Oxidation-reduction potential

The potential that appears when an inert electrode (an electrode made of an inert material that does not participate in the redox reaction itself) such as a platinum plate is immersed in a solution of an oxidant and a reductant that are in a conjugated relationship through electron transfer. Like the standard electrode potential, it is expressed as the potential difference with a standard hydrogen electrode. The oxidant is called Ox and the reductant is called Red.
Ox+ n eRed (1)
If the activities of Ox and Red are a _O and a _R , the redox potential E is E = E ₀ - ( RT / nF )log _e ( a _R / a _O ) (2)
Here, R is the gas constant, T is the thermodynamic temperature (absolute temperature), F is the Faraday constant, and E ₀ is the potential when a _R = a _O = 1, which is called the standard redox potential.

In general, oxidants in systems with high standard redox potentials are strong oxidants, and reductants in systems with low standard redox potentials are strong reducing agents. The potential given by the expressions (1) and (2) is sometimes simply called the reduction potential, and the potential given by swapping the left and right sides of (1) to reverse the sign of (2) is sometimes called the oxidation potential, but the reduction potential method is now widely used.

[Iwamoto Shinbu]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

電子移動で共役関係にある酸化体と還元体の溶液に、白金板のような不可侵電極（それ自身は酸化還元反応に関与しない不活性な物質の電極）を浸したときに現れる電位で、標準電極電位と同じく標準水素電極との電位差で表す。酸化体をOx、還元体をRedとし、それらが、
　　Ox＋neRed　(1)
の関係にあるとき、OxとRedの活量をa_O、a_Rとすれば、酸化還元電位Eは
　　E＝E₀－(RT/nF)log_e(a_R/a_O)　(2)
で与えられる。ここでRは気体定数、Tは熱力学温度（絶対温度）、Fはファラデー定数で、E₀はa_R＝a_O＝1の場合の電位であり、標準酸化還元電位という。

　一般に標準酸化還元電位の高い系での酸化体は強力な酸化剤であり、低い系での還元体は強力な還元剤となる。また、(1)、(2)の表現で与えられる電位を単に還元電位reduction potentialとよび、(1)の左右を交換して(2)の符号が逆転する表現で与えられる電位を酸化電位oxidation potentialということもあるが、現在では還元電位による方式が広く採用されている。

［岩本振武］

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