What occurs at a pH level of 5 in chlorinated water?

A pH level of 5 in chlorinated water is significant because it is within the range where hypochlorous acid is formed, and this compound is crucial for disinfection processes. Hypochlorous acid (HOCl) is the dominant species responsible for the antimicrobial effect when chlorine is introduced into water. Its effectiveness as a disinfectant is highly dependent on the pH level – with lower pH levels favoring its formation.

At a pH of 5, the concentration of hypochlorous acid is relatively high compared to higher pH levels, where the dominant species shifts to hypochlorite ion (OCl⁻), which is less effective as a disinfectant. Therefore, the lower pH leads to a higher availability of the active chlorine species that can kill or inactivate pathogens, making water treatment more effective.

Understanding pH's influence on chlorine chemistry is crucial for water operators to maintain effective disinfection and manage chlorine residuals efficiently.