366 ELEMENTARY CHEMICAL MICROSCOPY 



DETECTION. 



A. By Means of Ammonium Mercuric Sulphocyanate. 



See Zinc, Method A, page 307; Copper, Method A, page 



339- 



Mercury cobalt sulphocyanate 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 and does not interfere unless in ex- 

 cessively great amount. 



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 KN0 2 Co(N0 2 ) 3 i| H 2 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. 



