As the water pH increases, which substance will decrease in total amount?

As water pH increases, hypochlorous acid (HOCl) will decrease in total amount because hypochlorous acid is more stable and predominant at lower pH levels. At higher pH levels, the equilibrium shifts, and a greater proportion of hypochlorous acid converts to hypochlorite ion (OCl⁻), which is less effective as a disinfectant. This shift in the equilibrium can be attributed to the chemistry of chlorine in water; specifically, as the pH rises, the formation of hypochlorite ion increases, resulting in a reduction in the concentration of hypochlorous acid.

In contrast, chloramine and dichloramine are forms of chloramines formed when chlorine reacts with ammonia. Their levels can be affected by changes in pH, but they do not decrease in the same manner as hypochlorous acid does, since their stability can also be influenced by other factors in the water, like the amount of ammonia present. Sodium hypochlorite is a compound that dissociates to produce hypochlorous acid in solution, and thus its presence does not directly correlate to a decrease in hypochlorous acid without considering the pH's effects on the equilibria involved.