Blue Compounds of Cyanogen and Iron. 165 



2HS 



r-Tis 



The fact of the red cyanide being thus reducible is how- 

 ever a property which may in the above instance exercise a 

 considerable influence on the composition of the product, as 

 is shown by the following instructive experiment of Liebig. 

 To a boiling solution of ferridcyanide of potassium was added 

 a small quantity of protosulphate of iron, which was not nearly 

 sufficient to decompose all the cyanide. Ferrocyanide of po- 

 tassium, which remained in solution, and precipitate of prus- 

 sian blue (3Cfy4Fe), were formed: — 



4FeO 



r i 



This reaction leads to a new way of considering the above 

 analysed compound, which is adapted to its composition when 

 containing and also when free from potassium, for if one atom 

 more of protosalt of iron be added than assumed in the above 

 formula of decomposition, it forms, by decomposition with the 

 yellow prussiate, ferrocyanide of iron and potassium, which 

 divided in the already-formed prussian blue, produces a mix- 

 ture containing iron and potassium in the proportion found 

 by analysis : — f 3Cfy 4Fe 



4Cfy 6K\ J p - f'Fe 

 5FeO |H C Hk 

 L5KO 



On adding an excess of iron salt the second equivalent of 

 potassium is also replaced by iron, and a mixture of prussian 

 blue and ferrocyanide of iron formed, which contains cyanogen 

 and iron in the proportion of 6 to 5, as found. This view rests 

 upon the supposition that prussian blue (3Cfy4Fe) is always 

 formed by the decomposition at the ordinary temperature, as 

 was found to be the case at the boiling-point. In the con- 

 trary case we must assume that the potassium of the red 

 cyanide is simply replaced by iron : — 



2Cfy3K + 3FeCl = 2Cfy3Fe + 3KC1 



4Cfy 6K + 5FeCl - 4Cfy{^ 5Ka 



In the first case we have ferridcyanide of iron, in the se- 

 cond a double salt of the same with ferridcyanide of iron and 

 potassium. 



The separation of Fe 3 4 by potash seems to speak in favour 

 of the view that the precipitate is a mixture of prussian blue 

 and ferrocyanide of iron, as these substances would give this 

 reaction ; for if the compound were a ferridcyanide of iron, it 

 would be expected that potash would set free protoxide of 



