652- PRINCIPLES OF CHEMISTRY 



reaction, a smell-like that of bitter almonds, peculiar to pruario acid, 

 and acts as a most powerful poison. Although exceedingly stable 

 in a fused state, po'tassium cyanide easily changes when in solution. 

 Prussic acid is so very feebly energetic that even water decomposes 

 potassium cyanide. A solution of the salt, even without access of air, 

 easily turns brown and decomposes, and when heated evolves ammonia 

 and forms potassium formate ; this is easily comprehensible from the 

 representation of the cyanogen compounds which was developed in 

 Chapter IX., KCN -f 2H 2 O = CHKO a + NH 3 . Furthermore, as 

 carbonic anhydride acts on potassium cyanide with evolution of prussic 

 acid, and as potassium cyanate, which is also unstable, is formed by the 

 action of air, it will be easily seen that solutions of potassium cyanide 

 are very unstable. Potassium cyanide, containing as it does carbon 

 and potassium, is a substance which can act in a very vigorously re- 

 ducing manner, especially when fused ; it is therefore used as a 

 powerful reducing agent at a red heat. 13 The property of potassium 

 cyanide of giving double salts with other cyanides is very clearly shown 

 by the fact that many metals dissolve in a solution of potassium cyanide, 

 with the evolution of hydrogen. For example, iron, copper, and zinc 

 act in this manner. Thus 



4KCN + 2H a O + Zn =K a ZnC 4 N 4 + 2KHO + H a . 



one part of charcoal powder to the mixture of 8 parts of anhydrous yellow prussiate and 

 8 parts of potassium carbonate a mass is obtained which is free from cyanate, because 

 the carbon absorbs the oxygen, but in that case it id impossible to obtain a colourless 

 potassium cyanide by simple fusion, although this may be easily done by dissolving it in 

 alcohol. Cyanide of potassium 'may also be obtained from potassium thiocyanate, which 

 is formed from ammonium thiocyanate obtained by the action of ammonia upon bisulphide 

 of carbon (see works upon Organic Chemistry). Potassium cyanide is now prepared in 

 large quantities from yellow prussiate for gilding and silvering. When fused in large 

 quantities the action of the oxygen of the air is limited, and with great care the operation 

 may be successfully conducted, and therefore, on a large scale, very pure salt is some- 

 times obtained. When slowly cooled, the fused salt separates in cubical crystals like 

 potassium chloride. 



Pure KCN is obtained by passing CNH gas into an alcoholic solution of KHO. The 

 large amount of potassium cyanide which is now required for the extraction of gold 

 from its ores, is being replaced by a mixture (Bossier and Gasslaker, 1892) of KCN and 

 NaCN, prepared by heating powdered and dried yellow prussiate with metallic sodium : 

 K 4 Fe (CN) 6 + 2Na - 4KCN + 2NaCN + Fe. This method offers two- advantages over the 

 above methods : (1) the whole of the cyanide is obtained, and does not decompose with 

 the formation of N 2 ; and (2) no cyanates are formed, as is the case when carbonate of 

 potash is heated with the prussiate. 



13 A considerable quantity of potassium cyanide Is used in the arts, more particularly 

 for the preparation of metallic solutions which are decomposed by the action of a galvanic 

 current; thus it is very frequently employed in electro- silvering and gilding. An 

 alkaline solution is prepared, which is moderately stable owing to the fact that potassium 

 cyanide hi the form of certain double salts that is, combined with other cyanides is 

 far more stable than when alone (yellow prussiate, which contains potassium cyanide in 

 combination with-ferrous cyanide, is an example of this); 



