Chlorine speciation by pH
Definition
Free chlorine exists as two species in balance, hypochlorous acid and the hypochlorite ion, and the split between them sits right around pH 7.5. The two forms are not equally effective disinfectants.
At pH 7 free chlorine is mostly uncharged hypochlorous acid, which slips through cell walls and gives strong kill rates, while at pH 8.5 it is mostly hypochlorite, doing far less work for the same dose. A meter reads 1.0 ppm free chlorine at both pH values, but the chemistry behind the number differs. This is a concrete example of how a small pH shift changes which reactions are allowed to run.
Related concepts
pH as equilibrium position · pH controls chemistry
Part of The Blueprint of Water · 🧪 chemistry