pH vs ORP: What Each Measures and When to Use Them
pH and ORP electrodes look alike and are often wired to the same controller, but they answer different questions.
What pH measures
pH measures hydrogen-ion activity on a scale of 0 to 14. Below 7 the water is acidic, above 7 it is alkaline. Use pH to control neutralisation, coagulation and biological treatment, and to confirm your outlet meets discharge limits.
What ORP measures
Oxidation-reduction potential (ORP), measured in millivolts, shows the balance of oxidising and reducing agents in the water. A high positive ORP means strong oxidising power – useful as a proxy for disinfection effectiveness with chlorine or ozone. Negative ORP indicates reducing conditions.
Where to use each
| Application | Measure |
|---|---|
| Neutralisation of acidic or alkaline effluent | pH |
| Chlorine or ozone disinfection control | ORP (plus residual chlorine) |
| Cyanide oxidation | ORP and pH |
| Hexavalent chrome reduction | ORP and pH |
| Biological treatment | pH and DO |
Senvolt controllers such as the PH101 and ORP101 have relays to drive dosing pumps directly, and the PH102 can be configured for either parameter.