Magnesium Sulphite, MgSO₃

Crystals of MgSO₃.6H₂O are obtained by the action of sulphurous acid on magnesium, magnesium oxide, or magnesium carbonate, or by the double decomposition of magnesium sulphate and sodium sulphite. They dissolve in 80 parts of cold water and in 120 parts of hot, though the action of hot water apparently produces a basic salt. Basic magnesium sulphite, MgSO₃+2Mg(OH)₂, prepared by passing sulphur dioxide into water containing magnesia in suspension, has been used in sugar refining. The acid sulphite could not be prepared in the crystalline form. Evaporation at high temperature of the product of the reaction between sulphurous acid and magnesium hydroxide is said to yield crystals of MgSO₃.3H₂O. Rhombic crystals of MgSO₃.7H₂O were reported, which lost 1 molecule of water at 52° C.

The hexagonal crystals of the hexahydrate oxidise slowly to sulphate in air, lose their water above 200° C., decompose into oxide and sulphur dioxide when heated strongly, and become the monohydrate at 100° C. Sulphur dioxide slowly converts magnesia into the sulphite at about 326° C., which then decomposes at a slightly higher temperature.

The following double sulphites of magnesium and ammonium have been prepared: 3MgSO₃.(NH₄)₂SO₃.18H₂O, MgSO₃.(NH₄)₂SO₃.12H₂O, and MgSO₃.(NH₄)₂SO₃.6H₂O.