The constitution and formation of bleaching powder

Abstract "Bleaching powder" is made by the action of chloride gas on calcium hydroxide, the reaction being essentially: 2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2O. The constitution of the solid formed in this way has been the subject of investigation and speculation for many years, since its properties are not those to be expected of a mixture formed according to the above equation. In the first place, the reaction represented by the above equation never goes to completion; commercial bleaching powder usually contains 15-25% of calcium hydroxide and only about 35% chlorine available for oxidation; while even under special laboratory conditions it is extremely difficult to reduce the calcium hydroxide content below 5%. Secondly, the physical properties of bleaching powder are not those of a simple mixture of calcium hypochlorite, calcium chloride and unreacted calcium hydroxide; for instance, ordinary bleaching powder is not markedly deliquescent, and therefore does not contain free calcium chloride.In the account which follows, no attempt is made to summarize the whole of the previously published work; references are given only to publications which appear to have immediate connection with the issues raised here.