166 
time required for the pink precipitate to appear was not recorded. The 
samples were observed after standing 24 hours. From the results tabulated 
below it is apparent that the test is at least as delicate in the presence as 
in the absence of cyanide and that the results are obtainable much more 
quickly from the complex than from the simple ion. In the extreme dilutions 
of the simple ion the precipitate was frequently a single red crystal very 
minute and difficult to see. 
TABLE I. 
TIME. 
Molarity. els = | Meg. Ni per cc. Ratio Ni : H20 
NiSO, K».2Ni(CN),4 
0.0005 Immediate Immediate 0.02934 1 34,000 
00005 1 hour 3 min. 002934 Ne 340 , 000 
00001 24 hours 5 min. OO05S8T7 1 : 1,700,000 
.000009 24 hours 10 min. 000528 1 : 1,900,000 
000008 24 hours 20 min. 000470 1: 2,1307006 
000007 24 hours 30 min. 000411 1 : 2,480,000 
000006 24 hours | 1 hour 000352 1 : 2,840,000 
000005 24 hours 24 hours 000293 1 : 3,400,000 
OO0004 No ppt. 24 hours 000235 1 : 4,260,000 
.000003 No ppt. | No pink color 000176 1: 5,700,000 
000002 No ppt. No pink color OOO1LLT 
3. OXIDATION OF COBALTOCYANIDE JON TO COBALTICYANIDE ION. 
When KCN is added to a solution of cobalt salt, brown-red Co(CN)s 
is first precipitated and then redissolved to a brown solution of KyCo(CN),. 
On heating this soon changes to a pale yellow and the color change is generally 
assumed in manuals of analysis to indicate the completion of oxidation to 
cobalticyanide. We at first proceeded upon this assumption, but when the 
first drops of AgNO; were added to some of our complex cyanide solutions, 
soon after the color change took place, the solution darkened and addition 
of more AgNO; produced a dark-gray precipitate while solutions which had 
stood for several hours did not darken and gave a pure white precipitate. 
When one of the darkened solutions became distinctly opalescent, we sus- 
pected that colloidal silver had been formed. This was explainable by the 
assumption that AgNO, had been reduced by cobaltocyanide which was still 
present according to K,Co(CN), + AgNO; = K;Co(CN)- + Ag + KNOs. 
