Chlorine Speciation Equilibrium (HOCl / OCl- pH dependence)

Describes how free chlorine partitions between hypochlorous acid (HOCl, the strong biocidal form) and hypochlorite ion (OCl-, the weak form). As pH rises the equilibrium shifts toward OCl-, so biocidal effectiveness falls even with the same total residual.

$$\mathrm{HOCl}\rightleftharpoons {\mathrm{H}}^{+}+\mathrm{OC}{\mathrm{l}}^{-}$$

Variables

Symbol	Meaning	Unit
HOCl	Hypochlorous acid, the more effective oxidizing and biocidal form	mg/L or percent of free chlorine
H+	Hydrogen ion, sets solution pH	mol/L (pH)
OCl-	Hypochlorite ion, the weaker biocidal form	mg/L or percent of free chlorine

Worked example

A cooling tower drifting from pH 7.5 to pH 8.5 sees HOCl fall from about 50 percent to about 8 percent of free chlorine, losing roughly 85 percent of chlorine effectiveness with no change in feed rate.

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Part of The Blueprint of Water