THE ANALYSIS OF SILVER CHROMATE. 151 



DISCUSSION OF RESULTS. 



In comparing the analytical results, it is to be noted first that the compositions 

 of the different samples agree within less than 0.0 1 per cent, as the following 

 averages show. 



zAgBr : AgjCrO^ aAgCl : Ag2Cr04 



Sample! 1. 13 205 Sample II 0.86409 



Sample II 1. 13 204 Sample IV 0.8641 1 



Sample III 1. 13208 

 Sample IV 1.13214 



If anything. Samples I and II show a somewhat lower percentage of silver 

 than Samples III and IV. These samples were made from ammonium chromate 

 which contained a slight excess of chromic acid. This excess of acid accumulated 

 in the solution during the precipitation of the silver chromate, so that the pre- 

 cipitate formed under distinctly acid conditions, although the acidity was not 

 sufiicient to present any danger of the formation of dichromate. Samples III 

 and IV, on the other hand, since they were made from potassium chromate, 

 which is markedly hydrolyzed, were formed imder distinctly basic conditions, 

 and the precipitation or occlusion of basic salts is to be feared. Such occluded 

 basic salts would tend to raise the percentage of silver in the chromate. How- 

 ever, Sample IV yielded slightly higher results than Sample III, while on account 

 of the method of precipitation the reverse is to be expected; for Sample III was 

 precipitated by adding the silver nitrate to the chromate, while Sample IV was 

 precipitated by adding the chromate to the silver solution, the mother-liquor 

 remaining neutral in both cases. Too much emphasis should not be laid upon 

 the slight apparent difference in the composition of the different samples of 

 salt, since the variations in the experiments with the same sample are as large 

 as the differences between the samples. Hence the average result from the dif- 

 ferent samples is employed in the final calculations, all the analyses being 

 given equal weight in each series. 



In addition to the specimens of silver chromate, the preparation and analysis 

 of which have been described, two other interesting samples were prepared. 

 One was formed by adding a 0.04 normal silver nitrate solution to a solution 

 of chromic acid of similar concentration. On account of the solubility of silver 

 chromate in nitric acid solutions, precipitation was only partial. The precipi- 

 tate was washed and dried, and upon analysis was found to contain so little silver 

 that the presence of a small proportion of dichromate was certain, a result which 

 is hardly to be expected in the light of Sherrill's experiments. 



The second sample was prepared by heating ammoniacal solutions of silver 

 chromate in platinum vessels, the chromate being gradually precipitated as the 

 ammonia was expelled. This material yielded somewhat irregular results, 

 which on the whole indicated too high percentages of silver, and hence the 

 presence of basic salts, a result which could have been predicted from a consid- 

 eration of the conditions of preparation. 



