Which parameter must be closely monitored when performing chlorine testing methods?

The parameter that must be closely monitored when performing chlorine testing methods is pH. Chlorine's disinfecting ability is significantly influenced by the pH level of the water. At different pH levels, chlorine exists in different forms: free chlorine (hypochlorous acid and hypochlorite ion) and combined chlorine. Hypochlorous acid is the more effective form of chlorine for disinfection, and its concentration is highest at a pH level around 7.2 to 7.6. If the pH is too high or too low, it can alter the effectiveness of chlorine as a disinfectant, leading to insufficient disinfection or the formation of undesirable chlorination byproducts.

In contrast, while temperature, conductivity, and salinity can influence various chemical processes in water, they are not as critical to chlorine effectiveness as pH. Temperature can affect the rate of chlorination reactions but does not change the chlorine's effectiveness in the same specified way. Conductivity and salinity are more relevant to other aspects of water quality and treatment rather than specifically monitoring chlorine efficacy. Therefore, keeping a close watch on pH is essential for ensuring optimal chlorine performance in water treatment processes.