The hydrolytic constant of hypochlorous acid has apparently not been determined but as the acid is weaker than carbonic acid, which has a hydrolytic constant of 1 × 10^{-4}, the value is probably between 1 × 10^{-3} and 1 × 10^{-4}. From the formula _x_^{2}/(1 - _x_)_v_ = _k__{_wv_} in which 1 mole of pure Ca(OCl)_{2} is dissolved in _v_ litres, _x_ is the fraction hydrolysed, and _k__{_wv_} is the hydrolytic constant, complete hydrolysis occurs (_x_ = 1) when _v_ is not greater than 1 × 10^{4} litres. This is equivalent to a concentration of not less than 7.1 p.p.m. of available chlorine. Solutions of pure hypochlorites are alkaline in reaction because of the excess of hydroxyl ions (minimum concentration 1 × 10^{-4}). In solutions of bleach the hydrolytic action is retarded by the OH' due to the free base, and accelerated by the excess of H^{.} caused by the dissociation and partial hydrolysis of CaCl_{2}; the final result is determined by the relative proportions and the effect of the free base usually preponderates. The addition of any substance that reduces the OH' concentration enables hydrolysis to proceed to completion and affords a rational explanation of the fact that solutions of bleach, on distillation with such weak acids as boric acid, yield a solution of hypochlorous acid. It also explains why the addition of an acid is necessary in Bunsen's method (_vide_ p. 79) of analysing hypochlorite solutions. It has been stated that when hydrochloric acid is employed the increase in the oxidising power is due to the action of the acid upon calcium chloride but this never occurs under ordinary conditions; weak acids such as carbonic or acetic will give practically the same result as hydrochloric acid in solutions of bleach of the strength used in water treatment. The slightly higher result obtained with strong acids is due to the decomposition of chlorates. The effect of dilution alone is shown by the data given below. A 2 per cent bleach solution, containing very little excess base, was diluted with distilled water and the various dilutions titrated with thiosulphate after the addition of potassium iodide. In one series the solutions were titrated directly, and after acidification in the other. The results[A] were as follows: HYDROLYSIS OF BLEACH SOLUTION -----------------------------------+----------------------- Strength of Solution. Grams Bleach | Direct Titration × 100 Per 100 c.cms. | --------------------. | Acid Titration -----------------------------------+----------------------- 2.0 | 30.8 0.2 | 34.3 0.1 | 41.8 0.02 | 67.5 0.002 | 100.0 -----------------------------------+-----------------------