Wastewater Glossary: Oxidation-Reduction Potential (ORP)

For a body of water, multiple redox couples usually exist, forming a complex redox system; its oxidation-reduction potential (ORP) is the combined result of redox reactions among various oxidizing and reducing substances [2]. Regardless of the reaction form, oxidation means loss of electrons and reduction means gain of electrons—both are invariably accompanied by electron transfer. When a platinum electrode is inserted into a reversible redox system, it accepts electrons from the system and forms a half-cell whose potential corresponds to the system's reducing power. The potential measured by pairing it with a standard hydrogen electrode is the ORP of that system.

For a body of water, multiple oxidation-reduction potentials usually exist, forming a complex redox system. Its ORP is the combined result of redox reactions among various oxidizing and reducing substances [1]. Although this indicator cannot serve as a measure of the concentration of a specific oxidizing or reducing substance, it helps reveal the electrochemical characteristics of the water body and analyze its properties, making it a comprehensive indicator.

Using a platinum electrode as the indicator electrode and a saturated calomel electrode as the reference electrode, an original cell is formed with the water sample. The ORP of the platinum electrode relative to the saturated calomel electrode is measured with an electronic millivoltmeter or a general-purpose pH meter, and then converted to the ORP relative to the standard hydrogen electrode for reporting.

Formula: Ψn = Ψind + Ψref

Ψn — ORP of the measured water sample, mV;

Ψind — measured ORP of the water sample, mV;

The ORP of a water body must be measured in situ.

In practice, especially for most biological systems, no recognizable reaction occurs without enzymes and electron carriers. Besides direct potential measurement, ORP can also be obtained from equilibrium-constant calculations or by using redox indicators. In general, electron transfer within organisms proceeds from lower to higher ORP—for example, along the sequence NAD → flavin enzyme → cytochrome C system → O2. However, due to enzyme specificity and inhibition this sequence is not always followed, and depending on reactant concentrations a system with a lower standard potential may even oxidize one with a higher potential. In biological redox systems, polyphenols and cytochromes C and a sit near 200–300 mV, cytochrome b and flavin enzymes at 0 to −100 mV, NAD at −330 mV, and ferredoxin at −420 mV. In living cells, aerobic cells have higher potentials and anaerobic cells lower potentials; enzyme activity, cellular assimilation capacity, and microbial growth and development can all be influenced by ORP.

Ψref — electrode potential of the saturated calomel electrode at the measurement temperature, mV, obtainable from physical-chemistry handbooks.

ORP is affected by factors such as solution temperature, pH, and the reversibility of the chemical reaction.

ORP is related to oxygen partial pressure and is also affected by pH. At low pH the ORP is high; at high pH the ORP is low [3].