Which form of chlorine is most effective in inactivating bacteria?

Hypochlorous acid is the most effective form of chlorine for inactivating bacteria due to its superior disinfection properties. When chlorine is dissolved in water, it partially ionizes to form hypochlorous acid and hypochlorite ions. Hypochlorous acid is a neutral molecule that can easily penetrate bacterial cell membranes, making it more effective at disrupting cellular processes and leading to bacterial inactivation.

In contrast, substances like sodium hypochlorite and chlorine gas can contribute to disinfection but are not as effective as hypochlorous acid. Sodium hypochlorite, for instance, serves as a source of chlorine but must first dissociate in water to create hypochlorous acid and hypochlorite ions. Chlorine gas, while potent, can pose hazards related to its handling and toxicity.

Sodium hydroxide, on the other hand, is not a disinfectant but rather a strong alkali used to adjust pH levels in water treatment processes. It does not have any bactericidal properties.

Thus, hypochlorous acid stands out due to its ability to effectively penetrate and neutralize bacteria, making it the most effective choice for disinfection in water treatment.