At which pH level does chlorine disinfect the best?

Chlorine disinfection is most effective at low pH levels, typically around 6 to 7. At this pH range, chlorine exists primarily in the form of hypochlorous acid (HOCl), which is a much stronger disinfectant than hypochlorite ion (OCl-) which is present at higher pH levels. The hypochlorous acid can penetrate microbial cell walls more readily and effectively kill pathogens.

As the pH increases, the proportion of hypochlorous acid decreases and the concentration of the less effective hypochlorite ion increases, reducing the overall effectiveness of chlorine as a disinfectant. Therefore, maintaining a low pH level enhances the disinfection process by ensuring a higher concentration of hypochlorous acid is available to engage with and neutralize harmful microorganisms.