Halogens and Halophores

2.5.1 Chlorine and Chlorophores. Chlorine has been used since 1827 to disinfect drinking water and it remains the leading agent for this purpose because it is cheap and effective. Nevertheless, a variety of chlorine-generating compounds (chlorophores) and related compounds have been used in drinking water. Chlorine dioxide (C102) has been used in recent years for drinking water disinfection and wastewater treatment. This highly reactive compound cannot be manufactured and shipped in bulk, but it is prepared at the site of consumption by treating a sodium chlorite solution with chlorine as indicated (Equation 13.1). Inorganic chloramines were used in the 1930s and early 1940s to improve the taste of drinking water, but their use was largely discontinued because of their inferior disinfectant properties. Combining chlorine and ammonia in water results in a mixture of species including C1NH2, C12NH, and C13N. Approximately 25 times as much of these species is needed as is chlorine and the contact time required is 100 times as long (26).More recently, the addition of inorganic chloramines to water is being reconsidered because they prevent the formation of carcinogenic trihalomethanes from pollutants (27).

The germicidal species formed in water solutions of chlorine is hypochlorous acid (Equation 13.2). Based on this finding, a variety of inorganic hypochlorites were introduced as disinfectants. They include sodium hypochlorite, calcium hypochlorite dihy-drate, lithium hypochlorite, and chlorinated trisodium phosphate [Na0Cl-4(Na3P04)]. The water solutions of sodium hypochlorite are available in different concentrations of available chlorine including the following: 0.4-0.5% (Dakin's solution used on wounds); 5.25% (Chlorox and related preparations); and 12-15% (liquid bleach).

Organic compounds such as amides, sulfon-amides, imides, and amidines form reasonably stable N-chloro derivatives, which may be produced in bulk quantities by treating appropriate nitrogen compounds with HOC1. In water they slowly release HOCl and regenerate the parent nitrogen compound (Equation 13.3). Among the iV-chloro-sulfonamides, chloramine-T (sodiumjo-tolu-enesulfonchloramide, 1) has been used for

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