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.