Initial Wt.Final Wt.GainCorresponding Error in At. Wt.
I.2.954692.95650.00081−.24
II.2.677172.67835.00117−.39
III.3.002953.00422.00127−.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.

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.

Other Methods

A great deal of time was spent in trying to effect a partial synthesis of cadmium bromide in exactly the same manner as had been used in case of cadmium sulphate. No results were obtained because cadmium bromide is slowly volatile at 150°C, the temperature used, and retained some hydrobromic acid ever after more than 100 hours of drying. Some work was done in trying to establish the ratio between silver and Cadmium by dropping a weighed piece of cadmium into a solution of silver sulphate, the reaction being:

Cd + Ag2SO4 = CdSO4 + 2Ag