Basic ferrous sulphate, FeSO₄.FeO

Basic ferrous sulphate, FeSO₄.FeO, is apparently obtained in solution when a 30 per cent, solution of ferrous sulphate is electrolysed between iron electrodes by a very weak current in the dark. After a few weeks the liquid becomes dark green in colour, but upon exposure to light it decomposes, depositing ferrous hydroxide in the absence of air.

The acid sulphate, FeSO₄.5H₂SO₄.5H₂O, is obtained when excess of sulphuric acid is added to a concentrated solution of ferrous sulphate. It yields iridescent crystals, with a brilliant lustre, but which are unstable, decomposing at 95° to 97° C. into the monohydrate. Thus: -

FeSO₄.5H₂SO₄.5H₂O = FeSO₄.H₂O + 5H₂SO₄.4H₂O.

Other acid sulphates, namely 2FeO.3SO₃.2H₂O, FeO.2SO₃.H₂O, and FeO.4SO₃.3H₂O, have been described.

The following acid double sulphates have been prepared: -

2FeSO₄.Cr₂(SO₄)₃.H₂SO₄.2H₂O,
2FeSO₄.Al₂(SO₄)₃.H₂SO₄,
2FeSO₄.2ZnSO₄.H₂SO₄,
2FeSO₄.2ZnSO₄.H₂SO₄, and
FeSO₄.NiSO₄.H₂SO₄.