concentration of Cl^) is already practically nonexistent at around pH 4.0 and above. In fact, it is even practically nonexistent at pH's less than 4, except when the pH is close to zero and chloride concentration of 0.1 gmmol/L; but, 0.1 gmmol/L is equal to 3,500 mg/L, which is already a very high chloride concentration and will not be encountered in the treatment of water and wastewater. Practically, then, for conditions encountered in practice, at pH's greater than 4.0, [HOCl] predominates over Cl^uq).

Now, using Equation (17.10), let us calculate the distribution of HOCl and OCl-. Note that from the previous result, HOCl predominates over Cl2(aq) above pH 4.0, Cl2(aq) being practically zero. Thus, above this pH, the distribution of the chlorine disinfectant species will simply be for HOCl and OCl- . Expressing Equation (17.10) in the form of the equilibrium equation,

Taking logarithms, rearranging, and simplifying,

pKa is the negative logarithm to the base 10 of Ka.

Table 17.4 shows the ratios of [OCl-]/[HOCl] and [HOCl]/[OCl-] as functions of pH using Equation (17.14). This table shows that HOCl predominates over OCl-at pH's less than 7.5. Also considering Table 17.3, we make the conclusion that for all practical purposes, HOCl predominates over all chlorine disinfectant species

Ratios of -[Oct; and as Functions of pH

Hnr r

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