338 CONSIDERATION OF CARBON COMPOUNDS. 



boiled with water, and from the solution thus formed the crystals 

 separate on cooling. 



The nitrogen and carbon of the organic matter (heated as 

 above stated) combine, forming cyanogen, which enters into 

 combination first with potassium and afterwards with iron. 



Potassium ferrocyauide forms large, translucent, pale, lemon- 

 yellow, soft, odorless, non-poisonous, neutral crystals, easily 

 dissolving in water. 



Analytical reactions : 



1. Ferrocyanides heated on platinum foil burn and leave a 

 residue of (or containing) ferric oxide. 



2. Ferrocyanides heated with concentrated sulphuric acid 

 evolve carbonic oxide; with diluted sulphuric acid liberate 

 hydrocyanic acid; with concentrated hydrochloric acid liberate 

 hydroferrocyanic acid. 



3. Soluble ferrocyanides give a blue precipitate with ferric 

 salts : 



3(K 4 FeCy 6 ) + 2(Fe 2 C1 6 ) = 12KC1 + Fe 4 .3FeCy 6 . 



Potassium Ferric Potassium Ferric fer- 



ferrocyanide. chloride. chloride. rocyanide. 



The blue precipitate of ferric ferrocyanide, or Prussian blue, 

 is insoluble in water and diluted acids, soluble in oxalic acid 

 (blue ink), and is decomposed by alkalies with separation of 

 brown ferric hydrate and formation of potassium ferrocyauide. 

 The addition of an acid restores the blue precipitate. 



4. Soluble ferrocyanides give with cupric solutions a brownish- 

 red precipitate of cupric ferrocyanide. 



5. Soluble ferrocyanides produce, with solutions of silver, 

 lead, and zinc, white precipitates of the respective ferrocyanides. 



6. Ferrocyanides give with ferrous salts a white precipitate 

 of ferrous ferrocyanide, soon turning blue by absorption of 

 oxygen. 



Potassium ferricyanide, K 6 Fe 2 (CN) 12 (Red prussiate of potash). 

 Obtained by passing chlorine through solution of potassium 

 ferrocyanide : 



2(K 4 FeCy e ) + 2C1 ^KCl -f K 6 Fe 2 Cy rr 



Potassium Chlorine. Potassium Potassium 



ferrocyanide. chloride. ferricyanide. 



While apparently this decomposition consists merely in a 

 removal of two atoms of potassium from two molecules of 



