Reductive Bleaching

Sodium Hydrosulphite

Formula: Na2S2O4

Molecular weight:174.10
sodium dithionite is also used as a bleaching agent in reductive bleaching processes.
Examples are bleaching of mechanical paper pulp and the bleaching of cotton and wool. It especially reduces carbonyl and alcohol functional groups, which are responsible for the colour of the textile fabric. Unfortunately these processes are not 100 % effective. Therefore a reductive bleaching process is often preceded by an oxidative one (with hydrogen peroxide, sodium hypochlorite or sodium chlorite).
Suphur 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

1.Sulphur dioxide reacts with water to liberate nascent hydrogen.

2.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.