Active chlorine is the part of the chlorine in the pool that actually kills bacteria, viruses and algae (the actual disinfectant). Chemically, it is hypochlorous acid, abbreviated HOCl. Everything else in the water that is also called “chlorine” disinfects much more weakly or not at all.
Influence of the pH value
When chlorine is added to the water – whether as liquid chlorine, tablets, granules or via salt electrolysis – it does not turn completely into “active chlorine” (HOCl). An equilibrium is established between two forms:
- HOCl (hypochlorous acid) – this is the active chlorine, the actual disinfecting agent.
- OCl⁻ (hypochlorite ion) – this is only very weakly effective and hardly contributes to killing germs.
How much of the chlorine present (the “free chlorine” reading, DPD1) exists as HOCl and how much as OCl⁻ depends in the first instance on the pH value. The equilibrium is reversible: lowering the pH produces more HOCl; raising it turns HOCl back into OCl⁻.
Typical shares
- at pH 6.5, about 85% is present as HOCl,
- at pH 7.0, still about 74%,
- at pH 7.5, only about 50%,
- at pH 8.0, less than 20%.
A pool with pH 7.6 and a measured chlorine level (DPD1) of 1.0 mg/l therefore has significantly less disinfecting effect than the same pool at pH 7.0 – even though the DPD1 test will show the same chlorine content in both cases.
Influence of cyanuric acid
In addition, cyanuric acid comes into play when organic chlorine is used (the usual slow-dissolving tablets and granules based on dichlor or trichlor). Cyanuric acid protects chlorine from rapid breakdown by sunlight – but this effect mainly arises because the cyanuric acid binds part of the chlorine to itself. This chlorine bound to cyanuric acid is initially not available as HOCl. A second equilibrium is established: part remains bound to the cyanuric acid, part of the chlorine is released as HOCl.
As a rule of thumb
- at 10 mg/l cyanuric acid, about 40% of the chlorine is already bound to cyanuric acid,
- at 20 mg/l, about 50–55%,
- above 50 mg/l, already more than 65%; higher levels hardly bind any more.
Both effects combined
Both effects work together. A practical example: you dose 1.0 mg/l chlorine into water with 50 mg/l cyanuric acid and pH 7.4. About 65–70% of it is bound to the cyanuric acid, leaving 0.3–0.35 mg/l. Of this remainder, at pH 7.4 only slightly more than half is present as HOCl. So only about 0.15–0.2 mg/l is actually active in the water – even though a DPD1 test shows almost the full added amount of “1.0 mg/l”.
Why the DPD1 test shows more
The reason for the difference: on the one hand, the test buffers the water sample to a low pH (by definition 6.2 to 6.5); on the other hand, the oxidation of the indicator (which produces the colour of the sample) consumes the HOCl present, and the cyanuric acid immediately releases the remaining chlorine bound to it. The DPD1 test therefore measures the (theoretically) total available chlorine, not the HOCl that is currently working in the pool under real conditions.
What this means in practice
For continuous disinfection, what counts is therefore not the bare DPD1 reading alone, but the share that actually remains as HOCl after deducting the pH and cyanuric acid effects. If the pH is high or there is a lot of cyanuric acid in the water, the measured chlorine level must be correspondingly higher to reach this effective share.