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pH controls how much of your biocide exists in its active form. Chlorine in water forms hypochlorous acid (HOCl), which dissociates into the far weaker hypochlorite ion (OCl⁻) with a pKa of about 7.5. HOCl is roughly 70–80 times more effective at inactivating bacteria than OCl⁻. So at pH 7.4 about 56% of your free chlorine is the active form; at pH 9 only about 5% is. Bromine behaves differently — hypobromous acid has a pKa near 8.7, so it stays largely active at the alkaline pH most cooling towers run at.
When chlorine is added to water it hydrolyses to hypochlorous acid, which then partially dissociates:
HOCl ⇌ H⁺ + OCl⁻ (pKa ≈ 7.5 at 25°C)
Both species are counted as “free chlorine” by your test kit. That is the trap: the test tells you how much chlorine is present, not how much of it is working. Between roughly pH 6.5 and 8.5 the two forms coexist, and the balance shifts sharply with small pH changes.
Because pKa ≈ 7.5, the split moves fast across the range a cooling tower operates in:
| pH | Approx. HOCl (active) | Practical consequence |
|---|---|---|
| 5–6 | ~100% | Nearly all active — but far too corrosive to run a system at |
| 7.4 | ~56% | Roughly half your free chlorine is doing the work |
| 8.0 | Minority HOCl | Efficacy falling quickly; dose creep begins |
| 9.0 | ~5% | ~95% is the weak OCl⁻ form. Chlorine is largely wasted |
And the efficacy gap between the two species is not marginal. Studies put HOCl at 70–80 times more effective than OCl⁻ at inactivating bacteria — HOCl is uncharged and can cross the microbial cell membrane, whereas the negatively charged OCl⁻ is repelled by it.
Cooling water tends to drift alkaline. Scale-control programmes often hold pH high on purpose, and CO₂ stripping in the tower pushes it higher still. A tower sitting at pH 8.5–9.0 is a normal sight in Hong Kong.
At that pH, a chlorine residual that looks healthy on paper may be doing very little. The usual response — raise the dose — produces higher chemical cost, more corrosion, more by-products, and still poor control. The problem is not the dose. It is the pH.
This is one of the most common reasons a Legionella count comes back high on a system that “had chlorine in it”. The chlorine was there. It just was not in the active form.
Bromine chemistry is directly analogous — hypobromous acid (HOBr) dissociates to hypobromite (OBr⁻) — but the pKa sits near 8.7 rather than 7.5. Above pH 7.5 the HOCl fraction declines rapidly; the equivalent decline for HOBr does not begin until above roughly pH 8.7.
In plain terms: at the alkaline pH where chlorine has mostly given up, bromine is still largely in its active form. That is the core reason bromine-based programmes are favoured for high-pH industrial cooling water.
Oxidisers are not the only option, and the pH sensitivity differs by chemistry. Broadly:
The practical point is not to memorise a table. It is that “biocide” is not one thing. Choosing the wrong chemistry for your pH is the same mistake as under-dosing, and it costs the same.
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Not really. Raising the dose at pH 9 mostly adds cost, corrosion risk and by-products, because roughly 95% of the free chlorine is present as the weak hypochlorite ion. Correcting pH, or switching to a bromine-based chemistry with a higher pKa, addresses the actual cause.
Because hypobromous acid has a pKa near 8.7 versus about 7.5 for hypochlorous acid. At the alkaline pH typical of cooling water, bromine remains largely in its active undissociated form while chlorine does not.
Chlorine is most active below pH 7, approaching 100% hypochlorous acid at pH 5–6. That range is too aggressive to operate a real system at, which is why most programmes accept a compromise pH and manage the trade-off, or use a different biocide.
Because a free chlorine test counts both hypochlorous acid and the hypochlorite ion, so it tells you how much chlorine is present, not how much of it is working. At pH 9 about 95% of that free chlorine is the weak hypochlorite form. Measuring ORP alongside the residual shows the oxidising power actually present, which catches the pH problem a chlorine test misses.
We will test the water, check the biocide against the actual pH, and tell you what it needs.