Potential equilibrium electrode

We will discuss two types of electrode potentials metal/metal-ion and redox potentials. 

in the relative scale of potential where , = 0 the electrode potential is equal to the measured cell voltage , we obtain 

The term A / (Pt,M) appears in all measurements and thus does not influence the order of the measured electrode potentials. It is the potential difference that appears when two dissimilar conductors come into contact. Since the Fermi energies of two different metals are in general different, flow of electrons occurs that tends to equalize the Fermi energies, i.e., their chemical potential. The Fermi level is either (1) the uppermost (the top) filled energy level in a partially occupied valence band of electrons in a solid or (2) the boundary between the filled states and the empty ones in a band of electrons in a solid (Chapter 3). This electron flow charges up one conductor relative to the other and the contact potential difference results (Fig. 5.3). 

CONCENTRATION DEPENDENCE OF EQUILIBRIUM CELL VOLTAGE THE GENERAL NERNST EQUATION 

The Nemst equation can be derived by considering a general cell reaction with A,B,. . . reactants and M,N,.. . products 

Fundamentals of Electrochemical Deposition, Second Edition. By Milan Paunovic and Mordechay Schlesinger Copyright 2006 John Wiley Sons, Inc. 

If El is taken as a reference electrode and set arbitrarily to = 0, then S = E. Thus, in the relative scale of potential where = 0, the electrode potential E is equal to the measured cell voltage , and we obtain 

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Redox Potential the equilibrium electrode potential of a reversible reduction-oxidation reaction, e.g. Cu /Cu, Fe /Fe, Cl /Cr. 

Equilibrium electrode potentials are readily established when metal electrodes are in contact with melts. However, two difficnlties arise in attempts to measnre them suitable, sufficiently corrosion-resistant reference electrodes must be selected, and marked diffusion potentials develop at interfaces between different melts. 

If current passes through an electrolytic cell, then the potential of each of the electrodes attains a value different from the equilibrium value that the electrode should have in the same system in the absence of current flow. This phenomenon is termed electrode polarization. When a single electrode reaction occurs at a given current density at the electrode, then the degree of polarization can be defined in terms of the over potential. The overpotential r) is equal to the electrode potential E under the given conditions minus the equilibrium electrode potential corresponding to the considered electrode reaction Ec ... 

However, the value of the equilibrium electrode potential is often not well defined (e.g. when the electrode reaction produces an intermediate that undergoes a subsequent chemical reaction yielding one or more final products). Often, an equilibrium potential is not established at all, so that the calculated equilibrium values must often be used. 

The equilibrium electrode potential is given by the Nernst equation (cf. 3.2.17),... 

In contrast to the equilibrium electrode potential, the mixed potential is given by a non-equilibrium state of two different electrode processes and is accompanied by a spontaneous change in the system. Besides an electrode reaction, the rate-controlling step of one of these processes can be a transport process. For example, in the dissolution of mercury in nitric acid, the cathodic process is the reduction of nitric acid to nitrous acid and the anodic process is the ionization of mercury. The anodic process is controlled by the transport of mercuric ions from the electrode this process is accelerated, for example, by stirring (see Fig. 5.54B), resulting in a shift of the mixed potential to a more negative value, E mix. 

Standard emf standard equilibrium electrode potential equilibrium electrode rest potential Non-equilibrium electrode potential... 

The overpotential tj is defined as E — fcr, where E, is the equilibrium electrode potential at the concentration [H+] and 0H that actually pertain in the system. In fact, since ()H is unknown normally, we actually define Er with reference to the values of [H +] and pH, through the Ncrnst equation ... 

From Eqns. 4-27 and 4-28, the equilibrium electrode potential, , is obtained for the transfer of silver ions and chloride ions at the silver-silver chloride electrode as shown in Eqn. 4-29 ... 

It appears from Eqns. 10-17 and 10-18 that the potential for the onset of the anodic photoexcited hole transfer in the m ygen reaction shifts itself from the equilibrium electrode potential toward the less anodic direction (cathodic direction)... 

The potential, E, for the onset of the photoexdted reaction relative to the equilibrium electrode potential E of the same reaction can also be derived in a kinetics-based approach [Memming, 1987]. Here, we consider the transfer of anodic holes (minority charge carriers) at an n-type semiconductor electrode at which the hole transfer is in quasi-equilibrium then, the anodic reaction rate is controlled by the photogeneration and transport of holes in the n-type semiconductor electrode. The current of hole transport, has been given by Eqn. 8-71 as a function of polarization ( - ,) as shown in Eqn. 10-20 ... 

Figure 10-17 shows the polarization ciirves for the cathodic hydrogen reaction (cathodic electron transfer) on a p-type semiconductor electrode of galliiun phosphide. The onset potential of cathodic photoexcited hydrogen reaction shifts significantly from the equilibrium electrode potential of the same hydrogen reaction toward the flat band potential of the p-type electrode (See Fig. 10-15.). 

Equation (5.9) is the general Nemst equation giving the concentration dependence of the equilibrium cell voltage. It will be used in Section 5.4 to derive the equilibrium electrode potential for metal/metal-ion and redox electrodes. 

The equilibrium potential of an electrode (e.g., M/M ) is defined in Section 5.2 as the voltage of the cell, Pt H2(l atm) H+(a = 1) M M, where a is the activity. Three issues have to be resolved to measure this equilibrium electrode potential (1) the selection of a reference electrode (2) the coupling of the reference electrode with the electrode whose potential is being measured, in this case M/M + and (3) the experimental method for the voltage measurement. 

A simple way of visualizing the procedure is to represent all equilibrium potentials on a single vertical axis (Fig. 7.176). Corresponding to any activity ratio aA/aD, there is an equilibrium electrode potential for the interface relative to the SHE. The same is true for the other activity ratio a A/a D. Thus, the separation between any two points yields the potential difference across a cell, with the activity ratios corresponding to the points. 

An interesting result has emerged. When a Zn/Zn2+ interface and a Cu/Cu2+ interface are built into an electrochemical cell or system, one can proceed from the equilibrium electrode potentials and the zero-current cell potential to predict at which interface there will be a tendency for deelectronation (oxidation) and at which a tendency for electronation (reduction), i.e., which electrode will function as the electron source and which as the electron sink. 

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