What is hypochlorous acid, and why is it more effective than chlorine?
Hypochlorous acid (HOCl) is the most effective disinfecting form chlorine takes in water. Thanks to its uncharged, small molecular structure it crosses the bacterial cell wall far faster than the hypochlorite ion, which is why it shows markedly higher biocidal efficacy at the same free-chlorine level — and is non-irritating to people at working concentrations.
In water treatment “chlorine” gets talked about as though it were a single substance. In practice, what chlorine actually does depends on which chemical form it takes in the water — and the most effective of those forms is hypochlorous acid.
What chlorine becomes in water
Once a chlorine-based disinfectant enters water it settles into an equilibrium between two forms: hypochlorous acid (HOCl) and the hypochlorite ion (OCl⁻). The factor that governs that equilibrium is pH.
The dissociation constant describing it was measured by Fair and colleagues in 1948: pKa ≈ 7.5.1 That single number is what makes it possible to calculate how much of a free-chlorine residual is genuinely in the active form.
pH is not a setting — it is the determining factor
The table below places two independent sources side by side. The left columns give the HOCl fraction calculated from Fair’s dissociation constant; the right column gives the CT value — required concentration × contact time — that the US Environmental Protection Agency mandates for 3-log Giardia inactivation.2
| pH | HOCl fraction | OCl⁻ fraction | Required CT (mg·min/L) | vs. pH 6 |
|---|---|---|---|---|
| 6.0 | 97.2% | 2.8% | 79 | 1.00× |
| 6.5 | 91.6% | 8.4% | 94 | 1.19× |
| 7.0 | 77.6% | 22.4% | 112 | 1.42× |
| 7.5 | 52.3% | 47.7% | 134 | 1.70× |
| 8.0 | 25.7% | 74.3% | 162 | 2.05× |
| 8.5 | 9.9% | 90.1% | 195 | 2.47× |
| 9.0 | 3.4% | 96.6% | 234 | 2.96× |
CT values at 10 °C, 1.0 mg/L free chlorine. HOCl fraction calculated from pKa 7.54.
What the table says is this: going from pH 6 to pH 9, the share of chlorine present in the active form collapses from 97% to 3%, while the dose required to achieve the same disinfection triples. One is a chemistry measurement from 1948, the other a regulatory table from 2003 — and they tell the same story.
Why the difference is so large
The answer is molecular. A bacterial cell wall carries a negative charge. The hypochlorite ion is also negatively charged, so it is repelled by that wall and struggles to cross it. Hypochlorous acid is uncharged and small — it passes through with almost no resistance and oxidises internal structures rapidly.
Free-floating bacteria and biofilm are not the same problem
The CT values above apply to microorganisms suspended in water. Biofilm attached to a pipe, tank or heat exchanger is a different matter entirely.
LeChevallier and colleagues measured surface-attached biofilm bacteria as 150 to more than 3,000 times more resistant to hypochlorous acid than unattached cells.4 The more consequential finding from the same study: the rate-limiting factor is transport of the disinfectant into the biofilm. Which is why raising the chlorine dose does not raise efficiency — it only raises the by-product load.
This is why a problem in a water line is rarely solved by “adding more chlorine”.
Same free chlorine, different outcome
An important conclusion follows: two solutions can report the same free-chlorine reading while one is far more effective than the other — because what governs efficacy is how much of that chlorine is in the hypochlorous acid form.
A hypochlorous acid solution generated at neutral pH can therefore deliver the same disinfection power at a lower total chlorine level. In pools that means less chloramine, less odour and less irritation; in drinking water it means less trihalomethane formation.
The body’s own disinfectant
Hypochlorous acid is not a laboratory invention. In the human immune system, neutrophils produce precisely this molecule to destroy pathogens. Reviews in the medical literature document that at use concentrations it is non-irritating to human tissue while retaining broad-spectrum activity.3
Why generation on site matters
Hypochlorous acid is an unstable molecule; over time it reverts to less effective forms. Products bottled and left on a shelf for months lose potency accordingly. Peak efficacy comes from fresh generation — which is what makes producing hypochlorous acid at the point of need, by electrochemical activation, the sound approach.