pH and redox: two quantities, one misunderstanding
On dosing systems both values sit side by side and are constantly confused. Yet they answer fundamentally different questions.
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Anyone running a pool dosing system or a water treatment plant meets two probes that look confusingly alike - and two displays, one counting in pH and one in millivolts. The short version: the pH value says how acidic the water is. The redox potential says how strongly it oxidises - in the pool: how effectively the disinfection is working right now.
What the pH value measures
The pH value describes the activity of hydrogen ions on the familiar scale from 0 to 14. It is measured with a glass electrode whose membrane responds to H⁺ ions - how that works is explained in the basics, and the interactive pH scale gives a feel for the orders of magnitude.
What the redox potential measures
The redox potential (ORP, oxidation reduction potential) describes the ratio of oxidising to reducing substances in the solution. Instead of a glass membrane, the ORP electrode carries a bare platinum or gold pin on which an electron equilibrium establishes itself. The reading comes directly in millivolts - in pool water typically between 500 and 850 mV. The more oxidisers such as free chlorine are at work, the higher the value climbs.
The interplay in the pool
Only together do both values make a disinfection control. Chlorine acts mainly as hypochlorous acid - and its share depends directly on the pH: at pH 7.0 considerably more of it is present than at pH 7.8, for the same amount of chlorine. The dosing system therefore first brings the pH into the target range of 7.0 to 7.4 and then holds the disinfection power via the ORP reading. In public pools, at least 750 mV measured against Ag/AgCl is the guide value for sufficient effect under the German standard DIN 19643.
Why ORP is not a chlorine meter
A high ORP value does not automatically mean a lot of chlorine - it means a lot of effect. If the pH falls, the redox potential rises for the same amount of chlorine; cyanuric acid as a stabiliser, on the other hand, pushes it down noticeably. To know the chlorine concentration in mg/l, there is no way around a photometric test. The ORP measurement complements it but does not replace it.
The electrodes compared
| Criterion | pH electrode | ORP electrode |
|---|---|---|
| Measured quantity | Acidity (H⁺ activity) | Oxidising power |
| Sensor | Glass membrane | Platinum or gold pin |
| Unit | pH 0-14 | Millivolts (mV) |
| Reference system | Ag/AgCl in KCl | Ag/AgCl in KCl |
| Calibration | Two-point with buffers | Function check with test solution only |
| Typical check values | pH 7 and pH 4/10 | e.g. 220 or 468 mV |
Checking instead of calibrating
An ORP electrode has no slope that could be adjusted - the potential establishes itself at the metal pin. Instead of a calibration there is only the function check: if the electrode shows the value of a test solution with known potential to within about ±20 mV in a few minutes, it is fine. If it reacts sluggishly or is far off, the metal pin is usually coated - then the matching cleaning helps, while mechanical polishing should only be done with means intended for it.
The reference-system trap when comparing numbers
Anyone comparing ORP values from tables or literature sooner or later stumbles over two worlds: practical values are measured against Ag/AgCl, scientific figures often against the standard hydrogen electrode (SHE). Around 200 mV lie between the two - the same pool state is called 750 mV in one and about 950 mV in the other. So before comparing, always clarify which reference system a number refers to.