167 
By adding AgNO; to freshly prepared solutions of cobaltocyanide we found 
that this reaction takes place very slowly in cold but rapidly in hot solutions. 
When the AgNO; 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 AgNO; 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 cobaltoeyanide 
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 ee. of 0.1 molar CoSO, solution was treated in a casserole with just 
enough KCN to dissolve the Co(CN)., 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 ce. with water at 85° and 
AgNO; added dropwise with vigorous stirring. Results are given below. 
TABLE IT 
Ce. 0.1 molar CoSO, Time Heated. Result. 
IO) clo GR GASP eRE REGRET, OEE Te Z Mines. 2.4 se ColloidalvAic- 
1D) cng CCL RER Obra a ence se ae Be Mle ye ao oe Orange soln.: gray ppt. 
1) eee ee ee ee See anime. seen 2 || Onan ecersolncerave ppt. 
NOs gk Bot oe Sateen Cae OE ME AR 5 min.........| No darkening of soln.; ppt. white. 
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. 
DrTECTION OF NICKEL IN 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. 
