Oxidation-reduction potential (ORP), or "redox", indicates the relative capacity of a solution to oxidize or reduce. For example, to minimize biological growth a cooling system is usually treated with an oxidizing biocide such as chlorine, chlorine dioxide, or bromine.
Maintaining the ORP in a designated range will ensure that the proper amount of oxidizing agent is present. At the other end of the spectrum, reducing agents are often used to remove oxygen or other compounds from process streams. ORP can be used to monitor the proper level of these agents as well.
ORP sensors measure the electrochemical potential between the solution and a reference electrode. Readings are expressed in millivolts with positive readings indicating increased oxidizing potential, and negative readings being increased reduction.
ORP sensors cannot indicate levels of specific compounds, only relative degrees of oxidation or reduction.