Sodium hydrogen ferri-tetrasulphite, Na₂H₂Fe₂(SO₃)₄SO₄

Sodium hydrogen ferri-tetrasulphite, Na₂H₂Fe₂(SO₃)₄SO₄.2H₂O, results on dissolving freshly precipitated ferric hydroxide in a concentrated solution of sodium hydrogen sulphite and sulphurous acid. On concentration over sulphuric acid a crystalline mixture of this salt with the preceding is obtained, and separation is effected by treating this mixture with a solution of sodium hydrogen sulphite. The salt crystallises out in bright, olive-green needles.