hypochlorite, NaOCl

A solution of the hypochlorite in water is obtained by the action of chlorine on an aqueous solution of sodium hydroxide at a low temperature -

2NaOH+Cl₂ =NaOCl+NaCl+H₂O.

Solutions containing a high percentage of sodium hydroxide yield a concentrated solution of the hypochlorite, provided additional alkali is introduced periodically to maintain the concentration of the solution. Under these conditions, the sodium chloride produced by the reaction is precipitated, and a solution of hypochlorite obtained which reacts with hydrochloric acid to yield up to 49.2 grams of chlorine per 100 c.c. of solution. The temperature of the reaction should be maintained below 27° C., but the very concentrated solutions obtained lack stability, the hypochlorite changing to a mixture of chloride and chlorate, one molecule being oxidized at the expense of another. From very concentrated solution sodium hypochlorite separates as a solid hydrate, its composition approximating more nearly to that of the heptahydrate than the hexahydrate. It is a very unstable, hygroscopic substance, but dehydration in a current of dry air at reduced pressure converts it into the solid anhydrous hypochlorite, which melts about 45° C. It is less hygroscopic than the hydrate, and contains 40 to 60 per cent, of available chlorine.

When the turbid liquid formed by heating the heptahydrate at 20° C. is cooled slowly to the ordinary temperature, large greenish-yellow, very deliquescent crystals of the pentahydrate are formed. They melt at 27° C., and are stable at ordinary temperature in absence of air.

Sodium hypochlorite is also manufactured by the electrolysis of sodium-chloride solution without a diaphragm, the solution being less concentrated than that prepared by the chlorine process from sodium hydroxide, but free from the excess of alkali characteristic of that prepared by the older method. The process is carried out either in the apparatus designed by Kellner, or in that of Haas-Oettel, sodium chlorate being a by-product (v. infra). It is noteworthy that electrolysis of sodium-chloride solution with an alternating current also produces sodium hypochlorite.

Thomsen gives for the heat of formation of sodium hypochlorite in aqueous solution from its elements the value 83.36 Cal., Berthelot 84.7 Cal. For the molecular depression of the freezing-point in aqueous solution Raoult found the value 3.38° C.

In aqueous solution sodium hypochlorite finds technical application in the bleaching of paper, linen, cotton, and straw. In direct sunlight, concentrated solutions rapidly lose their activity. Storage in colourless bottles accelerates the rate of decomposition, and in brown bottles retards it. The stability of the solutions is much increased by complete exclusion of light.