The rate of formation of _____ increases with increasing pH

• A. HAA5
• B. nitrates
• C. THMs
• D. organophosphates

Answer: (D)

Raising pH changes how chlorine reacts with compounds in water, and some phosphorus-containing substances become more prone to form when the solution is more alkaline. Organophosphate compounds often undergo base-catalyzed reactions, such as hydrolysis, that accelerate as hydroxide ions become more available. When pH increases, these reactions proceed faster, so more organophosphate byproducts can form during chlorination.

In this context, the rate of formation grows with higher pH because the chemical path to organophosphate disinfection byproducts is enhanced under alkaline conditions. Other candidate groups are known to form disinfection byproducts as well, but their formation trends with pH aren’t as consistently tied to a simple, base-catalyzed pathway as organophosphates are in the scenarios described.