Chlorine speciation

Definition

The pH-dependent equilibrium between hypochlorous acid and hypochlorite ion that determines how much of a free chlorine residual is in the effective biocidal form.

Free chlorine exists as HOCl and OCl- governed by solution pH, and HOCl is the far more effective biocidal form. The HOCl fraction falls sharply as pH rises, from roughly 90 percent at pH 6.5 to about 8 percent at pH 8.5. A cooling tower drifting from pH 7.5 to 8.5 loses roughly 85 percent of its chlorine effectiveness without any change in feed rate, which is why bromine is often preferred in alkaline systems.

Related concepts

Oxidizing biocides · Contact time · Residual

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Part of The Blueprint of Water · 🧪 chemistry