Chlorine or brome react in water and turn in to hypochlorous or hypobromous acid.
Hypochlorous acid (or hypobromous acid) are known as active chlorine and result in destruction of microorganisms. These acids are also decomposed to hypochlorite or hypobromite ions.
Cl2 (g) + H2O → HOCl + HCl
While, adding gaseous chlorine to water causes in pH reduction, based on the following reaction. Adding sodium hypochlorite to water causes in pH increase.
NaOCl (s) + H2O → HOCl + NaOH
Hypochlorous become in chemical equilibrium with hypochlorite ion.
HOCl ↔ OCl– + H+
Pace and quantity of this dissociation is a factor of pH (and of course temperature) and therefore disinfection strength of chlorine and sodium hypochlorite which is a function of the concentration of hypochlorous acid in the medium, changes with pH.
In pH=8.0, amount of hypochlorous acid is only 25% of all the dissolved chlorine in water. Thus, only 25% of chlorine’s disinfection strength in achieved. Putt differently, chlorine consumption should increase by a factor of 4.