Q. Describe Peroxomonosulphuric Acid?

It is also known as permonosulphuric acid. It is obtained by the action of chlorosulphuric

acid on cold anhydrous hydrogen peroxide :

H₂O₂ + ClSO₂.OH --------------> H-O-O-SO₂.OH +HCl

It may also be prepared by the action of concentrated sulphuric acid on 5% hydrogen peroxide:

H₂O₂ + H₂SO₄ --------------> H-O-O-SO₂.OH + H₂O

Peroxomonosulphuric acid may also be obtained by hydrolysis of peroxodisulphuric acid at 273 K or by grinding potassium peroxodisulphate crystals with sulphuric acid in a freezing mixture.

H₂S₂O₈ + H₂O-------------> H₂SO₅ + H₂SO₄

K₂S₂O₈ + H₂SO₄---------> H₂SO₅ + K₂S₂O7

Peroxomonosulphuric acid is a white crystalline solid, m. p. 318 K. However, the salts of this acid are unstable in the solid state. The acid is a strong oxidising agent. It oxidises SO₂ to SO₃, sulphites to sulphates, ferrous salts to ferric salts and liberates iodine immediately from a solution of potassium iodide:

2KI + H₂SO₅-----------> K₂SO₄ + H₂O + I₂

2FeSO₄ + H₂SO₅------------> Fe₂ (SO₄)3+ H₂O