Reversible electrode potential measurement

Taking the polarographic half-wave potential (Ex 2 - - 1.45 volts, versus standard caloriel electrode hence ECil 2 - - 1.18 volts) in l.OAf CN as the measure of the reversible electrode potential (64). 

The experimentally measured reversible electrode potential, E q, includes not only the above emf but also the potential difference at the metal-platinum contact. The electrons are the electromotively active particles at this junction, and it may be assumed that at equilibrium an electrical potential difference exists between the two metals which equalizes the electrochemical potential of the electrons in the two phases. As is well known, it is equivalent to the Volta potential difference and is given by the following ... 

The single thermodynamically reversible electrode potential as measured against the standard hydrogen electrode is equal to the emf of the cell,... 

It should be noted that in the case where only electrons are exchanged with the metal the value of the reversible electrode potential, Eeq, is independent of the electrode material. The value of the exchange current, however, depends on the metal in question. Although this kind of a reversible potential can only be measured on an inert electrode, in principle the potential and the exchange current may be established on any electron conductor. 

In the absence of activating unsaturated groups the hydrocarbons are exceedingly weak acids. The direct measurement of pA in such cases has not been achieved. Reversible electrode potentials have been used in conjunction with bond-dissociation energies via thermodynamic cycles (30-31) to estimate... 

In the application of electrode potential measurements to anaerobic digestion, the University of Michigan has pioneered (2, 6, 10). Physical chemists have stated that the system theoretically cannot generate sufii-cient electroactivity to make such measurements statistically precise further that thermodynamic reversibility, system equilibrium, and steady-state conditions in such biological systems are suflSciently inexact to permit potential results of meaningful nature lastly, that sludge contaminants are serious enough to distort materially all results (11). 

In addition, because the net reaction at converts Fe to Fe, the measured potential exhibits a slow drift. Such mixed potentials are of little worth in determining equilibrium values. Many important redox couples in natural waters are not electroactive. No reversible electrode potentials are established for N03T-N0 -NH4-H2S or CH4-CO2 systems. Unfortunately, many measurements of Eh (or pe) in natural waters represent mixed potentials not amenable to quantitative interpretation. 

If the oxygen partial pressure in the surrounding of an Ni,NiO mixture is greater than that calculated from Equations (25-54) and (25-11) then nickel is oxidized. In the reverse case, (the oxygen partial pressure is smaller) it is reduced. If a gas flows over a Ni,NiO mixture, then by comparing the electrode potential measured in the gas with that calculated from Equation (25-54) and taking temperature into account, we can decide whether oxidation of Ni or reduction of NiO occurs. 

If the crystal grains are decreasing in size, the reversible potential should shift to more negative values (Stockholm conversion) because of the increasing effect of surface free energy, known in vapor pressure or solubility measurements as the Kelvin effect. If the equivalent radius of a metal crystallite is r, the shift of the reversible electrode potential with respect to that of a large crystal grain r oo) will be... 

The absolute electrode potential of the fuel cell is difficult to measure. However, only the electrode potential difference between the cathode and anode is important in fuel cells. The reversible cell potential can be obtained from the difference between the reversible electrode potentials at the cathode and anode. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

Lenhard J (1986) Measurement of reversible electrode potentials for cyanine dyes by use of phase-selective second-harmonic AC voltammetry. J Imag Sci 30 27-35... 

Although thiolate-surface complexes form at electrodes [58] and electrode potential measurements under reversible conditions are not usually possible, a great deal of useful information has been obtained by various electrochemical methods [35,36]. However, thermochemical properties of sulfur-centered radicals in aqueous solution have generally been obtained from studies of redox equilibria, such as those by BonifSeic and Asmus [52] and Surdhar and Armstrong [59], by methods described in Wardman [60]. In this section it will be convenient to consider sulhdes, disulfides, and sufhydryl compounds in that order. Afso alkyl and aryl compounds will be treated separately. Reduction potentials will be identified by placing the reactants and products of the half cell reaction in parenthesis [60], for example, (RS, H /RSH) is the reduction potential for ... 

The hydrogen electrode is used as a reference for electrode potential measurements. Theoretically, it is the most important electrode for use in aqueous solutions. The reversible hydrogen electrode in a solution of hydrogen ions at unit activity exhibits a potential, which is assumed to be zero at all temperatures. 

Determination of E, the reversible electrode potential, and n, the number of electrons in the o-tolid-ine redox reaction, can be determined from the sequence of spectropotentiostatic measurements (Figure 1). For a reversible system. 

Spectropotentiostatic measurements with an OTTLE cell Determination of , the reversible electrode potential, and w, the number of electrons in a redox reactions, can be performed in a sequence of spectropotentiostatic measurements with an OTTLE cell. Generally, for the reversible system given in Equation [1], the [0]/[R] ratio at the electrode surface is controlled by the applied potential according to the Nernst equation ... 

In the thermodynamic treatment of electrode potentials, the assumption was made that the reactions were reversible, which implies that the reactions occur infinitely slowly. This is never the case in practice. When a battery deUvers current, the electrode reactions depart from reversible behavior and the battery voltage decreases from its open circuit or equiUbrium voltage E. Thus the voltage during battery use or discharge E is lower than the voltage measured under open circuit or reversible conditions E by a quantity called the polari2ation Tj. 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

For simplicity, they treated a solution containing only one supporting electrolyte in addition to dissolved metal ions. Then, using a reference electrode (indicator electrode) reversible to one of the three kinds of ions (a dissolved metal ion, and an anion and cation of the supporting electrolyte), they examined the difference in potential between the working and indicator electrodes. Furthermore, the expressions derived were rewritten to correspond to the actual case when the potential of the working electrode was measured with respect to a conventional reference electrode as follows ... 

The experimental setup is shown in Figure 9.23. The Pt-black catalyst film also served as the working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H2-02 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. The Pt catalyst-working electrode potential, Urhe (=Uwr)> was measured with respect to a reversible reference H2 electrode (RHE) via a Luggin capillary in contact with the Pt-free side of the Nafion membrane. 

Surface reactions that give rise to the electric crurent measmed depend on the electrode potential range and the type of measurements reported. The dominating reactions contributing to the voltammetric data in Fig. 13a-d are chemisorbed CO oxidation, oxidation of methanol to CO2 (on both scans), and chemisorbed CO formation on the reverse mn ... 

Thus, the Volta potential may be operationally defined as the compensating voltage of the cell of Scheme 16. However, it should be stressed that the compensating voltage of a voltaic cell is not always the direct measure of the Volta potential. The appropriate mutual arrangement of phases, as well as application of reversible electrodes or salt bridges in the systems, allows measurement of not only the Volta potential but also the surface and the Galvani potentials. These possibilities are schematically illustrated by [15]... 

If two zinc electrodes are set up in opposition to one another as in Figure 6.12 (A), the difference of potential between them, measured by a potentiometer or voltmeter, is zero. If an infinitesimally small external emf is applied to the electrodes so that A is positive and B is negative, a very small current flows round the circuit, and Zn atoms pass from A into solution as Zn2+ ions, and Zn2+ ions leave the solution and are deposited as Zn atoms on B. If the small emf is reversed so that B is positive and A is negative, the current flows in the opposite direction, and zinc is dissolved from B and deposited on A. An electrode such as the zinc electrode, which reacts thus to an infinitesimal applied emf, is known as a reversible electrode. The hydrogen electrode described earlier is a reversible electrode. If two molar hydrogen electrodes are set up in opposition to one another, Figure 6.12 (B), the... 

It is relevant to follow up the description given above on reversible electrodes with attention focused on the potential difference of a cell. The potential difference measured under reversible conditions, is called the electromotive force, or emf, of the cell, E. It is clear that if a cell is reversible then it is implied that the half-cells of which it is composed are also reversible. 

Oxygen electrode. In principle, a classical oxygen electrode in a liquid electrolyte would be possible if an electrode material were known on the surface of which the redox system 02/0H is electrochemically reversible however, Luther26 measured its standard potential from the following cell without a liquid junction ... 

Although from the thermodynamic point of view one can speak only about the reversibility of a process (cf. Section 3.1.4), in electrochemistry the term reversible electrode has come to stay. By this term we understand an electrode at which the equilibrium of a given reversible process is established with a rate satisfying the requirements of a given application. If equilibrium is established slowly between the metal and the solution, or is not established at all in the given time period, the electrode will in practice not attain a defined potential and cannot be used to measure individual thermodynamic quantities such as the reaction affinity, ion activity in solution, etc. A special case that is encountered most often is that of electrodes exhibiting a mixed potential, where the measured potential depends on the kinetics of several electrode reactions (see Section 5.8.4). 

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