How does chlorine residual relate to pH levels?

Chlorine residual is significantly influenced by pH levels because the effectiveness of chlorine as a disinfectant varies with changes in pH. Chlorine exists in two forms in water: molecular chlorine (Cl2) and hypochlorous acid (HOCl). At lower pH levels, there is a higher proportion of hypochlorous acid, which is a strong disinfectant. As pH increases, more chlorine exists as hypochlorite ion (OCl-), which is less effective as a disinfectant.

Therefore, in order to maintain effective disinfection and adequate chlorine residual at higher pH levels, it is necessary to increase the amount of chlorine added to the water. This means that as the pH rises, a higher chlorine residual is required to achieve the same disinfection efficacy. Understanding this relationship is crucial for water operators to ensure safe drinking water and adherence to regulatory standards.