60 Preparation of Substances 



tion becomes saturated. Cool the liquid 

 with stirring or allow it to stand until the 

 next exercise. Filter off the crystals and dry 

 them between filter paper. Save the mother 

 liquor for the tests which follow. 



Tests for copper ; Notebook. — Test some 

 copper sulfate by adding ammonium hy- 

 droxid, at first one drop, then in larger quan- 

 tity. The light blue insoluble substance 

 f ormd by the small amount of ammonia is a 

 basic copper sulfate; the blue solution con- 

 tains copper and ammonia together in one 

 ion, an ammonio-cupric sulfate Cu(NH3)4S04. 

 This solution contains very few copper ions — 

 only those that break away from this com- 

 plex. The formation of a blue solution is a 

 test for copper. To the blue solution add a 

 few drops of potassium ferrocyanide. There 

 being so few Cu ions the copper ferrocyanide 

 formd is not v sible. Break up the ammonio- 

 copper complex by adding acetic or dilute 

 hydrochloric acids. As soon as copper ions 

 are present in amount of about 0.002 per cent 

 the copper ferrocyanide begins to be visible. 



Add a few drops of potassium ferrocyanide 

 to a copper sulfate solution. The formation 

 of brown copper ferrocyanide is a test for 

 copper. 



