At what pH does water have the highest concentration of hypochlorous acid?

The correct choice is based on the chemistry of chlorine when it is used as a disinfectant in water treatment. Hypochlorous acid (HOCl) is formed when chlorine gas (Cl2) is dissolved in water, and it is the form that is most effective for disinfecting.

The pH of the water significantly influences the relative concentrations of hypochlorous acid and hypochlorite ion (OCl^-), which are the two primary species of chlorine that exist in solution. Hypochlorous acid is favored at lower pH levels, while hypochlorite predominates at higher pH levels.

As the pH increases, the dissociation of hypochlorous acid leads to the formation of more hypochlorite ion, decreasing the concentration of hypochlorous acid. Therefore, the highest concentration of hypochlorous acid typically occurs at a lower pH, around pH 5 to pH 6. While hypochlorous acid does exist in significant amounts at neutral pH (around pH 7) and exhibits disinfectant properties, it is considerably more effective and has a higher concentration in the lower pH range rather than at pH 10 or pH 12