Bleaching Action of Sulphur Dioxide

Dry sulphur dioxide does not have any bleaching action. However, if the gas is moist, or if the object to be bleached is moist, then bleaching takes place. Usually only delicate articles such as coloured flowers, silk, wool etc., are bleached by sulphur dioxide.

The bleaching action takes place in two steps.

First step

Sulphur dioxide reacts with water to liberate nascent hydrogen.

2H2O(l) + SO2(g) -----------------> H2SO4(aq) + 2[H]
water   sulphur dioxide       sulphuric acid   nascent hydrogen

Second step

The nascent hydrogen so formed, then adds to the coloured matter. As a result the colour is removed and bleaching takes place.

Coloured matter + Nascent hydrogen -------------------> Colourless matter

The bleaching by sulphur dioxide is not permanent. The reaction is a unstable and reversible one; when exposed to the atmosphere, the bleached matter gets oxidized, and regains its original color.

Bleached matter + Oxygen -------------> Original colour

Dipping the bleached article in dilute sulphuric acid can also restore the original colors.

Comparison of Bleaching Action of Sulphur Dioxide and Chlorine

Sulphur dioxide	Chlorine
Similarity
Bleaches only in the presence of moisture	Bleaches only in the presence of moisture
Differences
Bleaches by reduction 2H2O(l) + 2O2 --> H2SO4 (aq) + 2[H] or Water +Sulpur dioxide --> Sulphuric acid +Nascent hydrogen Coloured matter + [H] —> Colourless matter	Bleaches by oxidation H2O(1) + Cl2 (g) -------- > HCl(aq) + HOCl(aq) HOCl(aq) ---------> HCI(aq) + [O] Coloured matter + [O] —> Colourless matter
Bleaching is temporary. Colour is restored when exposed to air or treated with dilute sulphuric acid	Bleaching is a permanent. Original colour cannot be restored when exposed to air or when treated with dilute sulphuric acid
It is a mild bleaching agent. It does not destroy the material being bleached	It is a powerful bleaching agent. Many materials get destroyed when treated with chlorine
Used for bleaching delicate materials like silk, wool, straw etc.	Used for bleaching tougher materials like cotton, jute, wood pulp etc.

 

The properties of sulphur dioxide are summarized below (Fig. 7.7).