The most basic form of chlorine is Chlorine Gas (Cl2). This is usually the cheapest form of chlorination, yet somewhat complicated. Therefore, it is usually used only in large installations (municipal water supplies, etc.). When Cl2 is added to water the following reaction occurs:
Cl2 + H2O <----> HOCl (Hypochlorous Acid) + H+ + Cl- (Hydrochloric Acid).
K = 3 x 10-5 (pH = 4.5 at equilibrium)
Thus, the water pH swings to the acidic side.
At higher pH's, the hypochlorite ion (-OCl) is formed. The hypochlorite ion has a higher oxidation potential than hypochlorous acid, yet hypochlorous acid is a better disinfectant. The fact that hypochlorous acid has no charge allows it to penetrate microbial cell walls easier. Therefore, the lower the pH, the better disinfecting power of a chlorine solution due to hypochlorous acid formation. The hypochlorite ion is overall more reactive, being harder on membranes and other materials of construction.
Hypochlorites (household bleach), perhaps the most common form of small-scale chlorination, tend to form high pH solutions due to the formation of NaOH.
NaOCl + H2O = NaOH + HOCl
The dissociation constant regulates the acid form on the basis of this equation.
HOCI <------> H+ ClO - K = 3 X l0-8
At pH 7.53, concentrations of HOCI and -OCI are equal. At pH = 10, the predominant form is hypochlorite ion (OCl-).
Chlorine Dioxide (ClO2) is a form of chlorine that is finding new uses as a disinfection agent. it has unique properties that may prove to be valuable for the crossflow filtration industry. It has been found that C1O2, while being a strong disinfectant, does not necessarily attack other components it contacts. ClO2 does not form trihalomethanes (THM's) and will not formchlorinated compounds with organic substances. Several CIO2 based disinfectants are being marketed now where the CIO2 is produced on a demand-type basis. The triggering action for ClO2 production in these disinfectants is thought to be sugar-like substances, which are an integral part of bacterial cell walls. ClO2 formation from chlorite ion (via chlorous acid) is accelerated in the presence of bacteria or organics consisting of sugar-like compounds. These demand-release disinfectants, due to these phenomena, have a very low toxicity to mammalian cells. Another advantage of ClO2 is that it is also an effective disinfectant at high pH for most microbes. However, it is most effective at the lower pH range.
Chloramines, formed when hypochlorous acid and ammonia are present together in solution, produce long lasting residuals and do not form trihalomethanes, yet their disinfection capability is limited. Additionally, chloramines can cause tastes and odors in finished water (e.g., swimming pool odor is attributed to chloramines).
In summary, chlorine in most forms is a more effective disinfectant in the acidic range than the basic. However, the hypochlorite ion which is most prevalent in high pH solutions has the highest oxidation potential and is more corrosive to other components it comes in contact with. As a result, sanitation with chlorine in an acidic solution (pH ~7) is more effective and more desirable from a compatibility standpoint.