Ferrous potassium ferrocyanide, FeK₂[Fe(CN)₆]

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.