ORP Sensors – A Technical Education

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 known 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

  • = mV

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)

Examples of ORP(mV)/Ozone Levels (ppm) for Disinfection

ORP mv APPLICATION OZONE ppm
200-400 Aquaculture, cooling towers <0.06
500-600 Swimming pools, hot tubes <0.15
600-800 Water disinfection <0.4
800+ Water sterilization >0.4

 

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 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 of ORP electrodes works the same as a combination pH electrode. The measuring electrode generates a millivolt output based on the oxidizing or reducing reaction 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

 

Free Chlorine ORP/mV vs. pH

ORP mV vs. Ozone ppm