Electrochemical potential dependence

McMurry, J., and Kendall, D. (1999). An artificial transmembrane segment directs SecA, SecB, and electrochemical potential-dependent translocation of a long amino-terminal tail. J. Biol. Chem. 274, 6776—6782. 

Summary, Scanning probe microscopy studies of electrodes chemically modified with electroactive transition metal complexes are described. Emphasis is placed on scanning tunneling microscopy and electrochemical scanning tunneling microscopy studies of their structure and dynamics of formation and on electrochemical force spectroscopy studies of their electrochemical potential dependent chemical properties. 

Notice first of all that equations (5.56) for the relative fluxes hold for the charged components as well, but for these components, the quantities should be understood as the electrochemical potentials depending on pressure, temperature, chemical composition and electric parameters. The last are, in turn, described by independent equations. As to phenomenological equations for concentrated mixtures, they are cumbersome and demand a lot of empirical parameters. 

In this section, the methodology adopted by Filhol and Neurock is discussed in greater detail, particularly with respect to establishing the reference potential and determining electrochemical potential-dependent structural and energetic transformations at the metal olution interface. A more complete discussion of the approach can be found in the paper by Taylor et al. [26]. Methanol dehydrogenation over Pt(lll) is used once again as a case study to examine potential-dependent chemical reactions. 

The chemical potential pi, has been generalized to the electrochemical potential Hj since we will be dealing with phases whose charge may be varied. The problem that now arises is that one desires to deal with individual ionic species and that these are not independently variable. In the present treatment, the difficulty is handled by regarding the electrons of the metallic phase as the dependent component whose amount varies with the addition or removal of charged components in such a way that electroneutrality is preserved. One then writes, for the ith charged species. 

The most important class of redox indicators, however, are substances that do not participate in the redox titration, but whose oxidized and reduced forms differ in color. When added to a solution containing the analyte, the indicator imparts a color that depends on the solution s electrochemical potential. Since the indicator changes color in response to the electrochemical potential, and not to the presence or absence of a specific species, these compounds are called general redox indicators. 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

For a charged species, the situation is slightly more complicated. In this case, the movement of a molecule across a membrane depends on its electrochemical potential. This is given by... 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

Figure 6. Scheme of microwave-electrochemical setup showing time-resolved, space-resolved and potential-dependent measurement techniques, as well as combinations of these. 

Up to now only qualitative data have been available on potential-dependent MC measurements of electrochemical interfaces. When metals or other highly conducting materials are used, or when liquids are in play, special care has to be taken to allow access of microwave power to the active electrode/electrolyte interface. 

Conway, B. E. The Temperature and Potential Dependence of Electrochemical Reaction Rates, and the Real Form of the Tafel Equation 16... 

One might righteously ask why this close and preferential connection exists between the r vs and the r vs po dependencies. The answer is straightforward and has simply to do with the definitions of O and Fermi level EF (or electrochemical potential of electrons j (=EF))7 which are connected via ... 

The electrochemical potential difFetence across the membrane, once established as a tesult of proton translocation, inhibits further transport of teducing equivalents through the respiratory chain unless discharged by back-translocation of protons across the membtane through the vectorial ATP synthase. This in turn depends on availability of ADP and Pj. 

The energy of an ion in a given medium depends not only on chemical forces but also on the electrostatic held hence the chemical potential of an ion j customarily is called its electrochemical potential and labeled fi. The electrostatic potential energy of an ion j when reckoned per mole is given by ZjF, where / is the electrostatic (inner) potential of the phase containing the ion a plus sign for cations and a minus sign for anions. Hence, the electrochemical potential can be written as the sum of two terms ... 

The current is recorded as a function of time. Since the potential also varies with time, the results are usually reported as the potential dependence of current, or plots of i vs. E (Fig.12.7), hence the name voltammetry. Curve 1 in Fig. 12.7 shows schematically the polarization curve recorded for an electrochemical reaction under steady-state conditions, and curve 2 shows the corresponding kinetic current 4 (the current in the absence of concentration changes). Unless the potential scan rate v is very low, there is no time for attainment of the steady state, and the reactant surface concentration will be higher than it would be in the steady state. For this reason the... 

Alexander N. Frumkin pointed out in 1932 that an electrochemical reaction occnrring at different potentials can be regarded as an ideal set of chemical reactions of the same type, and suggested that the Brpnsted relation be nsed to explain the potential dependence of electrochemical reaction rates. On the basis of Eqs. (14.6) and (14.11), the relation for the activation energy becomes... 

In 1930, Max Volmer and Tibor Erdey-Griiz used the concept of a slow discharge step for cathodic hydrogen evolntion (slow discharge theory). According to these ideas, the potential dependence of electrochemical reaction rate constants is described by Eq. (6.5). Since hydrogen ions are involved in the slow step A, the reaction rate will be proportional to their concentration. Thus, the overall kinetic equation can be written as... 

The rate of an electrochemical reaction depends, not only on given system parameters (composition of the catalyst and electrolyte, temperature, state of the catalytic electrode surface) but also on electrode potential. The latter parameter has no analog in heterogeneous catalytic gas-phase reactions. Thus, in a given system, the potential can be varied by a few tenths of a volt, while as a result, the reaction rate will change by several orders of magnitude. 

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