Calcium Hypochlorite, Ca(OCl)₂.4H₂O

Calcium Hypochlorite, Ca(OCl)₂.4H₂O, may be obtained in feather-like crystals by freezing a solution of bleaching powder and carefully thawing the cryohydrate on a filter, or by evaporating a solution of bleaching powder over sulphuric acid in vacuo. Von Tiesenholt tried to obtain it by the action of chlorine monoxide on dry calcium hydroxide, but the product was not pure because the lime did not take up its equivalent of acid anhydride.

In 1906 the Grieslieim Elektron Company took out a patent for the manufacture of calcium hypochlorite by treating cold milk of lime with chlorine. By evaporation in vacuo a crystalline hypochlorite is first formed. This is washed and dried in vacuo to remove the water of crystallisation, as the salt is more stable in the anhydrous state. Under certain conditions basic hypochlorites may be formed. Two have been isolated of composition Ca(OCl)₂.2Ca(OH)₂, and Ca(OCl)₂.4Ca(OH)₂. The commercial anhydrous hypochlorite, known by the trade name of " Hyporit," is more stable than ordinary bleaching powder. It gives a clear solution in water and contains 80 to 90 per cent, of bleaching chlorine. The solution smells strongly of chlorine monoxide. It can be stabilised by introducing lime to the extent of 25 to 30 per cent. This settles to the bottom as " lime sludge," and, mixed with a certain amount of fresh lime, can be used for increasing the stability of a further quantity of solution. Magnesium hydroxide cannot replace the lime for this purpose. The stability is not much influenced by concentration and temperature.

Vincent and Gaillard pointed out that, as a purifying agent for drinking-water, calcium hypochlorite had certain practical advantages over alkali hypochlorites, because it could be conveniently used in compressed tablets. A litre of water, treated with a tablet containing 3.5 mgm. of available chlorine, should be ready for drinking twenty minutes after the addition.