Magnesium Sulphate

It occurs in nature as minerals kiesserite (MgSO₄.H₂O), Epsom salt (MgSO₄.7H₂O) and kainite (KCl.MgSO₄.3H₂O).

Preparation

It is formed by reacting magnesite (MgCO₃) or dolomite with dilute sulphuric acid.

MgCO₃ + H₂SO₄ →  MgSO₄ + H₂O + CO₂

MgCO₃.CaCO₃ + 2H₂SO₄ → MgSO₄ + CaSO₄ + 2CO2 + 2H₂O

Dolomite (Insoluble)

It is commercially prepared by boiling kieserite mineral in water. The crystals are obtained when the solution is cooled.

MgSO₄.H₂O + 6H₂O → MgSO₄.7H₂O

Properties

It is a colourless crystalline compound, soluble in water. The crystals are efflorescent and bitter in taste. It is isomorphous with ZnSO₄.7H₂O. It forms double sulphates with alkali metal sulphates, K₂SO₄.MgSO₄.6H₂O (Schonite).

Heating effect: When heated to 150°C, it changes to monohydrate. On further heating, it becomes anhydrous at 200°C. On strong heating, it decomposes into MgO.

MgSO₄.7H₂O MgSO₄.H₂O MgSO4MgO + SO₂ + ½O₂

Magnesium sulphate is reduced by lampblack at 800°C.

2MgSO₄ + C → 2MgO + 2SO₂ + CO₂

Uses

(i) as a purgative in medicine.

(ii) as a filler for paper.

(iii) as a mordant in dyeing and tanning industry.

(iv) in the manufacture of paints and soaps and in fire−proofing fabrics.