Disinfecting action of chlorine is mainly due to hypochlorous acid and to a small extent due to hypochlorite ions. Hypochlorous acid is 70 to 80 times more effective than hypochlorite ions. Chlorine acts best at a pH of around 7 because of the predominance of hypochlorous acid. When pH exceeds 8.5, chlorine is unreliable as a disinfectant. Ref: Park 21st edition, page: 661.