The energy change, measured in volts, required to add or remove electrons to or from an element or compound. The reference reaction is the removal of electrons from hydrogen in a standard hydrogen half-cell (i.e. H₂(gas) at 1 atm pressure delivered to a 1.0 M solution of H⁺ ions at 25 °C, into which a platinum electrode has been inserted): H₂ → 2H⁺ + 2e⁻, This energy change is given the value of zero. The oxidation potentials of other species are determined relatively by measuring the potential difference between a half-cell containing an aqueous solution of the oxidized and reduced forms of the test substance, and the standard hydrogen half-cell. For example, for Fe²⁺ → Fe³⁺ + e⁻, E^θ‎ = 0.77, for Mn²⁺ → Mn³⁺ + e⁻, E^θ‎ = 1.51. With decreasing values of oxidation potential, the reduced form of a couple (e.g. Fe²⁺) will itself reduce the oxidized form of a couple with a higher oxidation potential (e.g. Mn³⁺). The oxidation potentials obtained under these controlled conditions are called standard electrode potentials, or sometimes standard reduction potentials. Compare redox potential.