Page 508 - Water and wastewater engineering

DISINFECTION AND FLUORIDATION 13-3

Chlorine existing in the form of HOCl and/or OCl is defined as free available chlorine or free
chlorine.
Example 13-1. If 15 mg/L of HOCl is added to a potable water for disinfection and the final
measured pH is 7.0, what percent of the HOCl is not dissociated? Assume the temperature
is 25 C.

Solution:
a. The reaction is shown in Equation 13-2 . From Appendix A, find that the p K a is 7.54 and

.
K a 10 7 54 2 88 10 8
.
b. Writing the equilibrium constant expression

[H ][OCl ]
K a
[HOCl ]

and substituting the values for K a and [H ]

7
288 10 8 [ 10 ][OCl ]
.
[HOCl ]

Solving for the HOCl concentration

]
[HOCl 3 47 [OCl ]
.

Because the fraction of HOCl that has not dissociated plus the OCl that was formed
by the dissociation must, by the law of conservation of mass, equal 100% of the original
HOCl added:
[HOCl ] [OCl ] 100% (of the total HOCl added to the solution)

then

3 47.[OCl ] [OCl ] 100%

4 47.[OCl ] 100%

[OCl ] 100% 22 37%
.
.
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