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14-September-2008 18:38:37 - electrode potential Please wikify this article or section. Help improve this article by adding relevant internal links. January 2008 or section by expanding it. Further information might be found on the talk page or at requests for expansion. January 2008 Absolute electrode potential, in electrochemistry, 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.1 According to an IUPAC definition presented by Trasatti, the absolute electrode potential is the electrode potential of a metal measured with respect to a universal reference system without any additional metal-solution interface.2 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 EMabs = EMSHE + 4.44 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.3 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 \rightarrow M^+_solution + e^-_gas For the hydrogen electrode the absolute half-cell process would be \tfrac12 H_2 gas \rightarrow H^+_solution + e^-_gas 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 extendable 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.4 An alternative definition of the absolute half-cell entropy has recently been published by Fang et al. 5 who define it as the entropy of the following reaction using the hydrogen electrode as an example: \tfrac12 H_2 gas \rightarrow 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. Determination The basis for determination of the absolute electrode potential under the Trasatti definition is given by the equation: E^Mabs = \phi^M + \Delta ^M_S \psi where: EMabs is the absolute potential of the electrode made of metal M, V φM is the electron work function of metal M, V \Delta ^M_S \psi is the contact Volta potential difference at the metalM-solutionS 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: E^\ominus H^+/H_2abs = \phi^Hg + \Delta ^Hg _S \psi^\ominus_\sigma=0 - E^Hg_\sigma=0SHE where: E^\ominus H^+/H_2abs is the absolute standard potential of the hydrogen electrode σ=0 denotes the condition of the point of zero charge at the interface. The types of physical measurments 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 numberical values. However, there is no fundamental signficiance to this near agreement because it depends on arbitrary choices, such as temperature and definitions of standard states. See also Electrode potential Galvani potential Volta potential References ^ 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 ^ IUPAC Gold Book - absolute electrode potential ^ Alan L. Rockwood, Absolute half-cell thermodynamics: Electrode potential, Physcial Review A, Vol 33, No. 1, pp. 554-559, 1986. ^ Alan L. Rockwood, Absolute half-cell entropy, Physcial Review A, vol. 36, No. 3, pp. 1525-1526, 1987. ^ 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. Retrieved from http://en..org/wiki/Absolute_electrode_potential Categories: Electrochemistry | PotentialHidden categories: All pages needing to be wikified | Wikify from January 2008 | Articles to be expanded since January 2008 | All articles to be expanded Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page This page was last modified on 10 September 2008, at 18:37
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