156 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Double Cyanides. 



The alkali cyanides dissolve the insoluble cyanides of silver, gold 

 and other heavy metals forming the double cyanides : 



KCN + AgCN = KAg(CN) 2 . 



On adding a few drops of a I per cent, solution of potassium 

 cyanide to a few drops of silver nitrate solution, a white precipitate of 

 silver cyanide is formed : 



KCN + AgNO 3 = AgCN + KNO 3 . 



On adding more potassium cyanide solution, the precipitate dis- 

 solves, forming the double salt. The double salt is decomposed with 

 the formation of silver cyanide by adding dilute nitric acid : 

 KAg(CN) a + HNO 3 = KNO 3 + HCN + AgCN. 



The double cyanides are extensively used in electroplating ; on 

 -electrolysis the compound is decomposed with the formation of potas- 

 sium and Ag(CN) 2 ions at the cathode and anode respectively. The 

 double cyanide is reduced at the cathode, the silver being deposited. 

 Silver is used as the anode and is dissolved by the Ag(CN) 2 ions 

 forming 2AgCN, which is soluble in potassium cyanide giving the 

 double cyanide. The reactions are thus : 



KAg(CN) 2 -> K + Ag(CN) 2 Ag + Ag(CN) 2 - 2 AgCN 

 K + KAg(CN) 2 -> 2KCN + Ag " AgCN + KCN - KAg(CN) 2 . 



Tests for Cyanides. 



(1) The smell of hydrogen cyanide, either before or after acidifying 

 the solution with dilute nitric acid and warming, is an indication of 

 the presence of a cyanide. 



(2) The formation of silver cyanide, by adding silver nitrate to a 

 solution acidified with nitric acid, or better by holding a drop of silver 

 nitrate on a glass rod in the vapour of the solution in (i), also in- 

 dicates the presence of a cyanide. 



(3) The formation of Prussian blue by boiling the solution with a 

 ferrous salt and alkali, acidifying and then adding a drop of ferric 

 chloride is characteristic. 



(4) The formation of ferric thiocyanate by adding to the solution 

 a drop of ammonium sulphide, evaporating to dryness, acidifying with 

 hydrochloric acid and adding a drop of ferric chloride is the most 

 delicate way of detecting a cyanide. 



If a cyanide be present with other organic substances, e.g. in 

 stomach contents, etc., in cases of poisoning, the material is acidified 

 with a non-volative organic acid, such as tartaric acid, and distilled. 

 The distillate is tested for hydrogen cyanide as above. 



