How does pH influence disinfection efficiency in chlorine-based drinking water disinfection?

How does pH influence disinfection efficiency in chlorine-based drinking water disinfection?

• A. Lower pH favors HOCl, a more effective disinfectant; higher pH shifts to hypochlorite, reducing potency.
• B. Higher pH increases HOCl concentration and disinfectant potency.
• C. pH has no effect on chlorine-based disinfection.
• D. Disinfection efficiency is only influenced by chlorine concentration, not pH.

Answer: (A)

The key idea is that chlorine’s disinfection power depends on which chemical form is present, and pH dictates that balance. Hypochlorous acid (HOCl) is far more effective at killing microbes than the hypochlorite ion (OCl−). As the pH drops, the equilibrium shifts toward HOCl, boosting disinfection efficiency; as the pH rises, the balance shifts toward OCl−, reducing potency. The pKa of HOCl is around 7.5, so HOCl dominates at pH below this value and OCl− dominates above it. Therefore, a lower pH favors HOCl and a higher pH shifts to hypochlorite, which aligns with the chosen statement as the best description of how pH influences chlorine-based disinfection. The other ideas—pH having no effect, or disinfection depending only on chlorine concentration, or higher pH increasing HOCl—don’t fit the chemistry, since the effectiveness hinges on the HOCl vs OCl− balance driven by pH.

The key idea is that chlorine’s disinfection power depends on which chemical form is present, and pH dictates that balance. Hypochlorous acid (HOCl) is far more effective at killing microbes than the hypochlorite ion (OCl−). As the pH drops, the equilibrium shifts toward HOCl, boosting disinfection efficiency; as the pH rises, the balance shifts toward OCl−, reducing potency. The pKa of HOCl is around 7.5, so HOCl dominates at pH below this value and OCl− dominates above it. Therefore, a lower pH favors HOCl and a higher pH shifts to hypochlorite, which aligns with the chosen statement as the best description of how pH influences chlorine-based disinfection. The other ideas—pH having no effect, or disinfection depending only on chlorine concentration, or higher pH increasing HOCl—don’t fit the chemistry, since the effectiveness hinges on the HOCl vs OCl− balance driven by pH.