What effect does high pH have on chlorine efficacy?

High pH levels reduce chlorine's effectiveness as a disinfectant primarily because of the relationship between pH and the dissociation of chlorine in water. Chlorine works effectively as a disinfectant in its hypochlorous acid (HOCl) form, which is more prevalent at lower pH levels. As pH increases, the proportion of chlorine that exists as hypochlorous acid decreases and more is present as hypochlorite ion (OCl-). The hypochlorite ion is a less effective disinfectant compared to hypochlorous acid.

For example, at a pH of 7.5, about 75% of chlorine exists as hypochlorous acid, while at a pH of 8.5, that percentage can drop significantly. This shift in the chemical form of chlorine directly impacts its ability to kill bacteria and other pathogens efficiently. Therefore, maintaining an appropriate pH level is crucial for optimal chlorine disinfection performance in aquatic facilities.