Ferrous potassium carbonate, FeK₂(CO₃)₂

Ferrous potassium carbonate, FeK₂(CO₃)₂.4H₂O, may be prepared by mixing, out of contact with air, a concentrated solution of ferrous chloride with excess of potassium carbonate. At first a white precipitate of ferrous carbonate is formed, which soon dissolves to a greenish solution from which the double salt crystallises in nearly white scales.