Chlorine is the most commonly used water disinfectant in applications that vary from treatment of drinking water and wastewater, pools and spas sanitization, to food processing and sterilization. Chlorine present in water binds with bacteria, leaving only part of the original quantity (free chlorine) to continue its disinfecting action. If the free chlorine level is improper with respect to the pH, water will have an unpleasant taste and odor and the disinfecting potential of the chlorine will be diminished.

Free chlorine reacts with ammonium ions and organic compounds to form chlorine compounds with diminished disinfecting capabilities compared with free chlorine. Combined chlorine and free chlorine together result in total chlorine content.