Cicr 2 cr o2

Hypochlorous acid can also be generated by the reaction with water of the chlorine-containing compound isocyanuric acid, C3N303H3:

Either the trichloro derivative, in which each hydrogen is replaced by CI to give C3N3O jCl3, or the sodium dichloro derivative, C3N303Cl2Na, is used. In either case, the OH group from water combines with the chlorine to produce HOC1 and the hydrogen of H,0 becomes bonded to the nitrogen, giving isocyanuric acid, C3N303H3:

C3N303C13 + 3 H,0 ^^C3N303H3 + 3 HOC1

Since this process is an equilibrium, not all the compound is immediately converted to hypochlorous acid. As HOC1 is used up, both by its use as a disinfectant and through dissociation in sunlight of its ionic form, the equilibrium shifts to the right and more HOC! is produced. None of the various forms of isocyanuric acid absorb UV light, so its chlorine is "protected" against decomposition by sunlight. Since the bulk chlorinated forms of isocyanuric acid are expensive, it is common to supply hypochlorite from a cheaper source and to add isocyanuric acid as a stabilizer, temporarily reversing the above reaction to "store" the chlorine until it is needed.

Disinfection by Chlorine: By-Products and Their Health Effects

An important drawback to the use of chlorination in disinfecting water is the concomitant production of chlorinated organic substances, some of which are toxic, since HOC1 is not only an oxidizing agent but also a chlorinating agent. Examples of these important by-products are the group of halogenated acetic acids (haloacetic acids), such as CH2C1—COOH, which the U.S. EPA restricts to 60 ppb as an MCL annual average for drinking water, and halo-acetonitriles, such as CH2Cl—CN. Dichloroacetic acid, CHC12—COOH, is a more potent carcinogen than is chloroform.

If the water to be disinfected contains phenol, C6H5OH, or a derivative thereof, chlorine readily substitutes for the hydrogen atoms on the ring to give rise to chlorinated phenols: These compounds have an offensive odor and taste and are toxic. Some communities switch from chlorine to chlorine dioxide when their supply of raw water is temporarily contaminated with phenols to avoid the formation of chlorinated phenols.

A more general problem with chlorination of water lies in the production of trihalomethanes, THMs. Their general formula is CHX3, where the three X atoms can be chlorine or bromine or a combination of the two. The THM of principal concern is chloroform, CHCl j, which is produced when hypochlorous acid reacts with organic matter dissolved in the water (see Box 14-3). Chloroform is a suspected liver carcinogen in humans, and it may also give rise to negative reproductive and developmental effects. Its

BOX 14-3

The Mechanism of Chloroform Production in Drinking Water

Humic acids, with which HOC1 reacts to form chloroform, are water-soluble, nonbiodegradable components of decayed plant matter. Of particular importance are humic acids that contain 1,3-dihydroxybenzene rings. The carbon atom (#2) located between those carrying the —OH groups is readily chlorinated by IIOC1, as in this elementary case:


Subsequently the ring cleaves between C-2 and C-3 to yield a chain:

In the presence of the HOC1, the terminal carbon becomes trichlorinated, and the —CCl3 group, is readily displaced by the OH in water to yield chloroform:

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