Hypochlorous Acid
Hypochlorous acid (HOCl) is the active, bactericidal form of chlorine in pool water, responsible for killing pathogens.
Definition
Hypochlorous acid (HOCl) is the active, bactericidal form of chlorine in pool water, responsible for killing pathogens.
Typical Values: At pH 7.2: ~67% of FC is HOCl; at pH 7.4: ~55%; at pH 7.6: ~40%; at pH 8.0: ~22%
In Plain Language
When chlorine dissolves in water, it forms both HOCl and OCl- (hypochlorite ion). HOCl is approximately 80 times more effective as a disinfectant than OCl-. The proportion of HOCl is controlled entirely by pH — at pH 7.2, about 67% is HOCl; at pH 8.0, only 22% is HOCl. This is why high pH drastically reduces chlorine effectiveness even when the total FC reading is adequate.
Why It Matters
Understanding HOCl explains why pH management is inseparable from chlorine management in pools.
Typical Values
At pH 7.2: ~67% of FC is HOCl; at pH 7.4: ~55%; at pH 7.6: ~40%; at pH 8.0: ~22%
Last reviewed: 2026-06-01