The data below tabulates standard electrode potentials (E°), in volts relative to the standard hydrogen electrode (SHE), at: Variations from these ideal conditions affect measured voltage via the Nernst equation. Electrode potentials of successive elementary half-reactions cannot be directly added. However, the corresponding Gibbs free energy changes (∆G°) must satisfy where z electrons are transferred, and the Faraday constant F is the conversion factor describing Coulombs transferred per mole electrons. Those Gibbs free energies can be added. For example, from Fe + 2 Fe(s) (–0.44 V), the energy to form one neutral atom of Fe(s) from one Fe ion and two electrons is 2 × 0.44 eV = 0.88 eV, or 84 907 J/(mol ). That value is also the standard formation energy (∆Gf°) for an Fe ion, since and Fe(s) both have zero formation energy. Data from different sources may cause table inconsistencies. For example: From additivity of Gibbs energies, one must have But that equation does not hold exactly with the cited values.

Table of standard electrode potentials

Legend: (s) – solid; (l) – liquid; (g) – gas; (aq) – aqueous (default for all charged species); (Hg) – amalgam; bold – water electrolysis equations.