Frumkin electrical potential difference

Consider a system in which a potential difference AV, in general different from the equilibrium potential between the two phases A 0, is applied from an external source to the phase boundary between two immiscible electrolyte solutions. Then an electric current is passed, which in the simplest case corresponds to the transfer of a single kind of ion across the phase boundary. Assume that the Butler-Volmer equation for the rate of an electrode reaction (see p. 255 of [18]) can also be used for charge transfer across the phase boundary between two electrolytes (cf. [16, 19]). It is mostly assumed (in the framework of the Frumkin correction) that only the potential difference in the compact part of the double layer affects the actual charge transfer, so that it follows for the current density in our system that... 

The adsorption of either ions or neutral molecules on the electrode surface depends on qn, i.e., on the apphed electric potential. Correspondingly, the electric field at the electrochemical interface is an additional free-energy contribution that either favors or restricts the adsorption of species on the electrode from the ionic conducting phase. A variety of adsorption isotherms has been proposed to account for the behavior of different electrochemical systems. Among them are the Langmuir, Frumkin, and Temkin isotherms [2]. Frumkin and Temkin isotherms, at variance with the Langmuir one, include effects such as adsorbate—adsorbate or adsorbate—surface interactions. 

The thermodynamic analyses used in this chapter make use of the electrochemical potential. In this way the electrical aspects of the interfacial equilibria are clearly defined. Earlier work on this problem, especially that by Volta and Nernst, had led to different conclusions regarding the source of the EMF in an electrochemical cell [12]. This problem was resolved by Frumkin, essentially, by writing out the interfacial equilibria using electrochemical potentials. In this regard, all interfaces in the cell must be considered including those between different metals at the terminals of the cell. This was shown in the discussion of the thermodynamic basis of the Nernst equation. 

Frumkin noted that, at different potentials, an electrode reaction may be regarded as a series of similar reactions differing only in the value of AG , this difference being equal to nFA (in the case of electrode reactions, one should consider the overall free energy change, including electrical work in what follows, this value will be denoted AG n is the number of electrons transferred within an elementary act of a slow process step). Therefore, for electrochemical reactions, Eq. (13) takes the form AG = AGS + anF cf>. 

We saw that formal kinetic equations apart from kinetic parameters also contain surface concentrations Cj of electrically active species. It follows from the material presented in previous chapters that differs from the corresponding bulk values because a diffusion layer with certain concentration profiles forms at the electrode surface. Moreover, another reason due to which surface concentrations change is adsorption phenomena, which form a certain structure called a double electrode layer (DEL) at the boundary metal solution. It is clear that in kinetic equations, it is necessary to use local concentrations of reactants and products, that is, concentrations in that region of DEL where electrically active particles are located. The second effect produced by DEL is related to the fact that a potential in the localization of the electrically active complex (EAC) differs from the electrode potential. Therefore, activation energy of the electrochemical process does not depend on the entire jump of the potential at the boundary but on its part only, which characterizes the change in the potential in the reaction zone. In this connection, the so-called Frumkin correction appears in the electrochemical kinetic equations, which is related to the evaluation of the local potential i// [1]. 

A very typical experiment in which the formation of this surface oxide is evident, has been described by Balaschowa and Frumkin They placed a platinum wire in a solution, applied an electric field perpendicular to the wire, and measured its electrophoretic deflection at different polarizing potentials. In Fig. 36 curves 1 and 2, valid for HCl and H2SO4 (2 10- AT), show the normal zero point of charge at a positive polarization between 0.1 and 0 2 volts. At increasing positive potential the electrophoretic velocity first increases then goes through a maximum and even reverses in sign. This means that at an anodic polarization of somewhat more than 0.4 volt the formation of the platinum oxide layer will start. 

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