ORP stands for oxidation-reduction potential, which is a measure, in millivolts, of the tendency of a chemical substance to oxidize or reduce another chemical substance. The oxidation-reduction potential of a solution is a measure of the oxidizing or reducing power of the solution.

Redox reactions, or oxidation-reduction potential, have a number of similarities to acid-base reactions. Fundamentally, redox reactions are a family of reactions that are concerned with the transfer of electrons between species. Like acid-base reactions, redox reactions are a matched set. you don’t have an oxidation reaction without a reduction reaction happening at the same time. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. Each reaction by itself is called a “half-reaction”, simply because we need two half-reactions to form a whole reaction. In notating redox reactions, chemists typically write out the electrons explicitly:

This half-reaction says that we have solid copper (with no charge) being oxidized (losing electrons) to form a copper ion with a plus 2 charge. Notice that, like the stoichiometry notation, we have a “balance” between both sides of the reaction. We have one copper atom on both sides, and the charges balance as well. The symbol “e-” represents a free electron with a negative charge that can now go out and reduce some other species, such as in the half-reaction:

Here, two silver ions (silver with a positive charge) are being reduced through the addition of two (2) electrons to form solid silver. The abbreviations “aq” and “s” mean aqueous and solid, respectively.

Every oxidation or reduction can be characterized by a half-reaction, which gives all of the chemical substances participating in the reaction. The ORP of the solution depends upon the logarithm of the concentrations of the substances participating in the halfreaction. The ORP can be calculated using the Nernst equation. ORP is not a good method for measuring concentration due to its logarithmic dependence on concentration and its dependence on multiple solution components. The best use of an ORP measurement is in monitoring and controlling oxidation-reduction reactions. An ORP sensor consists of an ORP electrode and a reference electrode, in much the same fashion as a pH measurement.

The principle behind the ORP measurement is the use of an inert metal electrode (platinum, sometimes gold), which, due to its low resistance, will give up electrons to an oxidant or accept electrons from a reductant.

The reference electrode used for ORP measurements is typically the same silver-silver chloride electrode used with pH measurements. In contrast with pH measurements, some offset in the reference is tolerable in ORP since, as will be seen, the mV changes measured in most ORP applications are large.

Standard Reduction Potential

The standard reduction potential is the tendency for a chemical species to be reduced, and is measured in volts at standard conditions. The more positive the potential is the more likely it will be reduced. The standard reduction potential is in a category known as the standard cell potentials or standard electrode potentials. The standard cell potential is the potential difference between the cathode and anode. For more information view Cell Potentials. The standard potentials are all measured at 298 K, 1 atm, and with 1 M solutions.

Standard reduction or oxidation potentials can be determined using a SHE (standard hydrogen electrode). Universally, hydrogen has been recognized as having reduction and oxidation potentials of zero. Therefore, when the standard reduction and oxidation potential of chemical species are measured, it is actually the difference in the potential from hydrogen. By using a galvanic cell in which one side is a SHE, and the other side is half cell of the unknown chemical species, the potential difference from hydrogen can be determined using a voltmeter. Standard reduction and oxidation potentials can both be determined in this fashion. When the standard reduction potential is determined, the unknown chemical species is being reduced while hydrogen is being oxidized, and when the standard oxidation potential is determined, the unknown chemical species is being oxidized while hydrogen is being reduced.

Standard Electrode (Reduction) Potentials

The table below is a list of important standard electrode potentials in the reduction state. To determine oxidation electrodes, the reduction equation can simply be flipped and its potential changed from positive to negative (and vice versa). When using the half cells below, instead of changing the potential the equation below can be used without changing any of the potentials from positive to negative (and vice versa):

Below is a table of standard reduction potentials.