Preparation, properties and uses of epsom salt

Magnesium sulphate (MgSO

₄.7H₂O)

Magnesium sulphate heptahydrate is commonly known as epsom salt because it was formerly prepared from water of the Epsom springs in England.

Occurrence

Magnesium sulphate occurs in nature as,

Kieserite MgSO₄.H₂O

Epsomite MgSO₄.7H₂O (in certain gypsum deposits).

It is also present in certain mineral springs.

Preparation

Laboratory preparation

In laboratory, magnesium sulphate is prepared by dissolving magnesium, its oxide, hydroxide or carbonate in dilute sulphuric acid and evaporating the resulting solution, when colorless efflorescent crystals of the heptahydrate, MgSO₄.7H₂O, crystallize out.

From magnesite

Magnesium sulphate can also prepared by dissolving magnesite in boiling dilute sulphuric acid, and crystallizing the hydrate of magnesium sulphate.

From dolomite

Magnesium sulphate is also made by dissolving dolomite in boiling

dilute sulphuric acid.

Insoluble calcium sulphate is removed by filtration. MgSO₄.7H₂O crystals are obtained from the filtrate.

Properties

• Magnesium sulphate is a colorless, efflorescent crystalline substance having a bitter taste. It is isomorphous with ZnSO₄.7H₂O.

• Magnesium sulphate is readily soluble in water (35.5 g of the salt dissolves in 100 g of water at 20°C).

Action of heat

When heated to 150°C, magnesium sulphate heptahydrate changes to monohydrate, MgSO₄.H₂O, which at 200°C gives anhydrous magnesium sulphate. On further heating, it is decomposed forming the oxide.

Double salt formation

Magnesium sulphate readily forms double salts with alkali metal sulphates.

Example: MgSO₄.K₂SO₄.6H₂O.

Uses

• In medicine as mild purgative.

• In industry as a weighing material for cotton and silk.

• In fire-proofing fabrics.

• As a filler for paper.

• As a mordant- in dyeing and in tanning industry.

• In the manufacture of soaps and paints.