Oxides And Oxy-acids Of Chlorine : Hypochlorites, Action Of Chlorine On Alkalies

When a stream of chlorine is passed into a cold solution of caustic potash so that excess of alkali remains, a liquid smelling like chlorine, but with a difference, is obtained. This liquid, discovered by Berthollet in 1789, is more stable than chlorine water and was used under the name of eau de Javelle for bleaching. In England about 1798 the absorption was carried out with milk of lime. Tennant, of St. Rollox (Glasgow), in 1799 found that chlorine is absorbed by dry slaked-lime, and the product, called bleaching powder, gave a bleaching liquor on treatment with water.

Balard, in 1834, showed that these bleaching substances contain salts of hypochlorous acid, HOCl. The reactions lead to the formation of a mixture of a hypochlorite and chloride:

2KOH + Cl₂ = KCl + KOCl + H₂O;

2Ca(OH)₂ + 2Cl₂ = CaCl₂ + Ca(OCl)₂ + 2H₂O.

With caustic soda a solution containing sodium hypochlorite, NaOCl, and sodium chloride, is formed. This is also produced by adding sodium carbonate to a solution of bleaching powder, and filtering off the precipitated calcium carbonate:

Ca(OCl)₂ + CaCl₂ + 2Na₂CO₃ = 2NaOCl + 2NaCl + 2CaCO₃,

or more usually by the electrolysis of brine, so that the chlorine liberated at the anode is allowed to mix with the caustic soda produced at the cathode, and the liquid is kept cool.

Acids, even carbonic acid, e.g., atmospheric carbon dioxide, liberate the very weak hypochlorous acid from its salts; solutions of these smell of the free acid when exposed to air, and exhibit bleaching properties.

Expt. 1. - Pass chlorine into cold dilute caustic soda solution. Take a piece of Turkey red cloth and paint on it a device with a mixture of gum and tartaric acid. Dry the cloth in a steam-oven and then immerse it in the hypochlorite solution (containing a slight excess of alkali). The colour is discharged only where the acid was applied. Now pass a stream of carbon dioxide through the liquid: the colour is now completely discharged: NaOCl + CO₂ + H₂O = NaHCO₃ + HOCl.

The bleaching action of hypochlorous acid is due to oxidation: HOCl = HCl + O. Many colouring matters when oxidised yield colourless or feebly-coloured products. The remaining hypochlorous acid is removed by washing, and finally by treating with a substance such as sodium thiosulphate, which decomposes the hypochlorous acid and is hence called an antichlor. Paper pulp, prepared from wood, is bleached with sodium hypochlorite solution and acid.

Protein crystallography
Chemistry guide
Oxides And Oxy-acids Of Chlorine
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