ORP is an abbreviation for for Oxidation Reduction Potential, also know 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

Characterisitics of ORP:

• Non-specific measurement of total activity
• mV output allows automated control of chemical reactions
  

Typical examples of ORP uses include:

• 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 a pH measuring electrodes except a Nobel metal is used in place of the pH glass as the measuring element. Nobel metals are used because they will not enter into the chemical reaction taking place. Other Nobel 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 needed for ORP measurements

Examples of ORP vs. various Oxidizing Agents