Which factor most affects the disinfecting ability of chlorine?

The disinfecting ability of chlorine is significantly influenced by hydrogen ion concentration, also known as pH. Chlorine exists in different forms depending on the pH of the water. At lower pH levels, more of the chlorine is present in the form of hypochlorous acid (HOCl), which is a much more effective disinfectant than its ionized form, hypochlorite ion (OCl⁻), which predominates at higher pH levels.

When the pH is optimal, the efficacy of chlorine to inactivate microorganisms increases considerably. Disinfection is less effective at higher pH levels because the proportion of hypochlorous acid decreases, thereby reducing chlorine's ability to kill pathogens. Understanding this relationship is crucial for effectively using chlorine in water treatment processes.

Other factors, such as biochemical oxygen demand (BOD), temperature, and dissolved oxygen, can also impact disinfection processes but have less direct influence on the integral chemical form of chlorine and its relative effectiveness at different pH levels. BOD relates to the organic load in the water which does affect chlorine demand, but it is not as critical as pH in determining the inherent disinfecting ability of chlorine. Temperature can affect chemical reactions generally but is secondary to pH in