Oxidation Reduction Potential
|Oxidation Reduction Potential|
|Target:||300 - 450 mV|
|Danger:||< 50 mV
> 500 mV
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 Range (mV)||Oxidiser||Respiration||Product|
|>+50||oxygen (O2)||aerobic||nitrate (NO3-)|
|+50 to -50||nitrate (NO3-)
|hydrogen sulphide (H2S)|
|acids and alcohols|
|<-300||Carbon Dioxide (CO2-)
Acetic Acid (CH3COOH-)
Just like pH probes, ORP probes should also be calibrated. Techniques to do this are:
- putting quinhydrone into pH 7 and 4 calibration solutions to make standards of 86 and 263 mV, respectively, see Oxidation Reduction Potential (ORP)
- Zobell calibration solution standard of 231 mV, see YSI ® Zobell ORP Calibration Solution
- ORP and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping Magazine
- Redox; Reduction-Oxidation Potential, ORP by Bob Fenner - WetWebMedia
- Redox and ORP by Eric Edelman - Reefs.org
- Environmental Stability: Nature v. Aquarium: Photosynthetically Active Radiation, Ultraviolet Radiation, Dissolved Oxygen, and ORP, Part 2 - Oxidation-Reduction Potential (ORP) by Dana Riddle - Advanced Aquarist's Online Magazine
- wikipedia:Reduction potential
- Gerardi, M.H., The microbiology of anaerobic digesters, Wiley-IEEE, (2003).