Hypochlorite Disinfection at Very Small Water Systems
Chlorine chemistry in one page
Chlorine added to water forms hypochlorous acid, the effective disinfecting species, in
equilibrium with the much weaker hypochlorite ion. The balance is set by pH:
- Lower pH favours hypochlorous acid, so disinfection is more effective.
- Higher pH favours hypochlorite ion, so the same measured free residual disinfects less.
That is why pH matters to disinfection performance and not only to corrosion.
Free chlorine = hypochlorous acid + hypochlorite ion. Combined chlorine = chlorine bound to
ammonia and nitrogen compounds as chloramines, a much weaker disinfectant. Total chlorine =
free + combined. A reassuring total residual can conceal almost no free chlorine, so a system
relying on free chlorine must measure free residual specifically.
Chlori