It has often been assumed that hypochlorite solutions are decomposed on addition to water containing free or half-bound carbonic acid with the production of free chlorine, but no evidence has been adduced in support. Free chlorine can be separated from hypochlorous acid in aqueous solution by extraction with carbon tetrachloride and when this solvent is shaken with a carbonated hypochlorite solution it is found that only traces of chlorine are removed.
Hypochlorous acid reacts with hydrochloric acid with the evolution of free chlorine HClO + HCl = Cl2 + H2O but in very dilute solution the amount of free chlorine formed is exceedingly minute. Jakowkin[10] has shown that this reaction does not proceed to completion and that the concentration of free chlorine can be calculated from the equation HClO × H· × Cl′ = 320Cl2 in which the reactions are expressed in gram molecules per litre. The hydrogen ions and chlorine ions are obtained from the dissociation of carbonic acid (H2CO3 ⇄ H· + HCO3′) and chlorides (NaCl ⇄ Na· + Cl′) and also by the dissociation of hydrochloric acid produced by the interaction of hypochlorous acid and organic matter. HClO = O + HCl ⇄ H· + Cl′. If the formula of Jakowkin can be correctly applied to solutions containing fractions of a part per million of hypochlorous acid the free chlorine liberated by the addition of 1 p.p.m. of bleach to a water low in chlorides would be of the order 10-7-10-8 p.p.m.
Sodium hypochlorite is probably hydrolysed in dilute solution in a manner similar to that of bleach.
2NaOCl = NaCl + NaOH + HClO.
For solutions containing equal amounts of available chlorine, electrolytic sodium hypochlorite is more dissociated than bleach because of the absence of an excess of base, and this, together with the presence of sodium chloride, accounts for the slightly higher germicidal velocity obtained. The experience of pulp mills, with bleach and electrolytic hypochlorites, confirms this: the latter is a much quicker bleaching agent than bleach and it is often so rapid as to make it desirable to reduce the velocity by the addition of soda ash.
Regarding hypochlorite solutions a phenomenon of more scientific interest than of practical importance has been noted by Breteau[12] who found that alkaline solutions of sodium hypochlorite containing 0.94 per cent of available chlorine lost 3.6 per cent of their titer on dilution with 80 volumes of water; also that this loss was increased by the addition of small quantities of salt (sodium chloride) and more so by carbonates and bicarbonates. The author has noted similar losses on diluting bleach solutions and that the loss increased on standing. The loss can be explained by the decomposition of hypochlorous acid, in the presence of light, into hydrochloric acid and oxygen. 2HClO = 2HCl + O2
Chlorine Water. When a solution of chlorine in water is used as a germicide the chemical reactions that occur differ materially from those of hypochlorite solutions. On solution in water, hydration or solvation probably takes place with the production of heat. Cl2·Aq. = 2,600 calories. Chlorine water is comparatively stable but decomposes under the influence of light in accordance with the equation Cl2 + H2O = 2HCl + O; a similar reaction occurs in the presence of organic matter or any substance capable of oxidation. Chlorine water contains only minute traces of hypochlorous acid and there is no evidence that the endothermic reaction
Cl2·Aq + H2O = HClO·Aq + HCl·Aq
-2600 - 68,460 = -29,930 - 39,315 - 1815
occurs in a measurable degree.
From thermochemical considerations hypochlorous acid and chlorine water should be about equally active as oxidising agents.