241 



Kecovery of Silver from Silver Ciilortde Residues. 



By Frank C. Mathers. 



The silver from any silver residue or solution can be easily precip- 

 itated as the chloride. Some silver electro-plating experiments in this 

 laboratory gave silver chloride residues which were treated in various 

 ways for the recovery of the metallic silver. One of the schemes was so 

 satisfactoiy that it is described in this paper. 



Metallic zinc and hydrochloric or sulphuric acid will reduce silver 

 chloride to metallic silver. The objection to this method is that it intro- 

 duces any impurity which is In the zinc into the metallic silver. Also the 

 finely divided precipitate of silver is very difficult to filter and to wash 

 free from the zinc salts. 



If silver chloride is boiled in sodium hydroxide solution witli glucose 

 or other reducing sugar, it is reduced to metallic silver. The very serious 

 objection to this method is that the finely divided silver is exceedingly 

 dilRcult to filter and wash free from the sodum chloride. 



The method which has given the best results in this work is an 

 electrolytic reduction scheme. The silver chloride was filtered and was 

 washed free from soluble salts. The silver chloride, after drying, was 

 transferred from the filter paper to a porcelain crucible and fused with a 

 Bunsen burner. One end of a platinvim wire was dipped into the fused 

 mass just as it begun to solidify. This crucible, containing the silver 

 chloride, was suspended by the platinum wire into a dilute sulphuric acid 

 solution. This platinum wire was connected as cathode. A platinum foil 

 served as anode. The electric current should not be strong enough to heat 

 the solution, since this would cause platinum to dissolve from the anode. 

 After several hours of electrolysis, the crucible either drops away from 

 the partially reduced silver chloride or may be removed easily by pushing 

 with a rod. The electrolysis was continued until the large amount of 

 hydrogen evolved from the cathode showed that the silver chloride was 

 largely reduced. The electrolyte was changed, at intervals of several 

 hours, imtil the odor of chlorine could not be detected in the gases which 

 were given off. The reduced silver, which retained the shape of the cru- 

 [16—29034] 



