Chlorine gas used for disinfection combines with water to form hypochlorous acid (HOCl). The HOCl ionizes to form hypochlorite (OCl^–) in a reversible reaction:
HOCl ↔ H⁺ + OCl^– (k = 2.7 × 10^–8 at 20° C)
the equilibrium of which is governed by pH. the sum of HOCI and OCl^– is known as free chlorine residual and HOCl is the more effective disinfectant. The 90% fraction of HOCl in the free chlorine residual is available at a pH value.