At what pH level does chlorine become less effective as a disinfectant?

Chlorine is used as a disinfectant in water treatment, but its effectiveness is influenced by the pH level of the water. As the pH increases, particularly above 8.0, the equilibrium of chlorine species shifts. Chlorine exists in different forms depending on the pH, primarily as free chlorine (hypochlorous acid and hypochlorite ion). At higher pH levels, the proportion of hypochlorous acid, which is the form that is more effective for disinfection, decreases, and the hypochlorite ion predominates.

The hypochlorite ion is a weaker disinfectant compared to hypochlorous acid. Therefore, at a pH above 8.0, the overall effectiveness of chlorine as a disinfectant reduces significantly, making it less efficient in killing microorganisms and providing safe drinking water. This essential knowledge is critical for water operators as maintaining appropriate pH levels is vital for effective disinfection processes.