| 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.