Ferri-, Ferro- s. Iron

Ferri-, Ferro- s. Iron Ferricyanide s. Hexacyano-ferrate(III)... 

According to Werner s theory (see this series, Vol. X.), the formulae of hydrogen ferro- and ferri-cyanide should be written [Fe(CN)6]H4 and [Fe(CN)6]H3 respectively, the six cyanogen groups being co-ordinated with the iron atom, and constituting the nucleus around which hover the replaceable hydrogen atoms. 

The iron salts of ferro and ferricyanic acid are the compounds to which the names cyanogen and cyanide are due. Two of these salts are of deep blue color and the Greek word from which cyanogen and cyanide are derived is cyanos which means blue. The ferric ferro-cyanide, Fe4 "(Fe"(CN6)3, is known as Prussian blue and the ferrous ferri-cyanide, Fe3"(Fe" (CN)6)2, is Turnbull s blue. These compounds are formed when ferric salts in solution are treated with potassium ferro-cyanide and when ferrous salts in solution are treated with potassium ferricyanide. They are common qualitative tests for the two forms of iron salts. The compounds are also used as laundry blueing and are formed in the blue print process of photography. 

---