Preparation, properties and uses of bleaching powder

Bleaching powder is also called calcium chlorohypochlorite because it is considered as a mixed salt of hydrochloric acid and hypochlorous acid. It is represented as:

Preparation

It is manufactured by the action of chlorine on dry slaked lime, Ca(OH)₂ at 40^oC.

This is the Odling view about its formation. There is another view proposed by Clifford according to which bleaching powder is a mixture of calcium hypochlorite and basic calcium chloride.

The manufacture of bleaching powder is carried out in Backmann's plant as follows:

It consists of a vertical cast-iron tower. The tower is provided with a hopper at the top, two inlets near the base (one for chlorine and other for hot air) and an exit for waste gases near the top.

The tower is fitted with eight shelves at different heights each equipped with rotating rakes. The slaked lime is introduced through the hopper and it comes in contact with chlorine, which slowly moves upwards. Bleaching powder is collected in a barrel at the base. The chlorine used in the manufacture of bleaching powder should be dilute and the temperature should be maintained below 40^oC.

Properties

1. It is a pale yellow powder. It has strong smell of chlorine. It is soluble in water but a clear solution is never formed due to the presence of impurities.

2. On long standing it undergoes auto-oxidation into calcium chlorate and calcium chloride.

3. In the presence of cobalt chloride it looses its oxygen.

4. On account of the formation of nascent oxygen, it shows oxidising and bleaching properties.

5. It loses its chlorine by the action of dilute acids (in excess).

The amount of chlorine obtained from a sample of bleaching powder by treatment with excess of dilute acids or carbon dioxide is called available chlorine. A good sample of bleaching powder contains 35 38% available chlorine.

Uses of Bleacing Powder

It is used

(i) as a disinfectant and germicide especially in the sterilization of drinking water.

(ii) for manufacture of chloroform.

(iii) for making wool unshrinkable.

(iv) as an oxidising agent in industry.

(v) mainly as bleaching agent for cotton, linen and wood pulp.

However, delicate articles like straw, silk, ivory, etc., are not bleached by bleaching powder.