424 ELEMENTARY CHEMICAL MICROSCOPY 



a slide bearing a drop of silver nitrate. If no tiny prismatic 

 crystals are obtained and no clouding of the silver nitrate, cyan- 

 ides are absent. If a clouding of the drop results, make a fresh 

 test, this time substituting for the sulphuric acid, a saturated 

 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. 153, page 293. Evaporate very gently almost 

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

 proceed to test for ammonia. See Ammonium, Method A , page 

 332. With sulphuric acid cyanates yield carbon dioxide and 

 ammonium sulphate. 



Precaution. — Always make a blank test upon the reagents 

 to be sure of their freedom from ammonium slats. 



FERRICYANIDES. 



a. Give of Vapors when heated with sulphuric acid which 

 produce silver cyanide. See Cyanides, a, page 423. 



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



