360 CONSIDERATION OF CARBON COMPOUNDS. 



According to the U. S. P., this diluted acid is made either by the 

 decomposition of potassium ferrocyanide by diluted sulphuric acid in 

 a retort, the delivery-tube of which passes into a receiver containing 

 water, by which the liberated gas is absorbed, this liquid being after- 

 ward diluted with a sufficient quantity of water to make a 2 per cent, 

 solution, or it is made extemporaneously by the decomposition of 6 

 parts by weight of silver cyanide by 5 parts of hydrochloric acid, 

 diluted with 55 parts of water, allowing the silver chloride to sub- 

 side and pouring off the clear liquid. 



The diluted acid has the characteristic odor of bitter almonds, a 

 slightly acid reaction, and is completely volatilized by heating. 

 Whilst the pure acid is very readily decomposed by exposure to light, 

 etc., the dilute acid is fairly stable. 



Potassium cyanide, Potassii cyanidum, KCN = 65. The pure 

 salt may be obtained by passing hydrocyanic acid into an alcoholic 

 solution of potassium hydroxide. The commercial article, however, 

 is a mixture of potassium cyanide with potassium cyanate. It is 

 obtained by fusing potassium ferrocyanide with potassium carbonate 

 in a crucible, when potassium cyanide and cyanate are formed, whilst 

 carbon dioxide escapes, and metallic iron is set free and collects on 

 the bottom of the crucible. The decomposition is as follows : 



K 4 Fe(CN) 6 -f K 2 CO 3 = = 5KCN + KCNO + Fe + CO 2 . 

 Potassiiim Potassium Potassium Potassium Iron. Carbon 



ferrocyanide. carbonate. cyanide. cyanate. dioxide. 



Potassium cyanide, U. S. P., should contain at least 90 per cent, 

 of the pure salt ; it is a white, deliquescent substance, odorless when 

 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 and decomposes slowly in the cold, but rapidly on heating, 

 with the formation of potassium and ammonium carbonates : 



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



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. 



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, 

 slowly soluble in water of ammonia ; evolves cyanogen when heated, 

 metallic silver being left. 



