Chlorine in Water
When chlorine gas is diffused in water, the chlorine hydrolyzes to form hypochlorous acid (HOCl) and hydrochloric acid (HCl). The equilibrium equation for this reaction is as follows: Cl2 + H2O <=> H+ + Cl- + HClO. Hypochlorous acid is weakly bonded and will easily ionize, forming the positive H+ ion and the negative hypochlorite (OCl-) ion. The equilibrium equation of this ionization reaction is as follows: Cl2(aq) + H2O(l) <=> 2H+(aq) + Cl-(aq) + ClO-(aq).
Hypochlorous Acid
The ionization of hypochlorous acid is dependent on pH. At pH 7.5 there are roughly equal amounts of hypochlorous acid and the hypochlorite ion. In acidic environments, there is more acid and fewer ions. In highly alkaline environments, pH 9.5 or higher, hypochlorous acid is fully ionized to hydrogen (H+) and the hypochlorite (OCl-) ion.
Chlorine as a Microbiocide
A microbiocide is a chemical that is used to kill microorganisms. Chlorine is added to pool water, cleaning products and drinking water as a microbiocide. The hypochlorous acid formed by mixing chlorine in water is a powerful oxidizing agent that destroys microorganisms, including disease-causing bacteria.
Hypochlorous Acid vs. the Hypochlorite Ion
Both hypochlorous acid and the hypochlorite ion move through germ cell walls and oxidize the enzymes inside the germ. Oxidation of enzymes causes the death of the germ cell. However, germ cell walls carry a negative electrical charge, which significantly slows the movement of the negatively charged hypochlorite ion. Hypochlorous acid carries no charge, so it moves much faster through germ cell walls, making it much more effective as a microbiocide than the hypochlorite ion.
The Significance of pH
When chlorine is being used as a microbiocide, it is important to maintain a low pH in the treated water. In an acidic environment, hypochlorous acid levels remain higher than levels of hypochlorite ion, increasing the germicidal efficiency of the chlorine solution.