Oxidation Reduction Potential

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Water Parameter
Oxidation Reduction Potential
Unit(s): mV
Target: 300 - 450 mV
Danger: < 50 mV

> 500 mV

Tendancy: Decrease
Increased: .
Decreased: Metabolism
Testing: Rarely
Maintenence: Ozone


The oxidation reduction potential (ORP), which is also know as Redox Potential (where redox is an abbreviation of reduction-oxidation reaction), of water in a marine aquaria is a relative measure of its oxidising power.

ORP is the tendency of a chemical species to acquire electrons and thereby be reduced. Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species' affinity for electrons and tendency to be reduced. It is very similar to pH, which is a measure of the transfer of hydrogen ions between chemical species. The ORP of water is determined by the transfer of electrons between chemical species. And just like pH, it is not a measure of the capacity for the system for oxidation or reduction.

ORP and Metabolism[1]
ORP Range (mV) Oxidiser Respiration Product
>+50 oxygen (O2) aerobic


nitrate (NO3-)
+50 to -50 nitrate (NO3-)

nitrite (NO2-)



nitrogen (N2)
<-50 sulphate (SO42-) anaerobic

sulphate reduction

hydrogen sulphide (H2S)
<-100 organics anaerobic

acid formation

acids and alcohols
<-300 Carbon Dioxide (CO2-)

Acetic Acid (CH3COOH-)


methane formation

methane (CH4)


Just like pH probes, ORP probes should also be calibrated. Techniques to do this are:



  1. Gerardi, M.H., The microbiology of anaerobic digesters, Wiley-IEEE, (2003).