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The bleaching action of chlorine water may also be regarded as due to the hypochlorous acid it contains, although a considerable amount of free chlorine is present, since the reaction: Cl2 + H2O <=> HCl + HOCl <=> H۰ + Cl' + HOCl, is reversible.
The equations: Cl2 + H2O = 2HCl + O, and HOCl = HCl + O, show that hypochlorous acid, for the same weight of chlorine, has twice the bleaching activity of free chlorine. There is, therefore, no loss of bleaching activity when the chlorine is first absorbed by alkali, although half is converted into inert chloride. It is the available oxygen liberated from HOCl which causes the bleaching action.
If chlorine water is distilled, hypochlorous acid comes over with free chlorine, leaving aqueous hydrochloric acid. In this case the equilibrium: Cl2 + H2O <=> HOCl + HCl, is disturbed by the removal of the volatile constituent HOCl (or its anhydride, Cl2O: 2HOCl <=> Cl2O + H2O). The reaction goes on practically to completion. But if chlorine water is boiled in a flask under a reflux condenser, so that the distillate constantly flows back, it is not decomposed but remains unchanged (Richardson, 1903). In this case the equilibrium is not disturbed, since no constituent is removed from the sphere of action. In N/20 chlorine water about 30 per cent., and in N/100 about 70 per cent., of the chlorine is hydrolysed into HOCl and HCl.
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