When is HOCl produced in a water treatment process?

Hypochlorous acid (HOCl) is produced when chlorine-based disinfectants are added to water, specifically through the addition of either gas chlorine or sodium hypochlorite. This process occurs because the chlorine reacts with water to form hypochlorous acid, which is a primary disinfecting agent used in water treatment.

When gas chlorine is introduced, it dissolves in water and dissociates into hypochlorous acid and a hypochlorite ion, depending on the pH level of the water. Sodium hypochlorite, commonly found in liquid bleach, also dissociates in water to yield hypochlorous acid. This formation of HOCl is central to effective disinfection, as it is the active component responsible for inactivating pathogens.

The other options do not directly produce hypochlorous acid. Air addition does not chemically react to form HOCl, nor does potassium permanganate, which is a separate oxidizing agent with different applications. Boiling water may remove some contaminants but will not create hypochlorous acid from chlorine sources.