550 CONSIDERATION OF CARBON COMPOUNDS. 



has a very low conductivity, due to its extremely small degree of dissociation. 

 In a tenth-normal solution at 18 C., only 0.01 per cent, of the acid molecules 

 are dissociated, thus : 



HCN ^ H- f CN'. 



As in the case of the mercury salts, the poisonous character of hydrocyanic 

 acid and its salts depends upon the degree of dissociation into CN X ions. A 

 number of complex cyanides are known, in which the cyanogen groups are 

 combined with metals to form complex radicals, in which both the cyanogen 

 and the metals are masked, and do not respond to the usual analytical reagents. 

 The best examples of such compounds are the ferrocyanide and ferricyanide of 

 potassium, K 4 FeCN 6 and K 3 Fe(CN) 6 , respectively. These compounds are not 

 poisonous because they do not form CN X ions, being dissociated in solution 

 according to the following equations : 



K 4 Fe(CN) 6 ^ 4K- + Fe(CN) 6 "" 



Ferrocyanogen ion. 



K 3 Fe(CN) 6 ^ 3K- + Fe(CN) 6 '" 



Ferricyanogen ion. 



The alkali cyanides are decomposed by such a weak acid as carbonic acid, 

 hence they have the odor of hydrocyanic acid, due to the action of the carbonic 

 acid of the atmosphere. In aqueous solution they have a strong alkaline re- 

 action, due to hydrolysis : 



KCN ; K- + CN'\ _ TT PV 

 HOH ^ (OH)/ + H- / ' 



The action is due to the extremely weak dissociating power of hydrocyanic 

 acid (see Chapter 15). 



For peculiarities of mercury cyanide see chapter on Mercury. 



Potassium cyanide, Potassii cyanidum, KCN 64.7. 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, while 

 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 . 

 Potassium Potassium Potassium Potassium Iron. Carbon 



ferrocyanide. carbonate. cyanide. cyanate. dioxide. 



A mixture of pure potassium and sodium cyanides, free from cyanate, 

 is now manufactured on a large scale by heating together anhydrous 

 potassium ferrocyanide and metallic sodium : 



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



Potassium cyanide, U. S. P., should contain at least 95 per cent, of 

 potassium cyanide ; it is a white^ deliquescent substance, odorless when 



