At what pH does the highest concentration of hypochlorous acid occur?

The highest concentration of hypochlorous acid occurs at a pH around 5. This is significant because hypochlorous acid (HClO) is a crucial disinfectant in water treatment processes. The equilibrium between hypochlorous acid and hypochlorite ion (OCl-) is pH-dependent, with hypochlorous acid being the more effective form for disinfection.

At lower pH levels, such as 3, the concentration of hypochlorous acid increases, but it also becomes more unstable and can lead to the formation of chlorine gas, which is undesirable. As the pH increases toward neutral (around pH 7), hypochlorous acid begins to dissociate into hypochlorite ions, which, while still effective, are not as efficient as hypochlorous acid in terms of disinfection strength.

At pH levels above 7, particularly at around 9, the proportion of hypochlorite increases significantly, reducing the effectiveness of the solution as a disinfectant. Therefore, a pH of 5 strikes a balance by maintaining a high concentration of the more effective hypochlorous acid while still being stable enough for practical use in water treatment.