554 CONSIDERATION OF CARBON COMPOUNDS. 



metals (iron, etc.) entering into the radical are completely hidden. 

 Thus, the iron in ferro- or ferricyanides is not precipitated by either 

 alkalies, soluble carbonates, ammonium sulphide, or any of the com- 

 mon reagents for iron, and its presence can only be demonstrated by 

 these reagents after the organic nature of the compound has been 

 destroyed by burning it (or otherwise), when ferric oxide is left, 

 which may be dissolved in hydrochloric acid and tested for in the 

 usual manner. 



The principal iron-cynogen radicals are ferrocyanogen [Fe u 

 (CN),, 1 ]*, and ferricyanogen [Fe^CNy] 111 . These two radicals con- 

 tain iron in the ferrous and ferric state respectively, and form, upon 

 combining with hydrogen, acids which are known as hydroferroeyanic 

 acid, H 4 Fe(CN) 6 (tetrabasic), and hydroferricyanic acid, H 3 Fe(CN) g 

 (tribasic) ; the salts of these acids are termed ferrocyanides and ferri- 

 cyanides. (For dissociation of these, see p. 549.) 



Potassium ferrocyanide, Potassii ferrocyanidum, K 4 Fe(CN) 6 . 

 3H 2 O = 419.62 ( Yellow prussiate of potash). This salt is manu- 

 factured on a large scale by heating refuse animal matter (waste 

 leather, horns, hoofs, etc.) with potassium carbonate and iron (filings, 

 etc.). The fused mass is 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 afterward with iron. 



Potassium ferrocyanide forms large, translucent, pale lemon-yellow, 

 soft, odorless, non-poisonous, neutral crystals, easily dissolving in 

 water, but insoluble in alcohol. 



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 dilute sulphuric acid liberate hydrocyanic acid ; 

 with concentrated hydrochloric acid liberate hydroferroeyanic acid. 



3. Soluble ferrocyanides give a blue precipitate with ferric salts 

 (Plate L, 5) : 



3K 4 Fe(CN) 6 + 4FeCl 3 = 12KC1 + Fe 4 (FeC 6 N 8 ) 3 . 



Potassium Ferric Potassium Ferric ferro- 



ferrocyanide. chloride. chloride. cyanide. 



The blue precipitate of ferric ferrocyanide, or Prussian blue, is 

 insoluble in water and diluted acids, soluble in oxalic acid (blue 



