Redox electrode deviates from equilibrium potential

Figure 6.5. RedOx electrode deviates from the equilibrium potential two currents flowing in opposite directions. The lengths of the arrows represent a measure of the magnitude of the process per unit area per unit time.

Figures 6.5 and 6.6 illustrate the RedOx and the metal/metal-ion electrodes in a nonequilibrium state, respectively. When current flows through an electrode, its potential A(/>(/) deviates from the equilibrium value A(/>eq by the amount ry, which was defined by Eq. (6.1a). Thus, the nonequilibrium potential difference A(p in Eqs. (6.33) and (6.34) can be substituted by the nonequilibrium value A(/)(/) ...

Mass transport overpotential (or concentration polarization (overpotential)) tjc — is a departure of the - electrode potential (or cell potential), E, from the - equilibrium electrode potential (or zero-current potential), Eeq, caused by a -> faradaic current flow and the associated surface concentration, Cox(x = 0), deviation from the bulk concentration, Cox(°°)> of the reacting redox species, Ox... 

Standard potential values are usually those of ideal unimolal solutions at a pressure of 1 atm (ignoring the deviations of fugacity and activity from pressure and concentration, respectively). A pressure of 1 bar = 10 Pa was recommended as the standard value to be used in place of 1 atm = 101 325 Pa (the difference corresponds to a 0.34-mV shift of potential). If a component of the gas phase participates in the equilibrium, its partial pressure is taken as the standard value if not, the standard pressure should be that of the inert gas over the solution or melt. In a certain case, a standard potential can be established in a system with nonunity activities, if the combination of the latter substituted in the Nemst equation equals unity. For any sohd component of redox systems, the chemical potential does not change in the course of the reaction, and it remains in its standard state. In contrast to the common thermodynamic definition of the standard state, the temperature is ignored, because the potential of the standard hydrogen (protium) electrode is taken to be zero at any temperature in aqueous and protic media. The zero temperature coefficient of the SHE corresponds to the conventional assumption of... 

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