Chemistry of Breakpoint Chlorination

Chlorination provides the means of disinfection for many public drinking water systems as well as public and home swimming pools. Insufficient chlorination of ammonaceous or ammoniacal waters leads to the formation of a mix of chloramines (monochloramine, dichloramine, and even nitrogen trichloride) which irritate the skin and eyes and impart an unpleasant odor to the water.

At sufficiently high concentrations of active chlorine (hypochlorite, hypochlorous acid, and molecular chlorine), a phenomenon known as breakpoint chlorination occurs. In breakpoint chlorination, ammonaceous and ammoniacal materials are completely oxidized to dinitrogen and the active chlorine is simultaneously reduced to chloride. In order to accomplish breakpoint chlorination, swimming pools are shock-treated. Over the winter, a large amount of organic matter has accumulated in most swimming pools. At the beginning of the swimming pool season, the chlorine demand is too high for normal amounts of chlorine to completely destroy all the organic matter. Consequently, significant concentrations of chloramines and other partial oxidation products are present. In shock-treatment, a large amount of active chlorine is added so as to completely oxidize all of these products.

Although breakpoint chlorination is essential for good water disinfection, in many cases, it is a nuisance side reaction that makes monitoring other reactions difficult. Studies of nitrogen trichloride are often hampered by breakpoint chlorination because the loss of dinitrogen is difficult to monitor. Nitrogen trichloride is the last step in the chlorination of ammonia before breakpoint chlorination occurs. In practice, one finds that the generation of nitrogen trichloride is always accompanied by a small amount of breakpoint chlorination.

In Margerum research group, breakpoint chlorination is under studies for better understanding of the mechanism and kinetics. Oxidation of hydrazine by Cl2, HOCl, and OCl- are carried out to study the intermediates in the breakpoint chlorination process.
Last updated 18 June 1997, Qian Liu.
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