How does an increase in pH affect the disinfection power of free chlorine?

An increase in pH decreases the disinfection power of free chlorine, which is due to the chemical nature of chlorine in relation to pH levels. Free chlorine exists in two forms in water: hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). Hypochlorous acid is the form that is more effective as a disinfectant; it is associated with higher antimicrobial activity.

As the pH of water rises, the balance between these two forms shifts. Specifically, at higher pH levels, the proportion of hypochlorite ion increases while the concentration of hypochlorous acid decreases. This shift reduces the overall disinfection capability of free chlorine. Effective disinfection is optimal at a lower pH (around 7.0-7.2), where the concentration of hypochlorous acid is maximized. Therefore, higher pH results in a less effective disinfectant, which is why the choice indicating that an increase in pH decreases the disinfection power is correct.