A measure of the oxidation/reduction status of a natural water, sediment or soil. It is a relative electrical potential, measured with a potentiometer (e.g., a pH meter adjusted to read in volts) using an inert platinum electrode and a reference electrode (calomel or silver/silver chloride). E_H is reported in volts or millivolts, and is referenced to the potential for the oxidation of hydrogen gas to hydrogen ions (H⁺). This electron transfer reaction is assigned a potential of zero on the relative potential scale. The oxidation and reduction reactions in natural systems are pH dependent and the interpretation of E_H values requires a knowledge of the pH. At pH 7, the E_H in water in equilibrium with the oxygen in air is +0.76v. The lowest possible potential is 0.4v when oxygen and other electron acceptors are depleted and methane, carbon dioxide, and hydrogen gas are produced by the decay of organic matter.

See Also

Electron Acceptor and Donor