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Title: An examination of some methods employed in determining the atomic weight of Cadmium
Author: Bucher, John E.
Language: English
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*** Start of this Doctrine Publishing Corporation Digital Book "An examination of some methods employed in determining the atomic weight of Cadmium" ***
EMPLOYED IN DETERMINING THE ATOMIC WEIGHT OF CADMIUM ***
An Examination of some Methods Employed in Determining the Atomic Weight
of Cadmium.
A Thesis
Presented to the Board of University Studies of the Johns Hopkins
University for the Degree of Doctor of Philosophy.
by
John E. Bucher.
1894
Contents.
I. Introduction and Historical Statement 1
II. The Oxalate Method 3
Preparation of Pure Cadmium 3
Preparation of Nitric Acid 4
Purification of Water 4
Purification of Oxalic Acid 5
Preparation of Cadmium Oxalate 7
Procedure 8
Results 13
III. The Sulphide Method 16
Preparation of Hydrogen Sulphide 16
Preparation of Nitrogen 17
Mode of Procedure 18
Results 24
Discussion of the Results 24
Discussion of the Method 26
IV. The Chloride Method 33
Preparation of Cadmium Chloride 35
The Filters 48
Analytical Process 52
Results 57
Discussion of the Results 58
V. The Bromide Method 69
Preparation of Cadmium Bromide and Hydrobromic Acid 70
Method of Analysis 78
Results 80
Discussion of the Results 80
VI. Syntheses of Cadmium Sulphate 82
Results 90
Discussion of the Results 91
VII. The Oxide Method 94
Results 96
Discussion of the Results 97
Determination of Error 104
Discussion of the Oxalate Method 114
VIII. Other Methods 119
IX. Conclusion 122
Acknowledgement.
The author wishes to acknowledge his indebtedness for advice and
instruction to Professor Morse at whose suggestion and under whose
guidance this work has been carried on. He also wishes to express his
thanks for instruction to Professor Remsen in Chemistry, Professor
Williams in Mineralogy, Dr. Ames in Physics and Mr. Hulburt in
Mathematics.
Introduction and Historical Statement.
The atomic weight of cadmium has been investigated by a number of
chemists but the results obtained vary between wide limits. The work
described in this paper was undertaken with the object of finding the
cause of the discrepancy in some of the methods employed. A complete
historical statement has been given by Morse and Jones, (Amer. Chem.
Jour., 14. 261.) and it is only necessary, here, to give a summary for
the purpose of reference:
Ratio. At. Wt. Cd.
1818, Stromeyer, Cd : CdO 111.483
(Schweiggers Jour. 22, 336.)
1857, Von Hauer, CdSO_{4} : CdS 111.935
(Jour. f. Prakt. Chemie 72, 338.)
1859, Dumas, 1st series. CdCl_{2} : Ag 112.416
2d „ CdCl_{2} : Ag 112.007
(Ann. Chim. Phys. [3], 55, 158.)
1860, Lenssen, CdC_{2}O_{4} : CdO 112.043
(Jour. f. Prakt. Chem. 79, 281)
1882, Huntington and Cooke, CdBr_{2} : AgBr 112.239
„ „ CdBr_{2} : Ag 112.245
(Proceedings Amer. Acad. 17, 28)
1890, Partridge, 1st Series CdC_{2}O_{4} : CdO 111.816
„ 2d „ CdSO_{4} : CdS 111.727
„ 3d „ CdC_{2}O_{4} : CdS 111.616
(Amer. Jour. Sci.[3], 40, 377)
1892, Morse and Jones, 1st Method, Cd : CdO 112.0766
2d „ CdC_{2}O_{4} : CdO 112.632
1892, Torimer and Smithy CdO : Cd 112.055
(Zeit. f. Anorg. Chem. I. 364)
In this summary as well as in the rest of this paper the following
atomic weights are used:
Oxygen = 16.00
Sulphur = 32.059
Carbon = 12.003
Chlorine = 35.45
Bromine = 79.95
Silver = 107.93
The Oxalate Method
Preparation of Pure Cadmium.
“Cadmium met. puriss. galv. reduc”, obtained from Schuchardt, was used
for preparing pure cadmium. It was heated to redness in a current of
hydrogen which had been purified by washing with both acid and alkaline
solutions of potassium permanganate. This treatment converted the
metallic powder into a bar which could be distilled in a vacuum. The
metal was then distilled nine times in the same manner that Morse and
Burton, Amer. Chem. Jour. 12, 219, had distilled zinc. All distillations
were made slowly except the last one, which was made quite rapidly.
Preparation of Nitric Acid.
Whenever pure nitric acid was required, it was purified by distilling
against a platinum dish and collecting the distillate in a smaller one
of the same metal. The nitric acid used was dilute and free from
chlorine.
Purification of Water.
The water used in this work was purified by distilling twice from an
alkaline solution of potassium permanganate, always rejecting the first
part of the distillate. Whenever water was needed in the preparation of
a pure compound e.g. cadmium oxalate, oxalic acid, cadmium nitrate,
etc., it was subjected to the additional process of being distilled
against a large platinum dish which was kept cool by placing ice inside
it.
Purification of Oxalic Acid.
Commercial oxalic acid was heated with a fifteen percent solution of
hydrochloric acid until all was dissolved. The solution was then warmed
for twenty four hours. On cooling, crystals of oxalic acid separated out
and these were washed with a little cold water to remove the mother
liquor. They were then dissolved in hot ninety-five percent alcohol and
allowed to crystallize slowly on cooling. The acid was next crystallized
from ether in which it is only sparingly soluble. After this it was
boiled with water until the odor of ethyl acetate had disappeared.
Finally it was recrystallized three times from water and dried in the
air at ordinary temperatures.
Preparation of Cadmium Oxalate.
A weighed piece of cadmium was dissolved in nitric acid and the excess
of acid evaporated off. The nitrite was then dissolved in a large
quantity of water and an equivalent amount of oxalic acid in solution
added. The oxalate separated in a few moments as a crystalline
precipitate. It was collected on a porcelain filter and washed
thoroughly to remove nitric acid and ammonium nitrate. A considerable
amount of ammonium nitrate was formed during the solution of the cadmium
in nitric acid. The oxalate was finally dried in an air-bath for fifty
hours, at 150°C.
Procedure
Enough cadmium oxalate for a determination was placed in a weighing tube
which had been tared against a similar vessel and dried at 150°C. until
the weight remained constant. It was then poured into a weighed
porcelain crucible. The tube and its tare were now dried again at the
same temperature to constant weight in order to avoid any error
resulting from moisture being absorbed by the cadmium oxalate which
adhered to the weighing-glass. The crucibles used in these
determinations were arranged in the same manner as those employed by
Morse and Jones in their work on this method. A small porcelain crucible
on whose edge were placed three short platinum wires bent in the shape
of the letter U, was placed in a larger porcelain crucible. The platinum
wires prevented the lid from sticking to the crucible after heating and
also allowed the products of decomposition to escape. The glaze was
removed from the outside of the larger crucible with hydrofluoric acid
to avoid sticking when heated to a high temperature. A second pair of
crucibles arranged in the same manner was tared against the first one
and in all cases treated like it. After the oxalate had been poured into
the weighed crucible, it was decomposed by placing the crucible with its
contents in a cylindrical asbestus covered air-bath, and slowly raising
the temperature until the mass beams uniformly brown in color. In the
last five determinations, the temperature was not allowed to exceed
300°C and after from forty to eighty hours the loss in weight was about
ninety percent of the amount calculated for complete decomposition. In
the first four the temperature was much higher and the time employed
shorter. After the oxalate had been thus treated nitric acid was added
and the contents of the crucible dissolved completely. The crucible was
then transferred to a bath constructed by placing a larger porcelain
crucible in a still larger one of iron and filling the intervening space
with sand. It was slowly heated until the nitric acid had all evaporated
and the dry nitrate began to give off red fumes. The crucibles were then
removed to a similar bath containing iron filings instead of sand. This
bath was heated by means of a single burner as long as red fumes were
observed, and then for about five hours with a triple burner. Finally,
the crucibles were transferred to a nickel crucible in the bottom of
which a plate of unglazed porcelain was placed. The nickel crucible
which had previously been set tightly into a hole cut in an asbestos
board was then heated over the blast lamp for two hours. After this the
porcelain crucible and contents were weighed and then reheated for half
hour periods as before until three successive weighings remained
constant. This usually required from three to four hours of blasting. In
all determinations, the resulting product was tested for oxides of
nitrogen with potassium iodide, starch and hydrochloric acid, but none
was found. All weighings were reduced to the vacuum standard on the
assumption of 8.4 for the Sp. Gr. of brass, 21. for platinum, 3.31 for
the oxalate and 8.15 for cadmium oxide. The results are:
Cadmium Oxalate. Cadmium Oxide. At. Wt. Cd.
I 1.97674 1.26414 111.73
II 1.94912 1.24682 111.82
III 1.96786 1.25886 111.77
IV 1.87099 1.19675 111.77
V 1.98941 1.27242 111.79
VI 1.37550 .87994 111.85
VII 1.33313 .85308 111.95
VIII 1.94450 1.24452 112.04
IX 2.01846 1.29210 112.09
A glance at these results shows that there is a variation of .36 of a
unit and that the atomic weight in general increases with the number of
determinations. In the first four determinations, there may have been
loss of cadmium by reduction and subsequent volatilization, but in the
later determinations this is not probable. It is believed that the
greater part of the variation was due to imperfect dehydration of the
oxalate. This and other sources of error in this method will be referred
to later. The nickel crucible used gave a slight sublimate on heating,
even after fifteen hours’ blasting. This condensed on the porcelain
crucible as a brownish coating but, as both the crucible and its tare
were blasted for the same length of time, it did not seem to change the
difference of their weights. More than a dozen nickel crucibles were
tried but none was found not to give a sublimate. The amount was so
slight that no attempt was made to determine its nature.
The Sulphide Method.
This method is based on the conversion of cadmium oxalate into cadmium
sulphide by heating in a current of hydrogen sulphide. The method has
been used by Partridge. This result was 111.61 for the atomic weight of
cadmium.
Preparation of Hydrogen Sulphide
In the present work this gas was always prepared from potassium
hydrosulphide which was made from barium sulphide (commercial). Barium
sulphide was treated with dilute hydrochloric acid and the resulting
hydrogen sulphide washed thoroughly with a solution of potassium
hydrosulphide and then with pure water. It was then passed into a strong
solution of potassium hydroxide until the later was saturated. When it
was required, it was set free from this solution by adding dilute
sulphuric acid and again washing the resulting gas with a strong
solution of potassium hydrosulphide.
Preparation of Nitrogen.
Whenever a current of nitrogen was required, it was prepared by passing
air over a layer of hot copper gauze in a combustion tube. A short layer
of copper oxide was first introduced, then the copper gauze and finally
another layer of copper oxide. The air was dried with caustic potash
before entering the tube and the nitrogen obtained was also passed
through a long tube filled with lumps of this substance before being
used.
Mode of Procedure.
A number of weighing tubes 140 millimetres long and 13 millimetres
internal diameter were made especially for this work. They were always
used in pairs, one being kept as a counterpoise. A porcelain boat of
such dimensions as just to slide into the tube was placed in each one.
For a determination, a tube and its boat were tared with another tube
and boat, glass against glass and porcelain against porcelain until the
difference in weight was less than two tenths of a milligramme. Both
boats were heated in a current of hydrogen sulphide to incipient redness
for about one hour. The current of hydrogen sulphide was then replaced
by one of nitrogen, in which the boats were cooled, but while still warm
they were transferred to their weighing tubes and allowed to cool in a
dessiccator containing caustic potash and weighed. Before weighing, the
stoppers of the weighing tubes were loosened for a moment in order to
equalize the internal and external pressure. This treatment was usually
repeated two or three times and the difference in weight remained
perfectly constant. A portion of cadmium oxalate sufficient for a
determination was placed in the weighed boat and dried at 150°C. The
oxalate had been prepared exactly like that used in the oxalate method
which has already been described. The gas pressure in the laboratory
varied very much while this method was under investigation and great
difficulty was experienced in maintaining a constant temperature
although a thermoregulator was used. Sometimes a specimen of oxalate
which was supposed to be dry would lose several tenths of a milligramme
when the thermometer would only have gone up to 160°C or 165°C for an
hour by accident. Under these conditions the drying was so uncertain
that only four determinations were completed although many were started.
The boat containing the oxalate which had been dried and weighed was
placed on supports of unglazed porcelain in a combustion tube and a
current of dry hydrogen sulphide passed over it. As soon as the air was
expelled, the tube, which was in a combustion furnace, was slowly heated
until all the oxalate seemed to be decomposed and then raised to dull
redness. After this temperature had been maintained for about an hour,
the sulphide was allowed to cool to a temperature of about 200°C. and
the current of hydrogen sulphide replaced by dry nitrogen, using a
three-way stopcock. When nearly cold the boat was slipped into its
weighing-tube and weighed, the same precautions being used as when
weighing the empty boat.
At this stage the sulphide was always from one to two milligrammes
lighter than at the end of the determination. It was reheated for
periods of one hour until the weight remained constant. This generally
required from three to five hours. All weighings were reduced to the
vacuum standard on the basis of 4.5 for the Sp. Gr. of cadmium sulphide,
3.31 for the Sp. Gr. of cadmium oxalate, 8.4 for the Sp. Gr. of brass
weights and 21 for the Sp. Gr. of platinum weights.
The results are as follows:
Cd C_{2}O_{4} CdS At. Wt. Cd.
I. 2.56319 1.84716 112.25
II. 2.18364 1.57341 112.17
III. 2.11643 1.52462 112.05
IV. 3.13105 2.25582 112.12
The first three determinations were made exactly as above described, the
heating in hydrogen sulphide being done in a Bohemian glass combustion
tube. The hydrogen sulphide was dried with calcium chloride.
The fourth determination was made under somewhat different conditions.
The boat containing the weighed oxalate was placed in a combustion tube
which passed through an asbestus covered air-bath. The air was displaced
by a current of dry hydrogen sulphide and the bath slowly heated. When
the temperature had risen to 210°C. it was maintained there for three
hours, and then raised to 250°C. for three hours. The sulphide then
weighed 2.27 grammes, being 14 milligrammes heavier than when the
determination was finished. It was replaced in the tube and reheated in
a current of hydrogen sulphide at a temperature of 300°C. for four
hours. It was then transferred to a porcelain tube and heated to redness
for one hour. It then weighed 2.25437 grammes, being 1.45 milligrammes
lighter than at the end of the determination. The weight did not become
constant until it had been heated six hours more to redness in a current
of hydrogen sulphide. When this oxalate was slowly heated in H_{2}S, a
small amount of oxalic acid sublimed to the colder part of the tube,
but, in the other cases where the heating was more rapid, only carbon
monoxide, carbon dioxide, and water were observed.
Discussion of the Method.
When hydrogen sulphide is passed through a red-hot tube, sulphur is
deposited on the colder parts because at this temperature hydrogen
sulphide dissociates and the elements do not recombine on cooling. In
this work, a faint sublimate was noticed before coming to the zone of
sulphur deposit. On exposure to air, it deliquesced in a few minutes
forming small yellow drops which had a saline taste, and gave tests for
potassium and sulphur. The sublimate had a yellow color and was
evidently formed by the action of sulphur on glass. It seemed to do no
harm, but in the fourth determination an effort was made to avoid it by
using a porcelain tube instead of a glass combustion tube for heating to
redness in a current of hydrogen sulphide.
The fact that sulphide of cadmium was always too light after the first
hour’s heating in hydrogen sulphide proves that it must have contained
some oxide of cadmium even after this heating. Oxide of cadmium is
readily absorbed by the glaze on porcelain, and some error must have
been introduced in this way because it would not be converted into
sulphide after forming a silicate.
The effect of this would be to give a low result for the atomic weight
of cadmium. To get some idea of the magnitude of this error, the
sulphide was poured out of the boats used in the first and second
determinations. They were then warmed with nitric acid for a few
moments, washed in water, and heated over the blast lamp for a few
minutes. The boats used as tares were treated in exactly the same
manner. On weighing, the boats in which the oxalate in determinations I
and II had been decomposed, were found to be 1.12 milligrammes and .82
milligrammes heavier respectively than at the beginning of the
determinations. This would only introduce an error of .03 of a unit in
the atomic weight on account of the small difference in weight between
these amounts of oxide and equivalent amounts of sulphide. The boats
were warmed, as above mentioned, with nitric acid to remove any adhering
sulphide. This might have decomposed some cadmium silicate at the same
time, and the error due to cadmium oxide thus be found smaller than it
really is.
The following experiment was made in the hope of avoiding the formation
of cadmium silicate. The glaze was removed from the inside of a
porcelain boat by hydrofluoric acid followed by a thorough scouring with
sand and water. The boat was then heated in the flame of a blast lamp
for several minutes, tared against another boat which was not treated
with hydrofluoric acid. Both were heated to redness in a current of
hydrogen sulphide for an hour, cooled, weighed, and then heated in
hydrogen sulphide for another hour, and weighed again. The boat gained
1.7 milligrammes during this second heating, showing that a boat whose
glaze has been removed by hydrofluoric acid could not be used in this
method. Throughout this work, great care was taken to exclude the oxygen
of the air from the cadmium sulphide, while hot. The current of hydrogen
sulphide in which the cadmium sulphide is heated must not be too slow,
otherwise the sulphur in the dissociated gas will diffuse to the colder
parts of the tube and condense, the residual gas becoming very rich in
hydrogen. The hydrogen will then reduce some of the sulphide to metal,
causing loss by volatilization. One determination was lost in this way,
over two milligrammes of the sulphide being sublimed out, and it could
easily be detected on the side of the tube. It is believed that the
cause of the variations in the four determinations made by this method,
is due to imperfect dehydration of the oxalate. It did not seem
advisable to continue this part of the work any farther; therefore the
chloride method was taken up.
The Chloride Method.
Huntington had determined the ratios of CdBr_{2} to AgBr and also
CdBr_{2} to Ag very carefully, obtaining the result 112.24 for the
atomic weight of cadmium. Morse and Jones had obtained 112.07 for this
constant by the oxide method. The object of the work about to be
described was to find the cause of this discrepancy if possible. It was
thought advisable however to make some determinations of the ratio of
CdCl_{2} to AgCl before beginning the bromide method.
Dumas, in 1859, used cadmium chloride to determine the atomic weight of
the metal. He did not establish its ratio to silver chloride but to
silver by titration. He prepared cadmium chloride by dissolving the
metal in hydrochloric acid and melting the resulting product in a
platinum capsule for five or six hours. He made two series of three
determinations. The chloride used in the first series was yellow in
places and not completely soluble. The result was 112.476. The second
series was made with chloride which was perfectly white and soluble and
gave 112.007 for the atomic weight of cadmium. It is evidently more
reliable than the first series and Dumas himself concluded that the
atomic weight is very near 112.01.
Preparation of Cadmium Chloride
Four different specimens of cadmium chloride were used in this work and
from these specimens portions were taken for analysis. These portions
were treated differently in different analyses, therefore it will be
necessary to give a brief descriptions of them and mention the number of
the determinations, in which each one was used. Chloride of cadmium was
prepared in the following manner. A solution of pure hydrochloric acid
was prepared by passing a current of hydrochloric acid gas into pure
water which was contained in a porcelain crucible until no more was
absolved. The water used had been purified by distilling against a
platinum dish and the hydrochloric acid gas was obtained by heating
ordinary concentrated chemically pure hydrochloric acid in a distilling
bulb whose neck had been closed by fusion in order to avoid the use of a
cork or rubber stopper. Hydrochloric acid thus prepared will leave no
residue on evaporation when air is excluded (Stas, Aronstein’s German
translation, p. 111). A piece of platinum foil freed from iron by
heating in the vapors of ammonium chloride as recommended by Stas
(Aronstein’s translation, p. 112) was introduced and a piece of cadmium
laid on it. Solution begins at once, the hydrogen being liberated on the
platinum foil. During the later part of the process, heat was applied.
After all of the metal had dissolved, the solution was evaporated, the
platinum foil having previously been removed. The crystals of cadmium
chloride which separated were not dried but allowed to remain slightly
moist with hydrochloric acid. If no platinum foil is used, the solution
of the pure metal becomes exceedingly difficult, unless a very large
excess of acid is used. No objection can be raised to the use of
platinum foil for in making fifty grammes of cadmium chloride it cost
less than a tenth of a milligramme and even this could probably have
been avoided by using a somewhat larger amount of hydrochloric acid. The
foil was always kept submerged in the acid liquid. The moist crystals of
cadmium chloride were transferred to a combustion tube passing through
an asbestus covered air-bath, and dried in a current of hydrochloric
acid gas for several hours at 300°C. The hydrochloric acid gas had been
passed through a long calcium chloride tube to dry it, although calcium
chloride probably does not do this very thoroughly. The hydrochloric
acid gas was then replaced by a current of nitrogen prepared as has
already been described under the sulphide method. After the current of
nitrogen had been passing for about half an hour, the tube was allowed
to cool, and the chloride transferred to another combustion tube, one
end of which had been sealed in the flame of a blast lamp. The other end
was drawn out and attached to a Sprengel mercury pump. After exhausting,
the chloride was sublimed in the vacuum. This takes place at a moderate
temperature and the sublimate has a beautiful crystalline structure and
is perfectly white. The crystalline mass exposes so much surface that
water is taken up very rapidly when exposed to the air. This action is
so rapid that the crystals cannot be transferred to a weighing-glass
without introducing an appreciable error. The whole sample was
accordingly transferred to a stoppered glass bottle which was kept under
a bell jar with sticks of caustic potash. Three samples were prepared in
this manner, the first being used in determination one, the second in
determinations two to seven inclusive, and the third in determinations
eight to nineteen inclusive. The samples used in determinations twenty
and twenty-one were prepared in the following manner: About three
grammes of cadmium were placed in a combustion tube in which three
bridges (as in the distillation of pure cadmium) had been made. A
section may be represented thus
[Illustration]
The metal was placed in cavity A and a stream of chlorine passed through
the tube. The chlorine was prepared from potassium bichromate and
hydrochloric acid and dried by passing it through a long tube containing
calcium chloride. When the air had been displaced, the cadmium was
heated. It fused and began to burn to the chloride which partly flowed
over the bridge into cavity B and partly distilled over into this
cavity. When the reaction had ended, the current of chlorine was
replaced by one of dry nitrogen, and the tube was allowed to cool and
the chloride taken for analysis XX. The specimen used in analysis
twenty-one was prepared in exactly the same way, only the chlorine used
was obtained from manganese dioxide, sodium chloride and sulphuric acid,
and was dried with phosphorous pentoxide instead of calcium chloride.
The special treatment of the portions taken for analysis was as follows:
Those taken for determinations I, II and from XI to XIX inclusive were
placed in a platinum boat and put into combustion tube. A current of
hydrochloric acid gas obtained by heating the aqueous acid was passed
through the tube. The gas had been dried by calcium chloride. When the
air was displaced, the chloride was heated somewhat higher than its
fusing-point i.e. to incipient redness, and maintained there for a
length of time varying from a few minutes to more than an hour. The
hydrochloric acid was then displaced by a current of nitrogen, and the
chloride allowed to cool. The boat with the chloride, while still
slightly warm, was placed in a weighing-tube, cooled in a dedicator
containing caustic potash and weighed. The chloride thus prepared is
transparent and presents only a small surface to the air. It takes water
up so slowly that no error is introduced from this source. This was
tested in one case by allowing a boat containing some chloride thus
prepared to stand in the air for a certain length of time and noting the
increase in weight. It was quite slow. In several cases specimens of
chloride were tested for hydrochloric acid using tropaeolin as an
indicator. It was always found neutral. The portions used for
determinations III and VI to X inclusive were prepared in exactly the
same manner as the preceding ones except that the hydrochloric acid gas
in which they were fused was not dried but used just as it came from the
aqueous acid. In some cases the platinum boat in which the chloride was
fused was weighed before and after the fusion. The weight remained
unchanged.
For determinations IV and V, about six grammes of cadmium chloride were
placed in a platinum boat, and more than two-thirds of it distilled out
in a current of hydrochloric acid gas which had not been dried. Part of
the distillate was collected after cooling in nitrogen and used in
determination IV while the residue remaining in the boat was used for
determination V. The method of preparing the chloride used in
determinations XX and XXI has already been described.
The Filters.
Thinking that a Gooch crucible with a platinum sponge on the bottom in
place of asbestus would be desirable for this work one was accordingly
made and answered the purpose very satisfactorily. All determinations
were made by using such filters. C. E. Munroe (Chem. News, Vol 58, p.
101) has described the preparation of these filters. A platinum Gooch
crucible was placed on a filter paper and some ammonium platonic
chloride which had been thoroughly washed introduced by suspending it in
alcohol and then pouring this into it. The precipitate settles to the
bottom forming a uniform layer and the alcohol drains, off through the
filter paper. The crucible was then dried slowly in an air-bath. After
this it was transferred to a porcelain crucible and slowly heated until
decomposition was complete. In this manner a layer of platinum felt is
obtained which acts as a very efficient filter. Another layer of double
chloride was then decomposed as before so that if there were any
imperfections in the first layer they would be covered by the second
layer. The surface was smoothed down by means of a glass rod. To prepare
a good filter the drying and subsequent heating should be very slow. The
heating must not be at too high a temperature, otherwise the felt
becomes very compact and is useless for filtering purposes. Pressure
produces the same effect. The filters were always treated with strong
nitric acid, washed and reheated before being used, but in no case was
chlorine detected in the nitric acid after the washing, nor any loss in
weight of the crucible. An objection to the use of these crucibles for
the purpose named was found in the course of this work, but it will be
discussed later. The crucibles were always set in a large
weighing-glass, and another weighing-glass containing an equal amount of
platinum foil used as a tare, in weighing. This precaution was perhaps
unnecessary, but at least it did no harm.
Analytical Process.
The weighed cadmium chloride was dissolved by placing the boat
containing it in an Erlenmeyer flask containing water. The boat was then
washed, dried and replaced in its weighing-tube. On weighing again, the
loss in weight is equal to the weight of cadmium chloride taken. All
samples gave a perfectly clear solution except those used for
determinations XX and XXI. A drop of nitric acid (1:3) was added to each
solution except in determination XIV where the cubic centimetres were
added, and in XVI where ten cubic centimeters were added. A solution of
silver nitrate was then added to precipitate the chlorine. This as well
as the subsequent washing was done in a dark-room illuminated by a
single gas light whose rays had to pass through a strong solution of
neutral potassium chromate. The precipitate was contracted by warming on
the water-bath. It was then collected in the prepared Gooch crucibles
and washed. Before filtering, the flask containing the precipitate and
mother-liquor was allowed to cool. Silver chloride is soluble in water
to a considerable extent but is reprecipitated by adding an excess of
either silver nitrate or hydrochloric acid. Stas (Ann. de Chem. et Phys.
[4], 25, 22; [5], 3, 145; [5], 3, 289.) investigated this very
thoroughly. Cooke also did some work on it and used a dilute solution of
silver nitrate to wash the chloride thus preventing solution (Proc.
Amer. Acad. 17, 7.). In the above work, therefore, a solution containing
0.10 grammes of silver nitrate per liter was first used, followed by one
only one-tenth as strong, and finally pure water was used. Only two or
three washings could be made with water as the chloride went into
solution after this owing to the removal of the silver nitrate. The last
silver nitrate solution used is so weak that any error introduced by not
washing it out completely is insignificant. After washing, the silver
chloride was dried at temperatures varying from 150°C. to 300°C. to
constant weight. A glass air-bath was used in order to prevent products
from the burning gas from coming in contact with the chloride. It was
then weighed. The quantity of silver nitrate used in the determinations
was varied very much. The excess over what was required to precipitate
the chloride is given in the table of results in those cases in which it
is known. The quantity of water used in each determination is also given
where it is known. It is given in the number of cubic centimetres used
per gramme of cadmium chloride and does not include wash water. All
weighings are reduced to the vacuum standard on the basis that Sp. Grs.
of CdCl_{2} = 3.94 and AgCl = 5.5. The results are:
╒═════╤════════╤═══════╤════════╤═══════════════════╤═══════╤═════════╕
│ No.│CdCl_{2}│ AgCl│ H_{2}O│ Excess AgNO_{3}│At. Wt.│Melted in│
│ │ │ │per Grm.│ │ │ │
├─────┼────────┼───────┼────────┼───────────────────┼───────┼─────────┤
│ I│ 3.09183│4.83856│ │ │112.339│ Dry HCl │
│ II│ 2.26100│3.53854│ │ │112.329│ „ „ │
│ III│ 1.35729│2.12431│ │ │112.320│Moist HCl│
│ IV│ 2.05582│3.21727│ │ │112.339│ „ „ │
│ V│ 1.89774│2.97041│ │ │112.306│ „ „ │
│ VI│ 3.50367│5.48473│ │ 8.90│112.283│ „ „ │
│ VII│ 2.70292│4.23087│ 200│ 1.79│112.301│ „ „ │
│ VIII│ 4.24276│ 6.6398│ 300│ 8.10│112.387│ „ „ │
│ IX│ 3.40200│5.32314│ 300│ 18.95│112.368│ „ „ │
│ X│ 4.60659│7.20386│ 300│ 25.62│112.472│ „ „ │
│ XI│ 2.40832│ │ │ │112.434│ Dry HCl │
│ XII│ 2.19114│3.42724│ │ │112.433│ „ „ │
│ XIII│ 2.84628│4.45477│ 300│4.45 + 3cc. HNO_{3}│112.319│ „ „ │
│ XIV│ 2.56748│4.01651│ 300│ .10│112.399│ „ „ │
│ XV│ 2.31003│3.61370│ 300│.10 + 10cc. HNO_{3}│112.406│ „ „ │
│ XVI│ 1.25008│1.95652│ 300│ 4.66│112.319│ „ „ │
│ XVII│ 1.96015│3.06541│ 300│ 3.22│112.466│ „ „ │
│XVIII│ 2.29787│3.59391│ 300│ 4.27│112.448│ „ „ │
│ XIX│ 1.94227│3.03811│ 300│ 3.61│112.423│ Dry HCl │
│ XX│ 1.10976│1.73547│ │ │112.471│ „ „ │
│ XXI│ 1.63080│2.55016│ │ │112.476│ „ „ │
│ │ │ │ │ │———————│ │
│ │ │ │ │ Average│112.383│ │
└─────┴────────┴───────┴────────┴───────────────────┴───────┴─────────┘
Discussion of the Results.
In the first five determinations, the analytical operations were
conducted as nearly as possible alike, but the preparation of the
portions of cadmium chloride taken for analysis was varied very much as
will be seen by referring back to this part of this paper. The results
do not vary more than ±0.015 from their average. This is very strong
evidence of the purity of the chloride used for, if it contained any
impurity, we should have expected to vary the amount in the different
portions. After this, attention was paid especially to the analytical
process, for it was thought that there probably was some serious error
in the method, the result being higher than any that had previously been
obtained, if we exclude Dumas’ first series which he himself did not
accept. The conditions were varied in many ways to see how much the
result could be influenced, but under no conditions were results as low
as Huntington’s average (112.24) obtained. A number of errors were found
in the method during the work, but they seem to neutralize each other to
a great extent. The more important ones will now be given. Nearly every
filtrate including the corresponding wash water was examined for
chlorine after the silver and cadmium had been precipitated by hydrogen
sulphide. The excess of hydrogen sulphide was expelled by boiling, after
the addition of some nitric acid. In two cases an inverted condenser was
used. On adding silver nitrate a precipitate was always obtained showing
the presence of chlorine. Care was always taken to filter off sulphur
formed by the oxidation of hydrogen sulphide, before adding the silver
nitrate. The precipitate was never very heavy, and was not estimated
quantitatively. It is evident that cadmium nitrate exerts a solvent
action on silver chloride. In some cases a very large excess of silver
nitrate was added but it did not change the results markedly. Silver
nitrate itself dissolved silver chloride to some extent. The increase in
insolubility, if any, on adding an excess of silver nitrate is probably
counterbalanced by the increased error due to occlusion of nitrates in
the silver chloride. Stas (Aronstein’s Trans. p. 156) says it is
impossible to contract silver chloride or bromide in a solution
containing salts without there being occlusion and that the precipitate
can only be freed from them by dividing up the contracted mass by
shaking with pure water. This was not done here owing to the solubility
of silver chloride in pure water, and the complications introduced in
the analytical part. The occlusion of nitrates by the silver chloride
would lower the atomic weight found. The silver chloride obtained always
darkened on heating and contained cadmium, as was shown in the following
manner: The lump of silver chloride was attached to the negative pole of
a cell and electrolyzed in a bath containing dilute sulphuric acid. The
resulting metal was then dissolved in nitric acid and the silver
precipitated by adding hydrochloric acid. The filtrate was evaporated to
expel the nitric acid and the residue taken up with water and tested for
cadmium with hydrogen sulphide. An appreciable quantity was always
found. This method of examination does not show the occluded silver
nitrate. Another error which tends to lower the atomic weight found is
due to the platinum crucibles used for filtering. If a silver nitrate
solution is filtered through such a crucible there will be an increase
in weight due to silver being deposited. This takes place in acidified
solutions as well as in neutral ones. Washing with ammonia does not
remove the deposit, but strong nitric acid does, the washings giving a
test for silver. Whether the depositing of silver is due to the action
of spongy platinum in contact with the compact metal of the crucible or
to some impurity in the platinum sponge was not determined, but the
former seems by far the most probable. The increase in weight during the
time required for filtering a determination must have been quite small
however. The samples of cadmium chloride employed for determinations XX
and XXI were prepared by burning cadmium in a current of chlorine. The
glass tube used was attached somewhat and the solution of the chloride
was very slightly turbid in each case. The turbidity was so slight
however, that no very serious error could have resulted from it,
particularly as it was probably partly counterbalanced by the formation
of some potassium chloride. For more accurate work, it should have been
made and redistilled in a porcelain tube. These two samples were tested
for free chlorine with potassium iodide and starch paste, but none was
found. Some of the specimens of chloride prepared by fusion in a current
of hydrochloric acid were found to be neutral, using tropaeolin as an
indicator.
As nearly as can be judged, the above errors would probably
counterbalance each other to a great extent, and thus give a fairly
close approximation to the atomic weight of cadmium when the average of
all the determinations is taken. The value 112.383 thus obtained can
only be regarded as tentative.
The Bromide method.
Huntington (Proc. Amer. Acad. 11.28) working under the direction of J.
P. Cooke, determined the ratio of cadmium bromide to silver bromide and
using the total quantities for the calculation the result for the atomic
weight of cadmium is 112.239. He also determined the ratio of cadmium
bromide to silver, obtaining 112.245 for the atomic weight of cadmium.
The work which will now be described was carried out very much like the
work described under the chloride method. The ratio of cadmium bromide
to silver bromide was investigated.
Preparation of Cadmium Bromide and Hydrobromic Acid.
A large quantity of hydrobromic acid was prepared according to the
method described by Dr. Edward R. Squibb (Trans. of Med. Soc. of the
State of N. Y.). One part of water was added to seven parts of strong
sulphuric acid (Sp. Gr. = 1.83) and the mixture cooled. Then six parts
of potassium bromide were dissolved in six parts of hot water and the
diluted sulphuric acid added to this hot solution. It was set aside
until cold in order to allow the sulphate of potassium to crystallize
out. The crystals were drained on a filter-plate and quickly washed with
two parts of water. The mother-liquor and washing were then distilled
until no more acid was obtained on further heating. The acid thus
obtained was distilled three times from potassium bromide, twice from
cadmium bromide formed by adding a piece of pure cadmium to it, and
twice without the addition of anything. It was tested and found to be
free from sulphuric acid. Cadmium bromide was prepared from it, in
exactly the same way that the cadmium chloride used in the chloride
method was prepared from pure metal and hydrochloric acid. While the
crystalline mass of cadmium bromide was still moist, it was transferred
to a combination tube and dried at 300°C for several hours in a current
of nitrogen. It was then sublimed in a vacuum as the chloride had been.
This specimen served for the first three determinations. About nine
grammes of it was placed in a platinum boat in a combustion tube, and
part of it distilled in a current of nitrogen. The distillate, a portion
of which had been tested with tropaeolin and found neutral, was used for
determination I. The residue in the boat was used for determination II.
Another portion of the main sample was resublimed in a vacuum and used
in determination no. III. Cadmium bromide is not hygroscopic or at least
only slightly, therefore the sublimed cadmium bromide can be transferred
to a weighing-glass without taking up water. This cannot be done in the
case of the chloride. It is probable that the hydrobromic acid as above
prepared was perfectly free from hydrochloric acid. Chlorine in cadmium
bromide would cause the atomic weight to be found lower than it really
is. It was thought desirable, however, to prepare an acid which would
certainly be free from chlorine. The method described by Stas
(Aronstein’s German translation, p. 154.) was employed with the
additional precaution that the above purified acid was used to start
with and all reagents employed had been especially prepared so as to be
free from chlorine. Pure silver was prepared according to Stas’
description (see Aronstein’s translation, page 34, also page 104) by the
action of ammonium sulphite on an ammoniacal solution of silver nitrate
and copper sulphate. The silver was dissolved in nitric acid free from
chlorine, and then slowly added to a dilute solution of the
above-described hydrobromic acid, and the precipitated silver bromide
thoroughly washed. It was then digested for a long while in a strong
solution of potassium bromide, first in the cold, then by heating. The
potassium bromide had been made thus: Twice recrystallized potassium
hydrogen tartrate was heated in a platinum dish in a muffle furnace
until it was converted into carbonate, and the excess of carbon burned
off. It was then dissolved in water, filtered and neutralized with some
of the hydrobromic acid already described. The carbonate had been tested
for both sulphuric acid and chlorine with negative results. After the
silver bromide had been digested with the potassium bromide, it was
washed very thoroughly, suspended in water, and a current of hydrogen
sulphide passed into it. This converts it into sulphide hydrobromic acid
being liberated. The acid was drained off on a porcelain plate, and then
distilled a number of times. It was finally tested and found to be
perfectly free from sulphates and also did not contain free bromine.
Having started with an acid which was probably pure and subjected it to
these operations with reagents free from chlorine, there can be no doubt
as to the purity of the resulting acid. The hydrogen sulphide used was
prepared from potassium hydrosulphide as in the sulphide method, and
washed first with a solution of the hydrosulphide, then very thoroughly
with pure water. From the hydrobromic acid obtained, a specimen of
cadmium bromide was prepared as before and sublimed twice in a vacuum.
This specimen was used for determinations IV and V.
Method of Analysis.
The first three determinations were made exactly like those in the
chloride method. The last two were also made in the same manner, only
the washing of the precipitate was varied. After the silver bromide had
been contracted by warming on a water-bath it was washed by decantation
and then agitated violently with cold water to remove occluded nitrates,
but it was then so finely divided that it could not be filtered. The
artifice used by Stas to contract it a second time was to pass a current
of steam into the milky liquid. This was tried here, but for some reason
or other did not work very well, and considerable difficulty was had in
filtering it. The results of the five determinations are tabulated
below. All weighings are reduced to the vacuum standard on the basis of
Sp. Gr. of CdBr_{2} = 4.6 and Sp. Gr. AgBr = 6.62.
────────┬────────┬────────┬────────┬────────────┬────────┬────────────
No.│CdBr_{2}│ AgBr│ H_{2}O │Ex. AgNO_{3}│ At. Wt.│Remarks
────────┼────────┼────────┼────────┼────────────┼────────┼────────────
I│ 4.39941│ 6.07204│ │ │ 112.43│Distillate }
II│ 3.18030│ 4.38831│ │ │ 112.42│Residue }
III│ 3.60336│ 4.97150│ │ │ 112.45│Resublimed.
IV│ 4.04240│ 5.58062│ │ │ 112.29│
V│ 3.60505│ 4.97519│ │ │ 112.38│
────────┼────────┼────────┼────────┼────────────┼────────┼────────────
│ │ │ │ Average │ 112.394│
Discussion of the Results.
The first three specimens were prepared under widely different
conditions yet the results agree quite closely. The last two were
prepared from the repurified hydrobromic acid. If chlorine had been
removed during the second purification we should expect a higher result
but the results are lower. There seems to be hardly any doubt that this
is due to analytical errors rather than a change in the composition of
the bromide. Whether this be true or not, the five determinations all
fall within the limits obtained by the chloride method and confirms it
as fully as can be expected.
The errors of the method are the same as those of the bromide method,
only they are probably less in most cases. One filtrate was examined for
bromium, but none was found showing the method to be more perfect in
this respect.
Syntheses of Cadmium Sulphate.
It was next thought of examining the method based on the conversion of
cadmium sulphate into cadmium sulphide, which has been used by von Hauer
whose result is 111.94 for the atomic weight of cadmium, and more
recently by Partridge who obtained a much lower result, namely 111.73.
They dried cadmium sulphate in porcelain boats, and then reduced it to
sulphide by heating in a current of hydrogen sulphide. The reduction
begins in the cold and is probably complete or at least nearly complete
before the temperature is sufficiently high for cadmium sulphate to
decompose into cadmium oxide, for the sulphate is very stable with
respect to heat. This being the case, probably no error results from the
formation of a silicate of cadmium in this method. The main difficulty
in this method would be to prove that the cadmium sulphate used is free
from water. Neither von Hauer nor Partridge has done this because drying
a substance to a constant weight is not sufficient evidence of its
anhydrous character, especially if the drying is done at a constant
temperature. This has been shown very clearly in the case of copper
sulphate, by Richards (Proc. Amer. Acad. Sci. 26. 263.)
It was therefore decided to attempt the synthesis of cadmium sulphate,
hoping to be able to fix a minimum value for the atomic weight of
cadmium.
A piece of hard glass tube was closed at one end by fusion and the other
end drawn out into a small tube which was then bent twice at right
angles. The large part was cut off near the beginning of the smaller
tube, and the edges rounded by fusion. It was filled with dilute
sulfuric acid and heated for some time to remove soluble matter from the
glass. After removing this acid, a weighed piece of cadmium was
introduced and an excess of dilute sulphuric acid (1: 3) added. The tube
contained a small piece of platinum to aid the solution of the cadmium.
During the process of solution, the two parts of the glass tube were
held together by a rubber band, and the outlet of the smaller tube
dipped under pure water contained in a small tube closed at one end.
[Illustration: Fig. 2.]
A section of the arrangement is shown in figure 2. Solution was aided by
the application of heat. These precautions in dissolving the metal were
taken to prevent loss by spraying. After the metal had been dissolved,
the solution and the water through which the hydrogen had escaped were
transferred to a porcelain crucible. An equal amount of sulphuric acid
was then added to the tare and both were heated until fumes of sulphuric
acid ceased to come off. The crucible containing the dry sulphate was
next placed an a porcelain plate in a nickel crucible set in a hole in
an asbestos board. This was placed over the flame of a Bunsen burner, so
that the bottom of the nickel crucible was barely at a red heat. The
temperature on the inside of this bath was considerably lower. After the
weight had become nearly constant, the sulphate was tested for sulphuric
acid by means of standard alkali using tropaeolin as an indicator. It
was found acid, but so slightly that no attempt was made to estimate it.
Result = 112.35 is preliminary.
Another synthesis was made as follows: A platinum crucible, lid and
perforated cone were place in a large weighing-glass and tared with a
similar weighing-glass containing a platinum crucible, platinum foil
being added until the weights were equal. After these had been
accurately weighed, a weighed piece of cadmium was added to the one
containing the cone. The cone was inverted over the piece of metal on
the bottom of the platinum crucible. A considerable excess of dilute (1:
3) sulphuric acid was then added, the lid whose edge was bent down
placed on the crucible, and the weighing-glass stoppered loosely. This
was placed in an air-bath, and gently warmed during the later part of
the process of solution. There is no difficulty in getting complete
solution if a sufficient excess of acid is used. A vertical section of
the crucible and weighing-glass is shown in figure 3.
[Illustration: 3]
This arrangement avoids loss from spraying, and the necessity of
transferring the solution from a tube to a crucible as in the first
experiment.
An equal quantity of sulphuric acid was added to the crucible used as a
tare and evaporated. After the metal had been dissolved, the platinum
cone was lifted to one side and the excess of acid evaporated off. It
was then heated in a glass air-bath for a long time at a temperature
which was probably about 400°C. After the weight had become constant,
the amount of free sulphuric acid was estimated by titration with a
standard alkali using tropaeolin as an indicator. 1.25 milligrammes were
found and this weight was subtracted from that found at the balance.
Weighing were reduced to the vacuum standard, assuming the Sp. Grs. of
cadmium and anhydrous cadmium sulphate[1] to be 8.54 and 3.0
respectively. The results were as follows:
Footnote 1:
Could not find any record of its Sp. Gr., 3.0 is assumed.
Cd CdSO_{4} At. Wt.
I. (112.35 can only be regarded as a preliminary experiment)
II. 1.15781 2.14776 112.35
Discussion of the Results.
These results agree fairly well with those obtained by the chloride and
bromide methods. The second experiment is more trustworthy than the
first. In it, we started with pure metal and the manipulations were so
simple that no serious error could have been made in them. Hence it will
only be necessary to consider the end-product, i.e., the cadmium
sulphate. The titration showed that the sulphate was not basic owing to
loss of sulphur trioxide, and after deducting the weight of the excess
of sulphuric acid we must have left a weight of cadmium sulphate which
is equivalent to the metal employed. The question now is, did it contain
anything else and what would be its effect? Clearly the effect of water
or any other impurity would be to lower the atomic weight found, hence
the atomic weight must be at least as high as the experiment indicates.
As the cadmium sulphate is deposited, at least the later part of it is
from a strong sulphuric acid solution, it probably does not contain any
water and in this case would fix a maximum value as well as the minimum
value, and thus determine the atomic weight. It might be objected to the
second experiment that the sulphuric acid found may have been present as
SO_{3} and not as H_{2}SO_{4} as was assumed. This seems highly
improbable, and even if it were so the error introduced would be only
about .03 of a unit in the atomic weight. As the first determination was
found practically neutral, it does not apply to it at all. The most
probable conclusion from these experiments is that the atomic weight of
cadmium is about 112.35. A more thorough study of this method would have
been made if time had permitted it.
The Oxide Method.
As the chloride and bromide methods and the synthesis of cadmium
sulphate all lead to approximately the same high result, it seemed
probable that the oxide method which had given a much lower result
(Morse & Jones 112.07) must be affected by some error. Accordingly it
was examined in the manner about to be described. A set of crucibles was
prepared as described by Morse and Jones in their work on this method,
and in the present paper under the oxalate method. After they had been
heated in a nickel crucible over a blast lamp and weighed, a weighed
piece of cadmium was introduced into the smaller inside crucible, and
dissolved in nitric acid with the aid of heat. An equal quantity of
nitric acid was added to the tare. The acid was then evaporated off, and
the resulting nitrate converted into oxide exactly as has already been
described under the oxalate. The first experiment was made in this way
and the second one exactly like it, only the porcelain crucible used was
the one which had been employed in the first determination. The glaze
had been removed by the cadmium oxide of the first determination, and
before using for the second one the crucible was boiled out with nitric
acid, and heated to constant weight over a blast lamp as before.
Determinations III, IV and V were made in the same way except that the
small inner crucible was platinum instead of porcelain. All weighings
were reduced to the vacuum standard on the basis of 8.54 for the Sp. Gr.
of cadmium and 8.15 for the Sp. Gr of cadmium oxide and 8.4 for the
brass and 21 for the platinum weights.
The results are as follows:
Cd CdO At. Wt. Cd.
I. 1.26142 1.44144 112.11
II. .99785 1.14035 112.04
——————
Average 112.08
III. 1.11321 1.27247 111.84
IV. 1.02412 1.17054 111.91
V. 2.80966 3.21152 111.87
——————
Average 111.87
The oxides resulting from these determinations were always tested for
oxides of nitrogen, sometimes by using meta phenylene diamine and at
other times by sulphanilic acid and naphthylamine sulphate, but no
traces were ever found. The average of the determinations made in
porcelain crucibles is 112.08. Morse and Jones obtained the same figure
or, if their results are reduced to the vacuum standard, 112.06, by the
same method under the same conditions. The results of the determinations
made in platinum crucibles are equally constant, but their average is
111.88 being .20 of a unit lower. Therefore, more oxide is obtained when
platinum crucibles are used instead of porcelain ones. In two cases the
platinum crucibles were weighed at the end of the determinations after
the cadmium oxide had been removed. Their weight remained unchanged. The
most probable explanation of these facts seems to be that something is
retained in the oxide in both cases, but that the amount is greater in
the determination made in platinum crucibles than in those in which
porcelain ones were employed. We should expect this, because in
porcelain crucibles some of the oxide is absorbed forming a silicate,
and any volatile impurity must be expelled from this part of the oxide.
Not finding oxides of nitrogen, it was thought that gases probably
nitrogen and oxygen might be occluded although Richards and Rogers
(Amer. Chem. Jour. 15, 567.) had examined cadmium oxide prepared from
the nitrate and found only a trace of gas. Accordingly two specimens of
cadmium oxide obtained in the above determinations were powdered in an
agate mortar and boiled with water for some time in order to remove any
adhering air. They were then dissolved in dilute hydrochloric acid from
which the air had been removed by boiling. A small amount of gas was
found in each case but not nearly enough to account for the difference
of .31 unit in the atomic weight of cadmium between 112.38 and the oxide
method. In fact not more than about one sixth of the amount required was
found. It may be that the powdering of the oxide and then boiling up in
water may have been to severe a treatment, and that the greater part of
the occluded gas escaped during these processes. It seems that there is
at least some error due to occluded gases in methods involving the
decomposition of cadmium nitrate to oxide, but no satisfactory idea of
its magnitude could be obtained from these two experiments as carried
out.
The following experiments were then made and they seem to give definite
evidence not only of the existence of an error but also of its
magnitude. Carbonate of cadmium was made by dissolving pure cadmium in
nitric acid, adding an excess of ammonia and a small quantity of
ammonium carbonate. After standing for some time the cadmium carbonate
was filtered off and rejected. The filtrate was treated with an excess
of ammonium carbonate and the precipitated cadmium carbonate allowed to
digest in it for some time. After washing by decantation several times
the carbonate was transferred to a funnel containing a porcelain
filter-plate, covered with a piece of ashless filter paper of slightly
larger diameter, and washed thoroughly. with water. It was then
transferred to a platinum dish, care being taken to avoid contamination
with filter paper and heated gently to convert it into oxide. The
resulting oxide was powdered in an agate mortar, returned to the
platinum dish and heated to incipient whiteness for seven hours in a
muffle furnace. The temperature must not be too high, otherwise the
oxide will distill readily leaving no residue. The oxide is slightly
volatile at good red heat as was observed in trying to make a
determinant at this temperature by the oxide method. A weighed portion
of the oxide which had been prepared from the carbonate in the manner
described was dissolved in a weighed porcelain crucible and the
resulting nitrate converted into the oxide again by heat just as in the
oxide method. This constitutes experiment I. Experiments two and three
were made in exactly the same way except that a platinum crucible was
used instead of a porcelain one. The results are:
Initial Wt. Final Wt. Gain Corresponding Error in At. Wt.
I. 2.95469 2.95650 .00081 −.24
II. 2.67717 2.67835 .00117 −.39
III. 3.00295 3.00422 .00127 −.38
As we started with cadmium oxide, and, after passing to the nitrate,
converted it back into the oxide, the weight should remain unchanged if
the method is correct. However, this is not the case, but a large
increase in weight takes place. The increase is larger in a platinum
crucible than in a porcelain one, which accounts for the fact that a
lower value for the atomic weight is found by the oxide method when they
are used. The use of a porcelain crucible therefore diminishes the
error, but does not eliminate it. The explanation of this has already
been given. The oxides obtained in these three experiments were tested
for occluded gases in the manner already described, but only small
amounts were found. Both of those made in platinum crucibles were tested
for nitrate of cadmium with brucine and sulphuric acid with negative
results. To show that the impurity was not converted into an ammonium
salt when the oxide was dissolved in hydrochloric acid, a slight excess
of caustic potash was added to the solution, the precipitate allowed to
subside and the clean, supernatant liquid tested for ammonia with
Nessler’s reagent. No ammonia was found. In order to make these
experiments as severe a test as possible, a somewhat higher temperature
was employed than had in the five experiments described under the oxide
method. This was accomplished by boring out the stopcocks of the blast
lamp so that a larger supply of gas was furnished. The two oxides in the
platinum crucibles seemed to be constant in weight, but that in the
porcelain crucible seemed to lose in weight slowly. The weight given was
taken after four hours blasting, which is longer and at a higher
temperature than was used in any of the five determinations made by the
oxide method. If the cadmium oxide prepared from the carbonate retained
any carbon dioxide, it would lose weight in being dissolved and
reconverted into oxide. The above experiments therefore seem to furnish
very strong evidence that there is an error of at least −.24 unit in the
oxide method when porcelain crucibles are used and −.39 of a unit when
platinum ones are employed. if .24 of a unit is added to 112.07 the
result obtained when porcelain crucibles are used we get 112.31 and
adding .39 to 111.87 gives 112.26. Considering the small number of
experiments made, the fact that they were made in such a way as to give
a low value (numerically) for the error rather than a high one, and also
that the error is probably variable to some extent, especially when
porcelain crucibles are used, the corrected results agree as closely
with 112.38, the average of the chloride, bromide and sulphate
(synthetical) methods as could be expected. It must also be borne in
mind that 112.38 is only to be regarded as an approximation to the
atomic weight of cadmium. The increase in weight observed in converting
the nitrate back into oxide might also be explained by assuming that the
cadmium oxide used in the beginning of the experiments was richer in
metal than the formula CdO indicated and that the increase in weight is
due to this excess of metal being changed to oxide. The method of
preparation of the oxide from the carbonate and the known properties of
cadmium oxide render this view highly improbable, and the following two
observations render it untenable:
1st. If this were the cause of the increase, the amount of increase
would necessarily be the same in both platinum and porcelain crucibles,
which is not the case.
[Illustration]
2nd. Three grammes of cadmium oxide made from the carbonate were
dissolved in dilute hydrochloric acid from which the air had been
expelled by boiling. The oxide, which is very compact, was placed in a
glass bulb which had been blown at the end of a tube. After displacing
the air by filling the entire apparatus with recently boiled water, the
exit of the tube was placed under boiling dilute hydrochloric acid, and
the bulb heated until the water boiled. It was then turned over so that
the steam displaced nearly all the water. On removing the flame the
dilute hydrochloric acid at once filled the bulb. The exit tube was then
quickly placed under a narrow tube filled with mercury and inverted over
mercury in a dish. The bulb was then heated until the oxide had
dissolved. By this method the gas would be boiled out of the solution
and collected in the top of the narrow tube. As only a very small amount
of steam and dilute hydrochloric acid go over at the same time, there is
no danger of the gas formed being absorbed to any considerable extent.
It is well to put the oxide into the bulb before the tube is bend. If
the hydrochloric acid is too strong, it must be cooled before entering
the bulb as otherwise the reaction is too violent, and the experiment
may be lost. This experiment shows that there is no excess of cadmium
present in the oxide employed for no gas was found. If three grammes of
the oxide contained enough metal to take up .00126 grms. of oxygen,
.00016 grms of hydrogen should have been set free, and its volume under
ordinary conditions of temperature and pressure would have been about
1.9 cubic centimetres. This experiment would also have shown the
presence of carbon dioxide if any had been present.
Discussion of the Oxalate Method.
After having done the work which has just been described, we are in a
position to turn to the oxalate method, which is the first method
described in this paper. It involves the decomposition of cadmium
nitrate, and is therefore affected by an error from this source, only it
is not as large as in case of the oxide method. If 2.95650 grammes of
cadmium oxide prepared in a porcelain crucible contain .00081 grammes of
impurity, an error of −.24 of a unit would be introduced in the atomic
weight as determined by the oxide method or +.10 in case the oxalate
method were employed. That is the oxalate should give about 112.48 for
the atomic weight of cadmium, but it really gives a very much lower
result. Morse and Jones obtained 112.04 ± .035 by it, while Partridge
obtained 111.81 ± .035 by it. If we take 112.38 for the atomic weight of
cadmium, there appears to be a second error of .44 of a unit in the
method as used by Morse and Jones, while Partridge’s result indicates an
error of .57 of a unit. Partridge only moistened the oxide obtained from
the oxalate with a few drops of nitric acid before making the final
heating, and it seems probable therefore that he made no appreciable
error on account of the final oxide retaining products of decomposition
from cadmium nitrate. The most probable cause of this large error seems
probably to be incomplete dehydration of the oxalate, or reduction to
metal during the decomposition of the oxalate, and subsequent
volatilization of some of it, or a combination of both of these. The
nine determinations given in the earlier part of this paper of course
vary so much that they are of no value whatever in determining the
atomic weight. The reason that the first four are low is probably due in
part to sublimation of cadmium, for on dissolving the resulting oxide in
nitric acid a considerable quantity of metal was noticed in each case.
In the others, the temperature was kept lower, and the decomposition
took a longer time. No metal was observed on taking up in nitric acid.
To be certain of what the cause of error is would require some very
carefully conducted experiments, but as there are a number of much more
reliable methods for determining the atomic weight of cadmium, it does
not seem desirable to spend the time required in making them. It should
be mentioned that Lenssen, in 1860, first employed this method. He made
three determinations. 1.5697 grms of cadmium oxalate giving 1.0047
grammes of oxide, which gives a value of 112.043 for the atomic weight
of cadmium . The difference between the highest and lowest determination
was .391 of a unit.
Other Methods
A great deal of time was spent in trying to effect a partial synthesis
of cadmium bromide in exactly the same manner as had been used in case
of cadmium sulphate. No results were obtained because cadmium bromide is
slowly volatile at 150°C, the temperature used, and retained some
hydrobromic acid ever after more than 100 hours of drying. Some work was
done in trying to establish the ratio between silver and Cadmium by
dropping a weighed piece of cadmium into a solution of silver sulphate,
the reaction being:
Cd + Ag_{2}SO_{4} = CdSO_{4} + 2Ag
Silver nitrate cannot be used because it becomes reduced to nitrate even
at a temperature of 0°C., as was shown by its reducing action on
potassium permanganate, and by the reaction with meta-diamido benzene
after the reaction had been completed. The main difficulty with the
method is that air must be excluded in order to prevent oxidation and
solution of some of the precipitated silver. The silver is perfectly
free from cadmium if an excess of silver sulphate is used and the
precipitated metal digested with it for some time. Since this part of
the work was done, a paper by Mylius and Fromm (Ber. 1894, 630) appeared
in which one of the reactions studied was that of cadmium on silver
sulphate. They also found the resulting silver free from cadmium. The
method seems very promising, but the work had to be discontinued for
lack of time.
Conclusion.
I. The work on the oxalate and sulphide methods described in this paper
is of no value for determining the atomic weight of cadmium. It does not
even enable us to fix an approximate value.
II. There are a number of errors in the chloride and bromide methods as
they were used in this work, but they are not very large and partially
compensate each other. Their results, 112.383 and 112.396 respectively,
may be regarded as approximations to the true value.
III. The synthesis of cadmium sulphate as carried out is of especial
value in fixing a _minimum_ value for the atomic weight of cadmium. The
result is 112.35, agreeing closely with that obtained by the bromide and
chloride methods.
IV. There is an error in the oxide method due to products of
decomposition of the nitrate being retained. Direct experiments gave .39
of a unit for this when platinum crucibles were used and .24 of a unit
when porcelain ones were used. The calculated errors for porcelain and
platinum crucibles are .30 and .51 of a unit respectively, if 112.38 is
assumed as the atomic weight of Cadmium.
V. The average of the chloride, bromide, and sulphate methods is 112.38.
This result is to be regarded as _tentative_ and not as final since the
main object of this work has been to find the cause of the discrepancy
in some methods employed in determining this constant, rather than to
make an atomic weight determination.
Biographical Sketch.
John Emery Bucher was born near Hanover, Pa., August 17, 1872. He
entered Lehigh University in 1888 and graduated in 1891. During the past
three years he has been a graduate student in the Johns Hopkins
University.
Subjects: Chemistry, Mineralogy and Mathematics.
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TRANSCRIBER’S NOTES
1. Silently corrected obvious typographical errors and variations in
spelling.
2. Retained archaic, non-standard, and uncertain spellings as printed.
3. Enclosed underlined font in _underscores_.
4. Denoted subscripts by an underscore before a series of subscripted
characters enclosed in curly braces, e.g. H_{2}O.
*** End of this Doctrine Publishing Corporation Digital Book "An examination of some methods employed in determining the atomic weight of Cadmium" ***