Freeavailable Chlorine

When chlorine is injected into water, it dissolves quickly, hydrolyzing to form hypochlorous acid and chloride ions as follows:

chlorine gas hypochlorous acid

hydrogen ion

chloride ion

Hypochlorous acid further ionizes to form hypochlorite ions as follows:

hypochlorite

The extent of ionization depends greatly on the pH of the water and to a lesser extent on the temperature (see Figure 7.30.5). The HOCl and OCl— forms provide free-available chlorine.

The disinfection potential of chlorine is related to its oxidation properties. Hypochlorous acid and, to a lesser extent, the hypochlorite ion enter the cell walls of bacteria, oxidizing certain enzymes and other organic cellular material essential to the bacteria's life processes.

Chlorine is ordinarily purchased as a liquid compressed in pressurized tanks or cylinders. Small installations sometimes use solutions of sodium or calcium hypochlorite. These chemicals also ionize to produce the hypochlorite ion. For calcium hypochlorite, the following reaction oc-

curs:

calcium hypochlorite

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