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.