ORP, or Oxidation Reduction Potential, is the ratio of the oxidizers in the water compared with the oxygen reducers in the water. In terms of pool chemistry, ORP measures the sanitizer residual, which is the water's ability to oxidize, and thereby destroy, contaminants. In some areas, it is also called "Redox."
Reduction potential is the measure of a solution's tendency to acquire electrons (which reduces its oxidation) when new chemicals are introduced into it. Just as the transfer of hydrogen ions determines pH, the transfer of electrons determines the reduction potential. This is important for pool chemistry because heavily reduced water cannot oxidize contaminants effectively.
Thus, ORP does not measure the amount of free available chlorine in the pool's water, but it does measure the potential effectiveness of that chlorine as an oxidizer. That effectiveness is heavily dependent on the water's pH, which is why a higher pH will produce lower ORP readings.
ORP is measured in millivolts (mV) with a device called an ORP meter, which detects the tiny electrical currents generated when metal is placed in water that contains oxidizing and reducing agents. For pool water, the accepted minimum ORP level is 650 mV.
- ORP Explained
- Myron L Company Article on ORP (PDF)
- UC Davis Article on ORP and Water Disinfection (PDF)