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What Is "ORP"?
ORP is an abbreviation for Oxidation Reduction Potential, also known as REDOX, and is a useful measurement
for monitoring and controlling chemical reactions.
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Oxidation: addition of oxygen/reduction of electrons
Reduction: reduction of oxygen/addition of electrons
Characteristics of ORP:
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Non-specific measurement of total activity
mV output allows automated control of chemical reactions
Typical examples of ORP uses include:
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Ozone or chlorine control
Chromate reduction or cyanide destruction
Units of Measure for ORP = mV
How Does an Electrode Work?
An ORP measuring electrode is identical to pH measuring electrodes except a noble 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 noble 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 a 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.
Examples of ORP vs. various Oxidizing Agents:
ORP mV vs. Ozone ppm
ORP mv
APPLICATION
200400
Aquaculture, cooling
towers
OZONE ppm
<0.06
500600
Swimming pools, hot tubes
<0.15
600800
Water disinfection
<0.4
Water sterilization
>0.4
800+
ORP Applications
Water treatment (sewage) - prechlorination and dechlorination Metal
Finishing (CN destruct and chromate reduction)
Ozone treatment (Commercial Aquariums, water disinfection) Bleach
production
Poultry processing - disinfect the skin
Fruit and vegetable washing
Pulp bleaching (paper industry)
Chlorine addition (swimming pools, spas)
Information courtesy of Sensorex via website