Interfacial potential differences electrochemical potentials

An electric current flowing through an ITIFS splits into nonfaradaic (charging or capacity) and faradic current contributions. The latter contribution comprises the effects of both the transport of reactants to or from the interface, and the interfacial charge transfer, the rate of which is a function of the interfacial potential difference. By applying a transient electrochemical technique, these two effects can be resolved... 

Modified electrodes. Where relevant, we have followed the recent lUPAC directive on the recommended list of terms for chemically modified electrodes (CMEs) [1]. A CME is thus an electrode made up of a conducting or semiconducting material that is coated with a selected monomolecular, multimolecular, ionic or polymeric film of a chemical modifier and that, by means of faradaic reactions or interfacial potential differences exhibits chemical, electrochemical and/or optical properties of the film . 

The distinctive feature of electrochemical kinetics is the strong dependence of reaction rates on the interfacial potential difference. 

With any electrochemical technique to study kinetics, the electrode-solution interface is perturbed from its initial situation. The initial conditions may be such that the system is in a chemical equilibrium and this usually means that the interfacial potential difference is determined by Nernst s law holding for the two components O and R of a redox couple being present... 

The interfacial potential difference (pd) for the partition equilibrium interface is given by the equality of electrochemical potential in terms of all ions in equilibrium, equation (4). 

A distinguishing aspect in electrode kinetics is that the heterogeneous rate constants, kred and kox, can be controlled externally by the difference between the inner potential in the metal electrode (V/>M) and in solution (7/>so1) that is, through the interfacial potential difference E = electrode setup (typically, a three-electrode arrangement and a potentiostat), the E-value can be varied in order to distort the electrochemical equilibrium and favor the electro-oxidation or electro-reduction reactions. Thus, the molar electrochemical Gibbs energy of reaction Scheme (l.IV), as derived from the electrochemical potentials of the reactant and product species, can be written as (see Eqs. 1.32 and 1.33 with n = 1)... 

Under electrochemical equilibrium conditions (AG = 0), the interfacial potential difference is given by the Nemst equation (see Eqs. 1.34 and 1.36) ... 

Adsorption and oil-water potentials. Dean, Gatty, and Rideal2 discuss the thermodynamics and the mechanism of the establishment of interfacial potential differences by the adsorption of ions, or by the adsorption or spreading of a film containing dipoles. They show that, provided that one or more of the charged components can pass the phase boundary and come into equilibrium on both sides, the adsorption of the interfacial film will not by itself change the phase boundary potential. For the electrochemical potentials of those charged components which can pass the boundary are equal, at equilibrium, in the two phases, i.e. 

Traditionally, electrochemical equilibria are explained in terms of thermodynamic cell potentials. However, in electro analytical applications, such a description is of little use, because one almost always uses a non-thermodynamic measurement, with a reference electrode that includes a liquid junction. It is then more useful to go back to the basic physics of electrochemistry, i.e., to the individual interfacial potential differences that make up the total cell potential. This is the approach we will use here. 

The first and last terms are interfacial potential differences arising from an equilibrium balance of selective charge exchange across an interface. This condition is known as Donnan equilibrium (24, 51). The magnitude of the resulting potential difference can be evaluated from electrochemical potentials. Suppose we have Na" and as interfacially active ions. Then at the a/m interface,... 

The accumulation of electric charges (nontransferable across the interface in the absence of electrochemical process) on each part of the interface results from the existence of mobile charge carriers in the two phases in contact with each other and an interfacial potential difference [153]. The charge... 

In electrochemical kinetics, this model corresponds to the Butler-Vohner equation widely used for the electrode reaction rate. The latter postulates an exponential (Tafel) dependence of both partial faradaic currents, anodic and cathodic, on the overall interfacial potential difference. This assumption can be rationalized if the electron transfer (ET) takes place between the electrode and the reactant separated by the above-mentioned compact layer, that is, across the whole area of the potential variation within the framework of the Helmholtz model. An additional hypothesis is the absence of a strong variation of the electronic transmission coefficient", for example, in the case of adiabatic reactions. 

Two reference electrodes can then be immersed in the two solutions and the electromotive force (e.m.f.) measured in this electrochemical cell. The measured value consists of liquid junction potential(s), which can be maintained constant within a certain experimental error by maintaining a constant, relatively high ionic strength and a suitable composition of the solutions, and the interfacial potential differences at the sides of the membrane adjacent to the sample and standard solutions. As the potential difference between the membrane and the standard solution is constant, the overall change in the e.m.f. corresponds to the change in the composition of the sample solution. 

The expressions for the rates of the electrochemical reactions given in Section II. A have not taken into account the detailed structure of the interfacial region. In general, the solution adjacent to the electrode will consist of at least two regions. Immediately adjacent to the metal there will be a compact layer of ions and solvent molecules which behaves as a capacitor. A potential difference will be established between... 

It is possible to find a range in which the electrode potential is changed and no steady state net current flows. An electrode is called ideally polarized when no charge flows accross the interface, regardless of the interfacial potential gradient. In real systems, this situation is observed only in a restricted potential range, either because electronic aceptors or donors in the electrolyte (redox systems) are absent or, even in their presence, when the electrode kinetics are far too slow in that potential range. This represents a non-equilibrium situation since the electrochemical potential of electrons is different in both phases. 

Although the equilibrium principle was available (equality of electrochemical potential of each ion that reversibly equilibrates across an immiscible liquid/liquid interface), the elementary theory and consequences were not explored until recently (6). To develop an interfacial potential difference (pd) at a liquid interface, two ions M, X that partition are required. However,... 

Thus, local corrosion (and the term local may imply a size of a few atoms up to that of a millimeter) occurs whenever a region of a material surface, a, is connected electrically (through a flow of electrons in the underlying metal region, p, at which there are interfacial reactions exhibiting an electrochemical potential different from that at a. The different constituents of an alloy would tend to provoke such a situation or, for example, S inclusions in steel. 

When an ion transfers at the interface between water, W, and an organic solution, O, a current flows across the interface, and the potential difference at the interface varies depending on the amount of the ion transferred, since the ion carries a charge similarly to an electron. Therefore, the ion transfer reaction can be regarded as an electrochemical process. Another electrochemical process at the W/0 interface is the electron transfer, which proceeds when a reductant or an oxidant in W comes into contact with an oxidant or a reductant, respectively, in O at the interface, resulting in an interfacial redox reaction. 

Thus, the concentration of the diffusing species has the same value c at any t at f = 0 or for any r > 0 at x >. This is true for almost all electrochemical diffusion problems in which one switches on (at t = 0) the appropriate current or potential difference across the interface and thus sets up interfacial charge-transfer reactions which, by consuming or producing a species, provoke a diffusion flux of that species. 

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