MICROCHEMICAL REACTIONS OF COBALT 
413 
DETECTION. 
A. By Means of Potassium Mercuric Thiocyanate. 
See Zinc, Method A, page 353; Copper, Method A, page 
385. Apply the reagent by Method IV, page 303. 
Mercury cobalt thiocyanate Hg(CNS)2 • Co(CNS)2 sepa¬ 
rates as dark blue prisms, usually in irregular clusters. Its 
solutions have the tendency to supersaturate and it is therefore 
necessary to give the reaction considerable time, or even evapo¬ 
ration over the micro-flame may be advisable. Crushing the 
first crystals appearing near the circumference of the drop and 
drawing the fragments across often expedites the reaction. 
Nickel yields no crystals under ordinary conditions and does 
not interfere unless in excessively great amount. See Zinc. 
Precautions. 
The test drop should be neutral or only slightly acid with 
acetic acid, but must not be alkaline. 
Better results are to be obtained with mineral acid salts than 
with those of organic acids. 
EXPERIMENTS. 
The student should refer to his notes under Zn, where the results of his ex¬ 
perience with the reagent upon Co should be found. 
B. By Means of Potassium Nitrite. 
To the neutral or slightly acid drop add a fragment of 
potassium nitrite. Stir. Then warm and add a drop of acetic 
acid. 
Potassium cobalt nitrite 3 KNO2 • Co(N02)3 • i| H2O is im¬ 
mediately precipitated in the form of tiny cubes, so minute as 
to simulate an amorphous or finely-granular deposit. These 
crystals appear black by transmitted light, yellow by reflected 
light. From hot solutions there may sometimes be obtained 
crystals recognizable as cubes and octahedra. 
This test has its greatest value in a negative way since failure 
to obtain the very insoluble double nitrite may be considered as 
indicative of the absence of cobalt. 
