MICROCHEMICAL REACTIONS OF THE COMMON ACIDS 377 



solution of primary sodium carbonate; hydrocyanic acid will 

 be set free and will give a characteristic silver cyanide. 



b. Set free the vapors of the acid and expose to them a drop 

 of sodium picrate. A blood red solution results. 



CYANATES. 



a. To a drop of concentrated solution add at the center, a 

 tiny crystal of cobalt acetate. The crystal will be immediately 

 surrounded by a deep blue colored zone and a blue amorphous 

 precipitate. The blue zone increases in diameter and eventually 

 may reach the circumference of the drop. Upon evaporation 

 deep blue tetragonal dendrites, and tabular and prismatic crys- 

 tals of a compound corresponding to the formula K 2 Co(CNO)4 

 will appear. Note that to obtain this compound the cyanate 

 must be in excess. With sulphocyanates tested thus a deep 

 blue liquid is obtained on evaporation, but the blue dendrites 

 which may separate have a different habit. 



Cyanides yield no blue, but a brown color instead. Even a 

 small amount of cyanide will prevent the blue zone, but the 

 crystal will be blue surrounded by a yellow or brown zone. 



b. Treat a drop with dilute sulphuric acid in the distilling 

 apparatus, Fig. 130, page 245. Evaporate very gently almost 

 to dryness; add a few fibers of freshly ignited asbestos and pro- 

 ceed to test for ammonia. See Ammonium, Method A , page 286. 

 With sulphuric acid cyanates yield carbon dioxide and ammo- 

 nium sulphate. 



Precaution. Always make a blank test upon the reagents 

 to be sure of their freedom from ammonium salts. 



FERRICYANIDES. 



a. Give off Vapors when heated with sulphuric acid which 

 produce silver cyanide. See Cyanides, a, page 377. 



b. To the test drop add sodium acetate, then apply a solution 

 of Benzidine Hydrochloride 1 by Method /, page 251. Light blue 

 prisms and stars will soon appear. 



Ferrocyanides do not give this reaction. 



c. Give no color with dilute solutions of pure Ferric Salts. 



1 Behrens, Z. anal. CJaem., 13, 432. 



