Calcium hydrate and hydrochloric acid are both practically completely dissociated, i.e. there is a large and equal quantity of H· and OH′, and the product is much greater than Kw (ionic product of water), and hence there is a combination of these ions, leaving the solution neutral and no undissociated acid or base exists. This statement is only approximately correct as hydrochloric acid is slightly more dissociated than calcium hydroxide (ratio 9 : 8) and the solution is consequently slightly acid, i.e. the H· concentration is greater than 1 × 10-7.
Hypochlorous acid is only very slightly dissociated, especially in the presence of the OCl′ ion due to the dissociation of the Ca(OCl)2, as compared with Ca(OH)2 and hydrolysis of the Ca(OCl)2 proceeds with increased dilution. The action is best represented by the equation
2Ca(OCl)2 + 2H2O ⇄ CaCl2 + Ca(OH)2 + 2HOCl
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 x2⁄(1 - x)v = kwv in which 1 mole of pure Ca(OCl)2 is dissolved in v litres, x is the fraction hydrolysed, and kwv is the hydrolytic constant, complete hydrolysis occurs (x = 1) when v is not greater than 1 × 104 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 CaCl2; 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 Per 100 c.cms. |
| |||
| 2.0 | 30.8 | |||
| 0.2 | 34.3 | |||
| 0.1 | 41.8 | |||
| 0.02 | 67.5 | |||
| 0.002 | 100.0 |
[A] Corrected for the alkali produced by HClO + 2KI = KCl + KOH + I2.
Although every precaution was taken to exclude carbonic acid, a portion of the hydrolysis was probably due to this acid, which would remove calcium hydrate from the sphere of action and consequently alter the equilibrium. The above figures are only applicable to the particular sample used; other samples containing different excesses of base would yield different hydrolytic values. The results are in agreement with the hypothesis presented and confirm the theoretical deduction that very dilute bleach solutions are completely hydrolysed if no salts are present that will dissociate and increase the OH′ concentration. Hydrolysis is reduced by caustic alkalies and alkaline carbonates, and increased by acids and acid carbonates that reduce the OH′ concentration.
The effect of chlorides is anomalous and no adequate explanation for their action can be given at present. The addition of small quantities of sodium chloride (0.1 per cent) increases the hydrolysis of bleach solutions but much larger quantities tend to the opposite direction.