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.

Accurate ORP Sensor

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.


Why is ORP Measurement Needed?

ORP measurements are a cost effective method used to monitor a variety of processes. Common applications include disinfection using chlorine, ozone, and bromine. All of these species are strong oxidizers, so ORP is used as an indicator of sanitizing activity. ORP is also used in process applications to monitor destruction of chromate or cyanide, as well as in bleach production and wet scrubbers.

Common Applications:

  • Prechlorination and dechlorination in water/wastewater treatment
  • Cyanide destruction and chromate reduction in metal finishing
  • Ozone treatment process control
  • Bleach production
  • Poultry processing
  • Fruit and vegetable washing
  • Pulp bleaching
  • Chlorine addition in commercial swimming pools & spas