Chlorine Chemistry Explained

Chlorine is the disinfectant most frequently used for water and wastewater treatment. It was used first for industrial applications and to control odor in wastewater, in the early 1800s. The subsequent use of chlorine to disinfect water occurred by the mid-1800s. Industrial uses of chlorine include applications such as bleaching paper and controlling nuisance organisms in cooling towers.

Hydrochloric and hypochlorous acids are formed when chlorine is added to water. The disinfectant and form causing bleaching action, is hypochlorous acid.

Cl ₂ H ₂ + O → HCl + HOCl (Hypochlorous acid)

Depending upon variables such as pH, temperature and the amount of organic or ammonia nitrogen, other forms of chlorine in water may include hypochlorite ions (OCl–) and chloramines. Chlorine existing in water as hypochlorous acid or the hypochlorite ion is termed free available chlorine. Chloramines, including monochloramine (NH2Cl), dichloramine (NHCl2) and nitrogen trichloride (NCl3) are referred to as Combined Available Chlorine. Total Chlorine refers to the sum of free and combined available forms.