Which statement best describes the effect of rising pH on chlorine's disinfection ability?

• A. More effective
• B. Unchanged
• C. Unpredictable
• D. Less effective

Answer: (D)

Chlorine’s disinfection power depends on which form of chlorine is present in water. When pH rises, the balance shifts from hypochlorous acid (the stronger disinfectant) toward hypochlorite ion (the weaker form). Hypochlorous acid can penetrate microbial cells more effectively, while hypochlorite ion is less capable of doing so. As more chlorine becomes hypochlorite with higher pH, the overall disinfection ability decreases. So, rising pH makes chlorine less effective at killing microorganisms. At lower pH, HOCl dominates and disinfection is stronger; at higher pH, OCl- dominates and disinfection wanes.