At pH values above 7.5, the disinfectant capability of chlorine is

CONCEPT

Free chlorine in water exists mainly as hypochlorous acid (HOCl) and hypochlorite ion (OCl−), linked by the equilibrium HOCl ⇌ H+ + OCl−. The pKa is about 7.5, so pH controls their relative proportions.

HOCl is substantially more germicidal than OCl−. Therefore, any pH shift that lowers the HOCl fraction lowers disinfection effectiveness at the same free-chlorine dose.

APPLICATION

1. Above pH 7.5, the acid–base equilibrium shifts toward OCl−.

2. The fraction of the more effective HOCl form consequently decreases.

3. The same free-chlorine dose then has markedly lower germicidal action.

CONTRAST

• “Greatly enhanced” would require a major rise in the HOCl fraction, but alkaline conditions shift the equilibrium the other way.

• “Slightly enhanced” also assumes an increase in the more active HOCl form, which does not occur above the pKa.

• “Not changed” ignores the pH-dependent conversion of HOCl into the less germicidal OCl− form.

CROSS-CHECK

At pH 7.5 the two species are present in roughly equal proportions; raising pH makes OCl− predominant. This independently confirms that chlorine disinfection becomes much less effective.

Result: the disinfectant capability is greatly reduced.