Potassium ferri-sulphite, KFe(SO₃)SO₄

Potassium ferri-sulphite, KFe(SO₃)SO₄, is prepared by the action of potassium hydrogen sulphite upon ferric ammonium alum. It crystallises in slender needles, which are sparingly soluble in cold water. With hot water, ferrous and potassium sulphates pass into solution, leaving an insoluble yellow residue.