Absolute electrode potential

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).

Definition 

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, 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 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) + e−_(gas)

For the hydrogen electrode, the absolute half-cell process would be

12H_2 (gas) → H⁺_(solution) + e−_(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. To express this in volts one divides the                                                                                 Gibb’s free energy by the negative of Faraday’s constant.

Rockwood's approach to absolute-electrode thermodynamics is easily expendable to                                                                              other thermodynamic functions. For example, the absolute half-cell entropy has been                                                                               defined as the entropy of the absolute half-cell process defined above. An alternative                                                                               definition of the absolute half-cell entropy has recently been published by Fang et al.                                                                                                         who define it as the entropy of the following reaction (using the hydrogen electrode as an example):

12H_2 (gas) → H⁺_(solution) + e−_(metal)

This approach differs from the approach described by Rockwood in the treatment of the electron,                                                                                i.e. whether it is placed in the gas phase or in the metal.