Quick answer: Oxidation-reduction potential (ORP), also called redox potential, is an electrical potential measured in millivolts that reflects the tendency of a water system to support oxidation or reduction reactions.
What ORP is useful for
ORP can help track changes in oxidizing and reducing conditions in pools, treatment processes, aquariums, and environmental water. It is a system-level indicator; it does not identify every oxidizer or reducer and should not be treated as a direct concentration reading for one chemical.
Why context matters
ORP is affected by the chemical species present, their activity, pH, temperature, probe condition, and stabilization. In chlorinated water, pH changes can alter chlorine chemistry and therefore change how an ORP result should be interpreted.
How to measure ORP
- Use a clean, conditioned probe and a representative sample or flow location.
- Remove bubbles and allow adequate stabilization time; ORP can respond more slowly than temperature.
- Record ORP in mV with pH, temperature, time, and recent chemical additions.
- Use model-specific verification or reference procedures rather than assuming one universal calibration process.
Common reasons for unexpected readings
Probe fouling, a dry or aged reference junction, inadequate stabilization, strong electrical interference, poor circulation, recent dosing, pH change, and changing organic load can all affect ORP.
Important limitation
Do not use ORP alone to declare water disinfected, biologically safe, or suitable for drinking. Follow the testing and treatment requirements for the actual application.
Sources
Reviewed August 1, 2026. Educational content; application-specific targets should be confirmed for the actual system, crop, or aquatic species.