At what pH does chlorine destroy bacteria most rapidly?

Chlorine is an effective disinfectant used in water treatment to destroy bacteria, and its effectiveness is significantly influenced by the pH of the water. At a pH around 7.5, chlorine exists primarily in its hypochlorous acid form (HOCl), which is the more potent and active species for disinfection. This optimal pH range allows for a more rapid reaction between the chlorine and bacteria, leading to more efficient microbial inactivation.

As the pH increases towards higher levels, such as 8.5 and 9.5, the proportion of hypochlorous acid decreases, shifting towards hypochlorite ion (OCl−), which is less effective at a higher pH. Conversely, at a lower pH like 5.0, while hypochlorous acid concentration is high, the overall stability of chlorine can be affected, and there may be increased formation of chlorine gas, which is not desirable in a water treatment process.

Therefore, the pH of 7.5 represents the sweet spot for chlorine's bactericidal action, resulting in the most rapid destruction of bacteria in treated water.