CYANOGEN AND ITS COMPOUNDS. 335 



oxide escapes, and metallic iron is set free and collects on the 

 bottom of the crucible. The decomposition is as follows : 



2(K 4 FeCy 6 ) + 2(K 2 CO 3 ) = lOKCy + 2KCyO + 2Fe + 2C0 2 . 



Potassium Potassium Potassium Potassium Iron. Carbon 



cyanide. carbonate. cyanide. cyanate. dioxide. 



Potassium cyanide is a white, deliquescent salt, odorless when 

 perfectly dry, but emitting the odor of hydrocyanic acid when 

 moist. Potassium cyanides and other alkaline cyanides show a 

 tendency to combine with the cyanides of heavy metals, forming 

 a number of double cyanides, such as the cyanide of sodium 

 arid silver, NaCy.AgCy, etc. 



Silver cyanide, Argenti cyanidum, AgCN = 133.7 (Cyanide of 

 silver). A white powder, obtained by precipitating solution of 

 potassium cyanide with silver nitrate. It is insoluble in water, 

 slightly soluble in water of ammonia; evolves cyanogen when 

 heated. 



Mercuric cyanide, Hydrargyri cyanidum, Hg(CN) 2 (Cyanide of 

 mercury). A white, soluble, crystalline salt obtained by dissolv- 

 ing mercuric oxide in hydrocyanic acid; it evolves cyanogen 

 when heated. 



Analytical reactions for hydrocyanic acid. 



1. Hydrocyanic acid or soluble cyanides give with silver 

 nitrate a white precipitate of silver cyanide, which is sparingly 

 soluble in ammonia, soluble in alkaline cyanides or hyposul- 

 phites, but insoluble in diluted nitric acid. Concentrated nitric 

 acid dissolves it with decomposition : 



HCy + AgN0 3 = AgCy + HNO 3 . 



2. Hydrocyanic acid mixed with ammonium hydric sulphide 

 and evaporated to dryness forms sulphocyanic acid, which upon 

 being slightly acidulated with hydrochloric acid gives with 

 ferric chloride a blood-red color of sulphocyanate of iron. (Ex- 

 cess of ammonium sulphide must be avoided.) 



3. Hydrocyanic acid, or soluble cyanides, give, when mixed 



