THE ANALYSIS OF SILVER CHROMATE. 141 



PURIFICATION OF MATERIALS. 



The laboratory distilled water was twice redistilled, once from alkaline per- 

 manganate and once from very dilute sulphuric acid. In both distillations 

 block tin condensers were employed, no cork or rubber connections being nec- 

 essary. 



The preparation of pure neutral silver nitrate for the precipitation of silver 

 chromate followed the lines laid down in previous researches in this laboratory. 

 A large quantity of heterogeneous silver residues was reduced to metallic silver 

 by means of sticks of pure zinc in slightly acid solution. After the silver had 

 been washed with water until free from halogens, it was dissolved in nitric acid, 

 and the solution was filtered. Silver chloride was precipitated from the diluted 

 nitrate by means of hydrochloric acid, and the precipitate of silver chloride 

 was thoroughly washed. From this silver chloride, metallic silver was again 

 obtained by reduction with cane sugar in strongly alkaline solution. After 

 being washed until free from chloride, the metal was again dissolved in nitric 

 acid in a Jena glass flask. By reduction with ammonic formate (prepared from 

 redistilled formic acid and redistilled ammonia), the silver was once more ob- 

 tained in the metallic state. The crystals were then dissolved in the purest 

 nitric acid, and the nitrate, after concentration of the solution, was four times 

 recrystallized from the purest water in platinum until free from acid. In this 

 crystallization, and in all others, centrifugal drainage in a machine employing 

 platinum fimnels as baskets ^ was always used, in order to free the crystals 

 entirely from any adhering mother-liquor, the mother-liquors all being rejected. 



Hydrochloric acid was prepared by distilling the commercial chemically 

 pure acid, after dilution with an equal volume of water. 



The methods for obtaining pure bromine have been recently tested by one of 

 us,^ and the processes fovmd suitable for the purpose were employed here. A 

 considerable quantity of hydrobromic acid was prepared by passing a current 

 of pure hydrogen sulphide through a layer of bromine covered with water. The 

 hydrogen sulphide was generated by the action of dilute sulphuric acid on 

 ferrous sulphide, and was thoroughly scrubbed in gas-washing bottles and 

 towers containing water. After the precipitated mixture of sulphur bromide 

 and sulphur had been removed by decantation and filtration, the acid was 

 boiled, with the occasional addition of small portions of recrystallized potas- 

 sium permanganate. This was done to eliminate any iodine which might have 

 been present. 



The hydrobromic acid was then heated with the calculated quantity of re- 

 crystallized potassium permanganate, the bromine being condensed in a Jena 

 flask cooled with running water. In this way three-eighths of the bromine re- 

 mained behind as potassium and manganous bromides, the remaining five- 



* Richards: Jour. Atner. Chem. Soc, 27, no (1905). 



* Baxter: Proc. Amer. Acad., 42, 204 (1906); Jour. Atner. Chem. Soc, 28, 1327; Zeit. anorg. 

 Chem., so, 394. (See page 54.) 



