After dissolution in water and before being consumed to destroy micro-organisms, chlorine is first oxidized by oxidation compounds such as hydrogen sulfide, iron and manganese. Then, it reacts with organic compounds and ammonia in water. The remaining chlorine is then changed to hypochlorous acid and depending on pH of the solution, if hypochlorous acid is not dissociated into hypochlorite ion, it destroys micro-organisms.
Chlorine dioxide has a high oxidation capacity and is able to increase the Redox potential in low concentrations for the purpose of effective disinfection to the desirable point; however, its oxidation strength is low due to its molecular structure. This substance, opposed to chlorine, doesn’t participate in any chlorination substitution reaction and as a consequence it doesn’t leave any harmful chlororganic substance. Furthermore, due to its low oxidation intensity, causes in a much less corrosion in comparison with chlorine.
Chlorine dioxide can catch 5 electrons based on the following formula:
ClO2 + 4H+ + 5e– → Cl– + 2H2O
However, chlorine can only catch 2 electrons:
Cl2 + 5e– → 2Cl–
Consequently, oxidation capacity of chlorine dioxide is 2.5 times higher than that of chlorine, however, oxidation strength and its reactivity in much lower.
Unlike chlorine, chlorine dioxide doesn’t participate in chlorine substitution reactions. Thus, the harmful compounds resulting from this family of substances are not produced during using chlorine dioxide. Moreover, as a consequence of its low destructive (oxidation) power compared to chlorine, it doesn’t cause corrosion.
Oxidizing compounds such as ozone, hydrogen peroxide, and peroxide acid cause problems during usage such as instability, difficult application, and safety related issues. In comparison with other disinfectants, Chlorine dioxide has a much lower oxidation strength, which means that it reacts with a less number of compounds such as biologic compounds and ammonia.
At the same time, chlorine dioxide is strong enough to overcome sulfide bonds in the membrane of bacteria and other biological materials. Following plot show the chlorine dioxide’s high oxidation capacity and at the same time its low oxidation strength (reactivity and destructivity) in comparison with other oxidants.