158 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Potassium Ferricyanide. K 3 Fe(CN) 6 . 



Preparation. 



Potassium ferricyanide is formed by the oxidation of potassium 

 ferrocyanide by means of chlorine or bromine : 



On adding a slight excess of bromine water to some potassium 

 ferrocyanide solution and boiling off the excess of bromine the colour 

 changes from yellow to brown-red, and on evaporation of the solu- 

 tion red crystals of potassium ferricyanide are obtained. 



Properties and Reactions. 



Potassium ferricyanide, or red prussiate of potash, is a red 

 crystalline substance soluble in water, giving a reddish-yellow solution. 



(1) On adding ferric chloride to its solution it turns dark brown. 



(2) On adding a solution of a ferrous salt to its solution, a deep- 

 blue precipitate (Turnbull's blue) is formed : 



2 K 3 Fe(CN) 6 + 3 FeS0 4 = Fe 3 [Fe(CN) 6 ] 2 + 3K 2 SO 4 . 



(3) In alkaline solution it is decomposed into potassium ferro- 

 cyanide and oxygen ; it acts therefore as an oxidising agent : 



2K 3 Fe(CN) 6 + 2KOH = 2 K 4 Fe(CN) 6 + H 2 O + O. 



Thus, if some litharge be added to a solution of potassium 

 ferricyanide rendered alkaline with sodium hydroxide and warmed, it 

 becomes brown owing to the formation of lead peroxide, PbO 2 . The 

 solution may be tested for ferrocyanide by filtering, acidifying and 

 adding ferric chloride which gives a precipitate of Prussian blue. 



Alkyl Cyanides Nitriles. 



Preparation. 



When potassium cyanide is treated with an alkyl iodide (methyl, 

 ethyl, etc., iodide) the alkyl cyanide is obtained: 



KCN + CH 3 I = KI + CH 3 CN. 



These compounds are also formed by treating the amides of the 

 corresponding acid with phosphorus pentoxide : 



CH 8 CONH 2 = H 2 + CH 8 CN. 

 Properties and Reactions. 



The lower members of the series are liquids with peculiar smell, and 

 are more or less soluble in water ; the higher members are solids. 



Like hydrogen cyanide they are hydrolysed by acids into the cor- 

 . responding acid : 



CH 8 CN + 2H 2 O = CH 3 COOH + NH 3 . 



On reduction they are converted into amines : 

 CH. t CN + 2 H 2 = CH 3 . CH 3 . 



