Absolute electrode potential
Absolute electrode potential, in
electrochemistry, according to an IUPACdefinition [ [http://goldbook.iupac.org/A00022.html IUPAC Gold Book - absolute electrode potential ] ] , is the electrodepotential 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 [ Sergio Trasatti, "The Absolute Electrode Potential: an Explanatory Note (Recommendations 1986)", International Union of Pure and Applied Chemistry, Pure & AppL Chem., Vol. 58, No.7, pp. 955—966, 1986. http://www.iupac.org/publications/pac/1986/pdf/5807x0955.pdf (pdf)] , 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 electrolytein which the electrode is submerged.
This potential is difficult to determine accurately. For this reason,
standard hydrogen electrodeis 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: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. [ Alan L. Rockwood, "Absolute half-cell thermodynamics: Electrode potential", Physcial Review A, Vol 33, No. 1, pp. 554-559, 1986.] 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
For the hydrogen electrode, the absolute half-cell process would be
Other types of absolute electrode reactions would be defined analogously.
In this approach, all three species talking 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. [Alan L. Rockwood, "Absolute half-cell entropy", Physcial Review A, vol. 36, No. 3, pp. 1525-1526, 1987.] An alternative definition of the absolute half-cell entropy has recently been published by Fang et al. [Zheng Fang, Shaofen Wang, Zhenghua Zhang, and Guanzhou Qiu, "The electrochemical Peltier heat of the standard hydrogen electrode reaction", Thermochimica Acta, Vol. 473, pp. 40-44, 2008.] who define it as the entropy of the following reaction (using the hydrogen electrode as an example):
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.
The basis for determination of the absolute electrode potential under the Trasatti definition is given by the equation:
where::EM(abs) is the absolute potential of the electrode made of metal M, V: is the electron
work functionof metal M, V: is the contact (Volta) potential difference at the metal(M)-solution(S) interface, V.
For practical purposes, the value of the absolute electrode potential of the standard hydrogen electrode is best determined with the utility of data for an ideally-polarizable mercury (Hg) electrode:
where:: is the absolute standard potential of the hydrogen electrode:σ=0 denotes the condition of the
point of zero chargeat the interface.
The types of physical measurements required under the Rockwood definition are similar to those required under the Trasatti definition, but they are used in a different way, e.g. in Rockwood's approach they are used to calculate the equilibrium vapor pressure of the electron gas. The numerical value for the absolute potential of the standard hydrogen electrode one would calculate under the Rockwood definition is sometimes fortuitously close to the value one would obtain under the Trasatti definition. This near-agreement in the numerical value depends on the choice of ambient temperature and standard states, and is the result of the near-cancellation of certain terms in the expressions. For example, if a standard state of one atmosphere ideal gas is chosen for the electron gas then the cancellation of terms occurs at a temperature of 296 K, and the two definitions give an equal numerical result. At 298.15 K a near-cancellation of terms would apply and the two approaches would produce nearly the same numerical values. However, there is no fundamental significance to this near agreement because it depends on arbitrary choices, such as temperature and definitions of standard states.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Standard electrode potential — In electrochemistry, the standard electrode potential, abbreviated Eo, E0, or EO (with a superscript plimsoll character, pronounced nought), is the measure of individual potential of a reversible electrode (at equilibrium) at standard state,… … Wikipedia
Standard electrode potential (data page) — The standard electrode potentials are electrode potentials of half cells at equilibrium. They can be used to determine the potential of an electrochemical cell or galvanic cell, or a position of equilibrium for an electrochemical (redox) reaction … Wikipedia
Absolute — may mean:Philosophy * Absolute (philosophy), a concept in philosophy * Moral absolutes, the belief that there are absolute standards against which moral questions can be judgedMath and science * Absolute value, in mathematics, the value of a real … Wikipedia
potential — 1. Capable of doing or being, although not yet doing or being; possible, but not actual. 2. A state of tension in an electric source enabling it to do work under suitable conditions; in relation to electricity, p. is analogous to the temperature… … Medical dictionary
absolute potential — The theoretical true potential difference between an electrode and a solution of its ions, measured against a hypothetical reference electrode, having an absolute potential of zero, with reference to the same solution … Dictionary of automotive terms
Galvani potential — (also called Galvani potential difference, or inner potential difference, Δφ, delta phi) in electrochemistry, is the electric potential difference between two points in the bulk of two phases [ [http://www.iupac.org/goldbook/G02574.pdf IUPAC Gold … Wikipedia
Reference electrode — Reference electrodeis an electrode which has a stable and well known electrode potential.The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each… … Wikipedia
Standard hydrogen electrode — NHE redirects here. For the sodium hydrogen exchanger protein, see Sodium hydrogen antiporter. The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation reduction… … Wikipedia
Electric potential — Electromagnetism Electricity · … Wikipedia
Volta potential — (also called Volta potential difference, or contact potential difference, or outer potential difference, Δψ, delta psi) in electrochemistry, is the electric potential difference between two points ( 1 and 2 ) in the vacuum: point 1 close to the… … Wikipedia