167 



By adding AgN0 3 to freshly prepared solutions of cobaltocyanide Ave found 

 that this reaction takes place very slowly in cold but rapidly in hot solutions. 

 When the AgN0 3 was added dropwise, the hot solutions first became lighter 

 in color, then gradually turned orange and darkened until a gray precipitate 

 was formed. If the addition of AgN0 3 was stopped when the orange tint 

 appeared, no precipitate formed, but the solution darkened on standing and 

 became opalescent, showing that colloidal silver had formed. We found that 

 this phenomenon was regularly reproducible in solutions of cobaltocyanide 

 not less than 0.005 molar. These experiments clearly show that the oxidation 

 of cobaltocyanide is by no means complete when the color change takes place. 

 We next investigated the time required to complete the oxidation, taking 

 the failure to form metallic silver as evidence that the oxidation was essen- 

 tially complete. 



10 ce. of 0.1 molar C0SO4 solution was treated in a casserole with just 

 enough KCN to dissolve the Co(CN) 2 , the solution heated nearly to boiling 

 and continuously rotated in the casserole for a definite time to promote 

 oxidation. The solution was then diluted to 100 cc. with water at 85° and 

 AgN0 3 added dropwise with vigorous stirring. Results are given below. 



TABLE IT. 





Cc. 0.1 molar C0SO4 



Time Heat 



3d. 



Result. 







10. 





2 min 



3 min 



Colloidal Ag. 

 Orange soln.; gray ppt. 

 Orange soln. ; gray ppt. 

 No darkening of soln.; 



ppt. 





10 



10 





10. 















These results show that heating with constant agitation must be con- 

 tinued for some time after the change of color. Presumably the time re- 

 quired increases with the amount of cobalt present. 



Detection or Nickel ix Cobalt Salts. 



We next determined the minimum amount of nickel that could be de- 

 tected in varying amounts of cobalt by our silver method and, for com- 

 parison, by Treadwell's and the modified Liebig. 



