CYANOGEN COMPOUNDS 155 



Metallic Cyanides. 



Hydrogen cyanide resembles hydrochloric acid in behaviour, form- 

 ing salts with alkalies and metallic hydroxides. The alkaline salts 

 crystallise like sodium and potassium chloride ; the silver salt is white 

 and insoluble in water and acids, but soluble in ammonia. Silver 

 cyanide, unlike silver chloride, is decomposed by boiling with mineral, 

 acids forming hydrogen cyanide. 



The chief salt is potassium cyanide which is used extensively for 

 extracting gold and in electroplating. 



Preparation of Potassium Cyanide. 



Potassium cyanide is prepared by fusing potassium ferrocyanide: 

 K 4 Fe(CN) 6 = 4 KCN + N 2 + FeC 2 . 



On heating about I gm. of potassium ferrocyanide in a crucible to 

 redness, allowing to cool, extracting the mass with water and filtering,, 

 the solution will be found to contain potassium cyanide as shown by 

 the test on p. 156. 



In this reaction the whole of the nitrogen of the ferrocyanide is not 

 obtained as cyanide ; if potassium ferrocyanide be fused with potassium: 

 carbonate a mixture of c> anate and cyanide is formed : 



K 4 Fe(CN) 6 + K 2 CO 3 = sKCN + KOCN + CO 5 + Fe. 



If potassium ferrocyanide be fused with sodium, a mixture of potassium, 

 and sodium cyanides results : 



K 4 Fe(CN) 6 + 2Na = 4KCN + 2NaCN + Fe. 



The cyanides dissolve in water leaving the iron and are obtained by 

 evaporation. 



Large quantities of cyanide are now prepared by two other methods : 



(1) by heating sodium with charcoal in a current of ammonia at 400. 

 Sodamide is formed and converted into sodium cyanamide (p. 159) : 



2 NH 3 + Na, = 2 NaNH 2 + H 2 

 2NaNH 2 + C = Na 2 CN 2 + 2H 2 . 



On raising the temperature to 800 the sodium cyanamide and charcoal 

 react, forming sodium cyanide : 



Na 2 CN 2 + C = 2NaCN. 



Sodium cyanide is formed directly at 800 : 

 NaNH 2 + C = NaCN + H 2 . 



(2) by heating beet-sugar molasses to 1000. At this temperature the 

 trimethylamine is decomposed into hydrogen cyanide and methane : 



(CH 3 ) 3 N = HCN + 2 CH 4 . 



So t dium cyanide is prepared by passing the gases into sodium hydrate and' 

 evaporating the solution. 



Metallic gold dissolves in potassium cyanide solution in the presence of 

 air or other oxidising agent : 



2Au + 4 KCN + H 2 O + O = 2KAu(CN) 2 + 2KOH, 

 forming a double cyanide from which the gold is obtained by electrolysis. 



