Potassium ferri-tetrasulphite, K₄Fe₂(SO₃)₄SO₄

Potassium ferri-tetrasulphite, K₂Fe₂(SO₃)₄.K₂SO₄ or K₄Fe₂(SO₃)₄SO₄, is obtained by adding sodium hydrogen sulphite solution to one of potassium ferro-heptanitroso sulphide, KFe₄(NO)₇S₃, in alcohol and allowing to stand at the ordinary temperature for a couple of weeks. The salt separates as yellow, microscopic needles, insoluble in cold water. Acids dissolve the salt readily, and boiling with water effects the precipitation of ferric hydroxide.