Fraction of Free Chlorine as HOCl

Quantifies the active biocidal fraction of free chlorine as a function of pH. Used to estimate instantaneous lethality of a chlorine residual at a given operating pH (HOCl and OCl- are equal near pH 7.5, the chlorine pKa is about 7.5 at typical temperatures).

$$f_{\mathrm{HOCl}}=\frac{[\mathrm{HOCl}]}{[\mathrm{HOCl}]+[\mathrm{OC}{\mathrm{l}}^{-}]}=\frac{1}{1+10^{(\mathrm{pH}-\mathrm{p}{K}_a)}}$$

Variables

Symbol	Meaning	Unit
$f_{H}OCl$	Fraction of free chlorine present as HOCl	dimensionless (0 to 1)
[HOCl]	Concentration of hypochlorous acid	mg/L
[OCl-]	Concentration of hypochlorite ion	mg/L
pH	Solution pH	dimensionless
pKa	Acid dissociation constant of HOCl (about 7.5)	dimensionless

Worked example

At pH 7.5 (pH equals pKa) the fraction is 1 / (1 + 10^0) = 0.5, so HOCl is about 50 percent, matching the chapter table.

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Part of The Blueprint of Water