Oxidation Reduction Potential (ORP)

What is ORP?

Oxidation reduction potential (ORP), also known as REDOX, is a measurement that reflects the ability of a molecule to oxidize or reduce another molecule:

• Oxidation is the loss of electrons, so oxidizers accept electrons from other molecules
• Reduction is the gain of electrons, so reducers donate electrons to other molecules

Oxidation reduction potential is measured as a single voltage in millivolts (mV). Oxidizers have a positive ORP value, while reducers have a negative ORP value.

How is ORP Measured?

ORP is measured using an electrochemical sensor called an ORP or REDOX sensor. Similar to to pH sensors, the most common type of ORP sensor is a combination sensor with a measuring electrode and a reference electrode. The measuring cell, typically a noble metal like platinum or gold, detects changes in REDOX potential, while the reference provides a stable comparison signal.

Like a pH measurement, an ORP measurement is not a direct indicator of concentration, but rather an indicator of activity. However, in solutions with one active species, like pool water containing chlorine, ORP correlates to concentration.