134 ILLINOIS STATE ACADEMY OF SCIENCE 



water acts slowly on zinc to form the hydroxide. In the 

 case of the other metals the excess loss was not deter- 

 mined. 



Since the recovery of silver from ''hypo" fixing baths 

 is of commercial importance, most of the time was spent 

 in its study. For this purpose a spent "hypo" solution 

 was obtained from a local photographer. Before using 

 this solution, however, preliminary experiments were 

 performed with a fairly concentrated solution of silver 

 chloride in sodium thiosulphate solution. The method 

 used followed very closely that with ammonia solutions, 

 using the same metals. With all of the metals it was 

 found that the precipitation of silver began very slowly, 

 only after two or more hours, and that it proceeded regu- 

 larly to completion. With iron the end was reached 

 only after forty-eight hours, but with the others twenty- 

 four were all that was needed. Only slight traces of the 

 precipitate were insoluble in dilute nitric acid. In the 

 case of copper the silver deposited as a smooth layer 

 upon the surface of the copper sheets; with the other 

 metals the silver deposited very smoothly on the glass 

 walls of the containing vessels, forming a mirror more 

 or less perfect. 



After this preliminary experiment a liter of the spent 

 liquor, mentioned above, was treated with small rectan- 

 gular chunks of pure zinc lying at the bottom of the flask. 

 The silver slowly deposited as a mirror on the walls. 

 About four grams of silver were deposited for three 

 grams of zinc disappearing into the solution. This is 

 far less silver than is theoretically possible (3.3 :1), but 

 by using a larger surface of zinc suspended in the solu- 

 tion and by stirring the solution it is believed the amount 

 might be increased nearly to the theoretical. 



After the silver had become completely precipitated 

 from the spent liquor a part of it was treated with sod- 

 ium carbonate to separate the carbonate of zinc. After 

 filtration of the carbonate it was found that the slightly 

 acidified solution (with acetic acid) dissolved 62 grams 

 of silver chloride per liter. The part of the silver-free 

 solution from which the zinc had not been removed was 

 found to dissolve 70 grams of silver chloride per liter. 



