THE ANALYSIS OF CADMIUM BROMIDE. 2$ 



tating flask. In three analyses (4, 5 and 8), this procedure was varied by 

 adding the bromide to the silver nitrate. After being shaken for some time, the 

 solution was allowed to stand several days, with occasional shaking, until the 

 supernatant Hquid was clear. 30 c.c. portions of the solution were then tested 

 with hundredth normal solutions of silver nitrate and sodium bromide in the 

 nephelometer ^ for excess of bromide or silver, and, if necessary, either standard 

 silver nitrate or sodium bromide solution was added, and the process of shaking 

 and testing repeated, until the amounts of bromide and silver in the solution 

 were equivalent. If the solution was perfectly clear when tested, and contained 

 no considerable excess of bromide or silver, the test solutions were discarded, 

 since they contained only negligible amounts of dissolved silver bromide; other- 

 wise they were returned to the flask and a correction was appHed for the silver 

 bromide thus introduced. 



As soon as the exact end-point of the titration had been found, about 4 eg. 

 of silver nitrate in excess were added, to precipitate dissolved silver bromide, 

 and the flask was again shaken and allowed to stand until clear. The precipi- 

 tate of silver bromide was collected upon a weighed Gooch crucible, after it had 

 been washed by decantation about eight times with pure water. Then it was 

 heated in an electric air-bath, first for several hours at 140°, finally for an hour 

 at 200°, and, after it had cooled in a desiccator, it was weighed. In order to 

 determine how much moisture was retained by the precipitate, in each case it 

 was transferred as completely as possible to a clean porcelain crucible and 

 weighed; then the salt was fused by heating the small covered crucible, con- 

 tained in a large crucible, and again weighed. After fusion the silver bromide 

 was light yellow, with only a trace of darkening, showing that no appreciable 

 reduction had taken place. The asbestos mechanically detached from the 

 Gooch crucible, together with a minute quantity of silver bromide which oc- 

 casionally escaped the crucible, was collected from the filtrate and wash-waters 

 upon a small filter, the ash of which was treated with nitric and hydrobromic 

 acids before weighing. Although the filtrates and first wash- waters were essen- 

 tially free from dissolved silver bromide, the subsequent wash-waters usually 

 contained a trace of this substance. The amount of dissolved salt was deter- 

 mined with the nephelometer by comparison with standard bromide solutions. 

 Finally, the weight of silver bromide was corrected for the sodium bromide 

 introduced. 



Although in our analyses of cadmium chloride no evidence could be obtained 

 of appreciable occlusion of either cadmium or silver salts by silver chloride, 

 especial precautions were taken to avoid any possibiUty of such a difficulty in 

 this research. In the first place both the cadmium bromide and the silver 

 nitrate solutions were very dilute during precipitation, each one having a vol- 

 ume of about I Hter. In the second place the method of precipitation was 



* See page 11. 



