How does chlorine's disinfection effectiveness change with pH levels?

The effectiveness of chlorine as a disinfectant is influenced by the pH of the water, with its most potent form being hypochlorous acid (HOCl). At lower pH levels, particularly around pH 7.4, more hypochlorous acid is present compared to a higher pH level such as 8.2. This is significant because hypochlorous acid is much more effective at killing microorganisms than hypochlorite ion (OCl⁻), which predominates at higher pH levels.

At pH 7.4, the disinfection potential of chlorine is enhanced due to the increased availability of hypochlorous acid. In contrast, as the pH rises to 8.2, the balance shifts toward hypochlorite ion, which is less effective for disinfection purposes. Therefore, the answer indicating that chlorine's disinfection effectiveness is higher at pH 7.4 than at pH 8.2 accurately reflects the relationship between chlorine species and pH, establishing why maintaining an optimal pH in water treatment is essential for effective disinfection. The effectiveness of chlorine does not remain constant regardless of pH, nor does it vary with temperature in relation to the pH context