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.