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Concept
Absolute electrode potential
Summary
Absolute electrode potential, in electrochemistry, according to an IUPAC definition, is the electrode potential of a metal measured with respect to a universal reference system (without any additional metal–solution interface).
According to a more specific definition presented by Trasatti, the absolute electrode potential is the difference in electronic energy between a point inside the metal (Fermi level) of an electrode and a point outside the electrolyte in which the electrode is submerged (an electron at rest in vacuum).
This potential is difficult to determine accurately. For this reason, a standard hydrogen electrode is typically used for reference potential. The absolute potential of the SHE is 4.44 ± 0.02 V at 25 °C. Therefore, for any electrode at 25 °C:
where:
E is electrode potential
V is the unit volt
M denotes the electrode made of metal M
(abs) denotes the absolute potential
(SHE) denotes the electrode potential relative to the standard hydrogen electrode.
A different definition for the absolute electrode potential (also known as absolute half-cell potential and single electrode potential) has also been discussed in the literature. In this approach, one first defines an isothermal absolute single-electrode process (or absolute half-cell process.) For example, in the case of a generic metal being oxidized to form a solution-phase ion, the process would be
M(metal) → M+(solution) + _electron(gas)
For the hydrogen electrode, the absolute half-cell process would be
1/2H2 (gas) → H+(solution) + _electron(gas)
Other types of absolute electrode reactions would be defined analogously.
In this approach, all three species taking part in the reaction, including the electron, must be placed in thermodynamically well-defined states. All species, including the electron, are at the same temperature, and appropriate standard states for all species, including the electron, must be fully defined. The absolute electrode potential is then defined as the Gibbs free energy for the absolute electrode process.
Official source
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