Ferrous potassium ferrocyanide, FeK2[Fe(CN)6], is obtained as a white precipitate when potassium ferrocyanide reacts with an equi- molecular quantity of a ferrous salt: -
FeCl2 + K4[Fe(CN)6] = FeK2[Fe(CN)6] + 2KCl.
It readily absorbs oxygen, yielding Prussian blue.
Everitt's salt also has the formula FeK2[Fe(CN)6] and is isomeric with the previous salt. It is obtained when dilute sulphuric acid and potassium ferrocyanide are heated together: -
2K4[Fe(CN)6] + 3H2SO4 = 3K2SO4 + K2Fe[Fe(CN)6] + 6HCN.
It is a pale yellow crystalline powder which, upon oxidation with hydrogen peroxide, yields Williamson's violet. |