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.