Which statement best describes chlorine disinfection efficiency relative to pH?

• A. More effective at higher pH
• B. Unchanged by pH
• C. Only effective at acidic pH
• D. More effective at lower pH

Answer: (D)

Chlorine’s ability to disinfect depends on its chemical form in water. At lower pH, chlorine is mainly as hypochlorous acid (HOCl), which is a much more effective disinfectant and readily inactivates microbes. As pH rises, HOCl shifts to hypochlorite ion (OCl−), which is weaker at disinfection. So, chlorine works best at lower pH and becomes less effective as pH increases. This explains why the statement that chlorine disinfection is more effective at lower pH is the best description. Other factors like temperature and contact time also influence performance, but the pH trend remains: higher efficiency at lower pH.