COMPOUNDS CONTAINING NITROGEN. 551 



perfectly dry, but emitting the odor of hydrocyanic acid when moist ; 

 it is soluble in about 2 parts of water ; this solution has an alkaline 

 reaction but is unstable, decomposition soon taking place with the 

 formation of potassium formate and ammonia, along with other 



products : 



KCN + 2H 2 O = CHK0 2 + NH 3 . 



A solution of potassium cyanate decomposes slowly in the cold, 

 but rapidly on heating, with the formation of potassium and ammo- 

 nium carbonates : 



2KCNO + 4H 2 O = K 2 CO 3 + (NH 4 ) 2 CO 8 . 



Potassium cyanides and other alkali cyanides show a tendency to 

 combine with the cyanides of heavy metals, forming a number of 

 double cyanides, such as the cyanide of sodium and silver, NaCN. 

 AgCN, etc., which are soluble in water. Hence, precipitates formed by 

 addition of alkali cyanides to solutions of metallic salts, are dissolved 

 in excess of the reagent. Double cyanides of silver and gold are used 

 in commercial electroplating. A large proportion of the alkali cyanides 

 manufactured is used in extracting gold from its ores, especially in 

 Transvaal. In 1889 not more than 50 tons of cyanide per annum 

 were consumed, while in 1905 the consumption was about 10,000 tons, 

 one-third of which was used in Transvaal. 



Silver cyanide, Argenti cyanidum, AgCN = 132.96. A white 

 powder, obtained by precipitating a solution of potassium cyanide 

 with silver nitrate. It is insoluble in water, slowly soluble in ammonia 

 water, sodium thiosulphate, and potassium cyanide when heated it 

 evolves cyanogen, metallic silver being left. 



Mercuric cyanide, Hg(CN) 2 . A white crystalline salt, obtained by 

 dissolving mercurous oxide in hydrocyanic acid ; it is soluble in water 

 and alcohol and evolves cyanogen when heated. 



Mercuric oxycyanide, Hg(ON) 2 .HgO. (Basic mercuric cyanide.) This is 

 obtained by triturating mercuric oxide, dilute sodium hydroxide solution, and 

 mercuric cyanide until the mixture becomes colorless. The salt is purified by 

 washing with cold water, or recrystallizing from hot water. It occurs as a 

 white, crystalline powder, soluble in 17 parts of water, and turns red litmus 

 blue. It is recommended as a substitute for mercuric chloride, as it is claimed 

 to have greater antiseptic power, to be less irritating, and to have no corroding 

 action on steel instruments. 



Analytical reactions for hydrocyanic acid. 



(Potassium cyanide, KCN, may be used.) 



1. Hydrocyanic acid, or soluble cyanides, give with silver nitrate 

 a white precipitate of silver cyanide, which is sparingly soluble in 



