This article is a technical education piece to thoroughly describing OPR and its characteristics, typical applications, and ORP examples.
What Is “ORP”?
ORP is an abbreviation for Oxidation Reduction Potential, also know as REDOX, and is a useful measurement for monitoring and controlling chemical reactions.
Oxidation: addition of oxygen/reduction of electrons
Reduction: reduction of oxygen/addition of electrons
Characteristics of ORP
Non-specific measurement of total activity
mV output allows automated control of chemical reactions
Typical examples of ORP uses
Ozone or chlorine control
Chromate reduction or cyanide destruction
Units of Measure for ORP
Water treatment (sewage) – prechlorination and dechlorination
Metal Finishing (CN destruct and chromate reduction)
Ozone treatment (Commercial Aquariums, water disinfection)
Poultry processing – disinfect the skin
Fruit and vegetable washing
Pulp bleaching (paper industry)
Chlorine addition (swimming pools, spas)
Examples of ORP(mV)/Ozone Levels (ppm) for Disinfection
Aquaculture, cooling towers
Swimming pools, hot tubes
How Does an Electrode Work?
An ORP measuring electrode is identical to pH measuring electrodes except a nobel metal is used in place of the pH glass as the measuring element. Noble metals are used because they will not enter into the chemical reaction taking place. Other nobel metals such as gold or silver can be used, but platinum is the most commonly used.
The reference is identical to the one used in the pH electrode. It is an Ag/AgCl (silver/silver chloride) wire in 3.5M KCl saturated with AgCl (silver chloride). A second junction for protection of the reference wire is common in industrial electrodes and is referred to as a “Double Junction.” A combination ORP electrode works the same as a combination pH electrode. The measuring electrode generates a millivolt output based on the oxidizing or reducing reactions taking place while the reference electrode generates a constant millivolt output. The working range of an ORP electrode is +/- 2000mV. A pH meter with a millivolt scale or ORP transmitter or controller is used to display readings.
Temperature compensation is not used for ORP measurements. The correction factors are system and chemical dependent and are not easily determined.