------------------------- 18.3    "
                                    100.0 per cent.

Tanning experiments with this substance yielded white and soft leathers,
which were indistinguishable from those tanned with Neradol D.

A characteristic feature of this synthetic tannin is its behaviour in
concentrated form towards pelt, which is not attacked by it, but is
readily tanned even at such high concentrations. An explanation of this
is to be found in the large quantity of salts present in the product. A
disadvantage of this synthetic tannin is its complete incapability of
dissolving phlobaphenes, which is even so far extended as to precipitate
otherwise easily soluble tannins when adding it to solutions of the
latter in comparatively large proportions; here, again, the salts are
responsible for this behaviour, their large quantities effecting a
salting out of the natural tannins.

The class of aldehyde condensations also comprises that of inducing
condensation by means of sugars; if phenolsulphonic acid is heated with
glucose, a reddish-brown liquid results, which is soluble in water. The
solution exhibits reactions similar to those of Neradol D. It is,
however, not possible, by this method of condensation, to prepare as
highly concentrated products as is possible in the case of Neradol D,
since employing sugars as condensation agents means liberation of a
large volume of water. Analysis of this product, using the shake method,
gives a tannin content of 16.2 per cent; tanning experiments
demonstrated that the time of tannage, using a 2° Bé. solution, was the
same as that required by Neradol D, and yielded a leather, the surface
of which was reddish-grey, the inner layers being white, but which is
otherwise very similar to Neradol D tanned leather. [Footnote:
Austr. Pat, 69,375, 69,376, 69,377.]

Relatively to its capability of solubilising phlobaphenes, this product
exhibits similar properties to that obtained by merely heating
phenolsulphonic acid, to a slight extent only solubilising quebracho
extract, which, on diluting the mixture, is completely thrown out of

5. Condensation with Glycerol

Phenolsulphonic acid, when heated with glycerol, undergoes the process
of condensation, and forms a brown fluid, which, when brought to acidity
10, exhibits the following reactions:--

  Ferric chloride----------Brown-black coloration.
  Lead acetate-------------White precipitate, insoluble in
  Aniline hydrochloride----Slight precipitate.

Tanning experiments with this partly neutralised product resulted in a
very gradual conversion of the pelt into a greenish-grey coloured
leather; the colour, however, does not penetrate the pelt and is hence
caused by colloidally suspended impurities. If the solution is filtered
through a filter candle, a somewhat clearer solution results, but the
latter also tans very slowly and yields a brown coloured leather.

Analysis of the partly neutralised product reveals a tannin content of
17.6 per cent. A 2° Bé. solution of the non-neutralised product showed a
rapid tanning effect at first, when brought into contact with pelt, on
which it had a strong swelling effect, and to which it imparted a
greenish colour; the tanning effect, however, slowed down considerably,
after a few days, and the solution penetrated the pelt only very
gradually; this is probably due to the presence of large quantities of
colloidally suspended impurities, which, when the substance is partly
neutralised with the formation of salts of the sulphonic acids, are
brought into true solution and hence penetrate the pelt with greater



Badische Anilin u.(German abbreviation for "und") Soda-Fabrik
Boehringer & Sons

Chem. Fabrik Jucker & Co.

Deutsch-Koloniale Gerb u. Farbstoff Gesellschaft

Elberfelder Farbenfabriken

Fischer, E.

Gesellschaft f.(German abbreviation for "für") Chem. Industrie, Basle
















Alcohol figure
Alizarin yellow, in paste
Alkalies, reaction of, to Neradol D
Alum-neradol tannage
Alum tannage
Aminophenol, _p_-
Aniline dyes
Arylsulphaminoarylsulphonic acids
Arylsulphoxyarylsulpho acids


Bakelite solution
Benzoylamino 6-chloranthraquinone
Benzylsulphanilate sodium
Bismuth salts
Bleaching method for leather with Neradol D
Bromo-[Greek: b]-naphthol
Bromosalicylic acid


Carbomethoxyhydroxybenzoic acid,
Carbomethoxyhydroxybenzoic acid chloride,
Cerium salts
Ceruleoellagic acid
Cesium salts
Chestnut wood extract
Chloronaphthalenesulphonic acid
Chrome-Neradol D tannage
Chrome salts
Chrome tannage
Coal, bituminous
Coffee tannin
Combination tannage with Ordoval
Combination tannage with Neradol D
Condensation by heat
Condensation methods
Condensation with aldehydes
Condensation with glycerol
Condensation with phosphorus compounds
Condensation with sulphur chloride
Copper salts
Cresol-_p_-sulphonic acid, _o_-
Cresolsulphonic acid
Cresotinic acid


Detannisation with hide powder
Diaminonaphthylmethanedisulphonic acid
Di-[Greek: b]-oxynaphthoic
Di-[Greek: b]-resorcylic acid
Dichloronaphthylmethanedisulphonic acid
Dicresylmethanedisulphonic acid
Dicresylmethanedisulphonic acid purified electro-osmotically
Dicresylmethane sulphonate sodium
Didymium salts
Diferulic acid
Digallic acid
Digallic acid, [Greek: b]-
Digallic acid, inactive
Digallic acid, _m_-
Digalloylleucodigallic acid anhydride
Digentisinic acid
Dihydric alcohols, aromatic
Dihydroxybenzene, _m_-
Dihydroxybenzene, _o_-
Dihydroxybenzene, _p_-
Dimethylellagic acid
Di-_m_-oxybenzoic acid
Dinaphthylmethanedisulphonic acid
Dinitronaphthylmethanedisulphonic acid
Di-_o_-cumaric acid
Diorsellic acid, _o_-
Diorsellic acid, _p_-
Dioxyellagic acid
Dioxynaphthylmethanedisulphonic acid
Dioxytoluic acid
Diphenylmethanedisulphonic acid
Di-_p_-hydroxybenzoic acid
Diprotocatechuic acid
Disalicylic acid
Disyringic acid
Dithionaphthylmethanedisulphonic acid
Dividivi tannin
Dixylylmethanedisulphonic acid


Electro-chemical behaviour of Neradol D
Electro-osmosis of Neradol D
Ellagic acid
Ellagitannic acid
Empirical formula of tannin
Ester formula of tannin
Ethyl acetate figure


Fat-Neradol D tannage
Feruloyl-_p_-oxybenzoic acid
Flavellagic acid
Formaldehyde tannage


Gallate ethyl
Gallic acid
Galloyl-_p_-hydroxybenzoic acid
Galls, oak
Gall tannin
Generator tar


Hexahydroxyaurinecarboxylic acid
Hexoxydiphenyldicarboxylic acid
Hexoxydiphenylmethanedicarboxylic acid
Humic acid
Hydrolysis of tannins
Hydroxybenzoate sodium, _m_-
Hydroxybenzoate sodium, _p_-
Hydroxybenzoic acid, _p_-
Hydroxybenzoic acid


Indophenol reaction
Iron, reaction of, to Neradol D
Iron salts


Ketone formula of tannin


Lanthanum salts
Lead salts
Leather analysis in presence of Neradol D
Lecanoric acid
Leucodigallic acid
Leucoellagic acid
Luteic acid


Malletto tannin
Mangrove tannin
Melangallic acid
Mercury salts
Metellagic acid
Methylenedisalicylic acid
Methylenedisalicylic acid, brominated
Methylenedisalicylic acid, iodised
Molybdenum figure
Myrabolams, tannin


Naphthalenesulphonic acid, [Greek: b]-
Naphthol, [Greek: a]-
Naphthol, [Greek: b]-
Naphthol-[Greek: a]-methanesulphonic acid
Naphtholdisulphonic acid
Naphtholmonosulphonic acid
Naphtholsulphonic acid, [Greek: a]-
Naphtholsulphonic acid, [Greek: b]-
Neodymium salts
Neradol D
Neradol D tannin
Neradol N
Neradol ND
Neradol ND, neutral
Nitronaphthalenesulphonic acid
Nitrophenol, _o_-


Oak bark
Oak bark tannin
Official method of tannin analysis
Ordoval G
Orsellic acid
Orsellinoyl-_p_-oxybenzoic acid
Oxyazo reaction
Oxybenzoyl-_m_-hydroxybenzoic acid
Oxybenzoyl-_p_-hydroxybenzoic acid, _m_-
Oxybenzoylsyringic acid
Oxynaphthoyl-_p_-hydroxybenzoic acid, _a_-
Oxynaphthylmethanesulphonic acid
Oxyphenylmethanesulphonic acid


    58,405; 61,057; 61,061;
    64,479; 66,895;
    68,796; 69,194;
    69,375; 69,376;
    69,377; 70,162
    72,161; 111,408; 112,183;
    132,224; 181,288;
    184,449; 200,539;
    206,957; 211,403;
    262,558; 282,313;
    286,568; 290,965;
    291,457; 293,042;
    293,640; 293,693;
    297,187; 297,188;
    300,567; 303,640;
    305,516; 319,713;
    78,282; 78,797; 79,139
Pelts, action on, of Neradol D
Penta-[_p_-hydroxybenzoyl] glucose,
Pentacetyl-_m_-digallic acid
Pentacetyl tannin
Pentamethyldigallic acid, methyl ester
Pentamethyl-_m_-digalloyl chloride,
Pentamethyl-_m_-digallic acid
Pentamethyl-_m_-digallic acid methyl ester
Pentamethyl-_p_-digallic acid
Pentamethyl-_p_-digallic acid methyl ester
Pentamethoxybiphenylmethylolide carboxylic acid methyl ester
Pentoxybiphenylmethylolide carboxylic acid
Phenolsulphonate sodium
Phenolsulphonic acid
Phenolsulphonic acid anhydride
Phenol, tautomeric
Phenylcarboxylic acid
Phenylhydrazine derivatives of tannin
Phenylhydrazine ellagic acid
Phlobaphene-solubilising action of neradols
Phthalic acid
Picric acid
Platinum salts
Polydigalloylleucodigallic acid anhydride
Preparation of tannin infusion
Properties of leather tanned with Neradol
Protocatechuic acid
Protocatechuyl-_p_-hydroxybemoic acid
Purpuro tannin
Pyrogallic acid
Pyrogalloylcarboyl-_p_-oxybenzoic acid
Pyruvic acid




Reaction, Procter-Hirst
Reagents for Neradol D tannage
Resorcylic acid, [Greek: b]-

Rosins, acid
Rosolic acid
Rufigallic acid


Salicylic acid
Salicylic acid phenyl ester
Salicyl-_p_-hydroxybenzoic acid
Silver oxide
Solution salt
Structure of tannin
Sulphinic acid
Sulphite cellulose extract
Sulphite lye
Sulphonic acids, aromatic
Sulphonic chloride
Sulphur tannage
Sulphuric acid-free Neradol D
Sulphuric acid in leather
Syringoyl-_p_-hydroxybenzoic acid


Tannin action, real
Tannin analysis
Tanning matters
Tannin molecule
Tannin, pure
Test tannage
Tetragalloyl-[Greek: a]-methylglucoside
Tetramethylellagic acid
Thionaphtholsulphonic acid
Thiosulphonic acid
Thorium salts
Toluidoanthraquinone, l-_m_-
Total solids
Total solubles
Tribromopyrogallic acid
Tricarbomethoxygalloyl chloride


Vanadium salts
Vanillic acid
Vanilloyl-di-_p_-oxybenzoyl-_p_-hydroxybenzoic acid
Vanilloyl-_p_-hydroxybenzoic acid
Vanilloyl vanillin




Zinc salts
Zirconium salts

*** End of this Doctrine Publishing Corporation Digital Book "Synthetic Tannins, Their Synthesis, Industrial Production and Application" ***

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About ISYS® Search Software
Established in 1988, ISYS Search Software is a global supplier of enterprise
search solutions for business and government.  The company's award-winning
software suite offers a broad range of search, navigation and discovery
solutions for desktop search, intranet search, SharePoint search and embedded
search applications.  ISYS has been deployed by thousands of organizations
operating in a variety of industries, including government, legal, law
enforcement, financial services, healthcare and recruitment.