What is the relationship between pH and effective chlorine residual?

The relationship between pH and effective chlorine residual is fundamentally linked to the chemical forms that chlorine takes in water. As the pH increases, the proportion of hypochlorite ion (OCl−), which is a less potent form of chlorine, increases. This shift leads to a decrease in the concentration of hypochlorous acid (HOCl), the more effective disinfectant form of chlorine.

At lower pH levels, hypochlorous acid predominates, which has a greater ability to inactivate pathogens and thus results in a higher effective chlorine residual. Conversely, at higher pH levels, the efficiency of chlorine as a disinfectant diminishes owing to the dominance of hypochlorite ions. Therefore, when pH rises, the effective chlorine residual decreases. This dynamic is critical for water treatment operators to monitor and manage in order to ensure adequate disinfection and water quality.

Understanding this relationship is crucial for maintaining effective chlorine levels in water distribution systems, as it directly impacts the system's ability to effectively control microbial contamination.