Oxidation-Reduction Potential (ORP) provides an indication of a solution’s ability to oxidize or reduce another material. ORP is often also called “Redox”. ORP is a useful water quality parameter, particularly for processes such as water disinfection, odor control processes, and dechlorination processes prior to industrial or municipal discharge.
Oxidation and Reduction Reviewed
Oxidation is the loss of an electron. Reduction is the addition of an electron. There can be no oxidation without a simultaneous reduction. The electron lost from the oxidized element is gained by another element, which is reduced. The substance capable of donating an electron is called the reducing agent. The substance capable of accepting an electron is called the oxidizing agent. The most well known oxidation-reduction reaction is the formation of rust. Oxygen reacts with iron to form iron oxide (a.k.a. rust). During this process, iron is oxidized (loses an electron), and oxygen is reduced (gains an electron).
The higher the ORP value, the more potential a solution has to oxidize (steal electrons from) substances in solution. The lower the ORP value, the more potential a solution has to reduce (donate electrons to) other substances in solution. For example, chlorine is a strong oxidizer. Adding chlorine to a swimming pool will raise the ORP value of the water, and increase the water’s ability to destroy bacteria and microbes by oxidizing them (stealing electrons from them and altering their chemical make-up). ORP levels of 650-700 mV are generally recognized as strong enough to kill bacteria and sanitize water.
The addition of an oxidizer (chemical that donates electrons) will raise the ORP value. The addition of a reducer (chemical that accepts electrons) will lower the ORP value. Examples of oxidizers include chlorine, fluorine, ozone, hydrogen peroxide. Examples of reductants include sodium bisulfite and sulfur dioxide.
An ORP probe measures the oxidizing/reducing capability of a solution by measuring the electron activity. An ORP reading is non-specific to a particular redox reaction in solution. It is a measure of the ratio of the oxidized to reduced forms of all chemical species in solution. Because the chemical composition of a solution is typically unknown, ORP represents a non-specific measurement.
A two-electrode system is used to make an ORP measurement. In the WQ-ORP probe, these electrodes are combined into one probe body. The WQ-ORP consists of a relatively inert platinum electrode and a reference electrode. ORP is determined by the difference in potential between these two electrodes.
The platinum electrode has a low resistance and will easily give up electrons to an oxidant or accept electrons from a reductant. The electrode will continue to accept or give up electrons until it develops a potential, due to a build-up of charge from the loss or gain of electrons. This charge is equal to the ORP of solution. Nobel metals such as Platinum, Silver or Gold are used because they can sense the electron transfer but are not consumed in the reactions.