THE ANALYSIS OF CADMIUM CHLORIDE. II 



different specimens of silver chloride from analyses were separately dissolved in 

 ammonia and reprecipitated with hydrochloric acid, and the filtrates, after evap- 

 oration, were tested for cadmium. Negative results were obtained in both 

 cases, showing both that silver chloride does not occlude cadmium salts to 

 an appreciable extent and that the washing of the silver chloride had been 

 thorough. 



The determination of the silver chloride dissolved in the wash-waters was 

 the most diflScult step in the analysis. At first the last few washings, those which 

 had been carried out with pure water and which were the only ones which could 

 have contained dissolved silver chloride, were evaporated to small bulk and an 

 excess of silver nitrate was added. The precipitate of silver chloride, together 

 with any asbestos which had been displaced from the Gooch crucible, was 

 collected upon a small filter which was ignited and weighed. Owing to the com- 

 bined effect of organic matter and light upon these solutions the precipitate was 

 always too heavy. Hence this method was finally discarded. Four preliminary 

 results obtained in this way varied between 112.34 and 11 2.40 for the atomic 

 weight of cadmium. 



In order to avoid this error, in the next series the silver chloride dissolved in 

 the wash-waters was determined by precipitating the chloride in 25 c.c. 

 portions of the solution with an excess of silver nitrate, and comparing in a 

 nephelometer the precipitate produced with that from solutions prepared 

 from standard hydrochloric-acid solutions. At least two comparisons were 

 made in each analysis. 



The nephelometer employed for the estimation of slight opalescences has al- 

 ready been described in detail by Richards and Wells.^ All the precautions ne- 

 cessary for the accurate use of this instrument were carefully observed. The 

 two tubes to be compared were always of the same size. The source of light in 

 the nephelometer was so adjusted that tubes containing exactly equal amounts 

 of precipitate gave identical readings. It was found advantageous to insert a 

 plate of ground glass between the source of light and the test-tubes. In making 

 up the test solutions, great pains were taken that the concentration of electro- 

 lytes in the two solutions and the conditions of precipitation should be as 

 nearly as possible the same. Final readings were taken only after the ratio 

 between the two tubes had become constant. 



The weight of pure silver required exactly to combine with the chlorine in 

 cadmium chloride also was determined. From the weight of cadmium chloride 

 very nearly the necessary quantity of pure silver was calculated. This silver 

 was weighed out and dissolved, in a flask provided with a column of bulbs to 

 prevent loss of silver by spattering (see fig. 2) in distilled nitric acid diluted with 

 an equal volume of water. Ordinarily the silver was caused to dissolve so slowly 

 that practically no gas was evolved. Careful experiments have shown, how- 



^ Atner. Chem. Jour., 31, 235 (1904); 35, 510 (1906). 



