A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 16
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When chloride is not acceptable, the mercurous sulfate electrode may be used:Hg/Hg2SO4/K2SO4 (saturated, aqueous)(2.1.47)With a nonaqueous solvent, one may be concerned with the leakage of water from anaqueous reference electrode; hence a system likeAg/Ag + (0.01 M in CH3CN)(2.1.48)might be preferred.Because of the difficulty in finding a reference electrode for a nonaqueous solvent thatdoes not contaminate the test solution with undesirable species, a quasireference electrode(QRE)6 is often employed.
This is usually just a metal wire, Ag or Pt, used with the expectation that in experiments where there is essentially no change in the bulk solution, the potential of this wire, although unknown, will not change during a series of measurements.The actual potential of the quasireference electrode vs. a true reference electrode must becalibrated before reporting potentials with reference to the QRE. Typically the calibrationis achieved simply by measuring (e.g., by voltammetry) the standard or formal potential vs.the QRE of a couple whose standard or formal potential is already known vs.
a true reference under the same conditions. The ferrocene/ferrocenium (Fc/Fc+) couple is recommended as a calibrating redox couple, since both forms are soluble and stable in manysolvents, and since the couple usually shows nernstian behavior (19). Voltammograms forferrocene oxidation might be recorded to establish the value of £pc/Fc+ vs-tne QRE, so thatthe potentials of other reactions can be reported against £pc/Fc+- ^ *s unacceptable to reportpotentials vs.
an uncalibrated quasireference electrode. Moreover a QRE is not suitable inexperiments, such as bulk electrolysis, where changes in the composition of the bulk solu6Quasi implies that it is "almost" or "essentially" a reference electrode. Sometimes such electrodes are alsocalled pseudoreference electrodes (pseudo, meaning false); this terminology seems less appropriate.54Chapter 2. Potentials and Thermodynamics of Cells£0(Zn2+/Zn)NHESCE£0(Fe3+/Fe2+)-0.763-1.003.7-3.70-0.2424.5-4.50.24204.7-4.70.770.535.3-5.3E vs. NHE(volts)E vs.
SCE(volts)E vs. vacuum(volts)EF (Fermi energy)(eV)Figure 2.1.1 Relationship between potentials on the NHE, SCE, and "absolute" scales. Thepotential on the absolute scale is the electrical work required to bring a unit positive test charge intothe conducting phase of the electrode from a point in vacuo just outside the system (see Section2.2.5). At right is the Fermi energy corresponding to each of the indicated potentials. The Fermienergy is the electrochemical potential of electrons on the electrode (see Section 2.2.4).tion can cause concomitant variations in the potential of the QRE.
A proposed alternativeapproach (20) is to employ a reference electrode in which Fc and Fc + are immobilized at aknown concentration ratio in a polymer layer on the electrode surface (see Chapter 14).Since the potential of a reference electrode vs. NHE or SCE is typically specified inexperimental papers, interconversion of scales can be accomplished easily. Figure 2.1.1is a schematic representation of the relationship between the SCE and NHE scales. Theinside back cover contains a tabulation of the potentials of the most common referenceelectrodes.2.2 A MORE DETAILED VIEW OF INTERFACIAL POTENTIALDIFFERENCES2.2.1 The Physics of Phase PotentialsIn the thermodynamic considerations of the previous section, we were not required to advance a mechanistic basis for the observable differences in potentials across certainphase boundaries.
However, it is difficult to think chemically without a mechanisticmodel, and we now find it helpful to consider the kinds of interactions between phasesthat could create these interfacial differences. First, let us consider two prior questions:(1) Can we expect the potential within a phase to be uniform? (2) If so, what governs itsvalue?One certainly can speak of the potential at any particular point within a phase. Thatquantity, ф(х, у, z), is defined as the work required to bring a unit positive charge, withoutmaterial interactions, from an infinite distance to point (x, y, z). From electrostatics, wehave assurance that ф(х, у, z) is independent of the path of the test charge (21).
The workis done against a coulombic field; hence we can express the potential generally asФ(х, у, x)rx,y,zJ oo% -d\(2.2.1)2.2 A More Detailed View of Interfacial Potential Differences55where % is the electric field strength vector (i.e., the force exerted on a unit charge at anypoint), and d\ is an infinitesimal tangent to the path in the direction of movement. The integral is carried out over any path to (x, y, z). The difference in potential between points(У, / , z') and (x, y, z) is thenXУ Zф(х\ /, z') - ф(х, v, z) = f ' ' -d1(2.2.2)In general, the electric field strength is not zero everywhere between two points and theintegral does not vanish; hence some potential difference usually exists.Conducting phases have some special properties of great importance.
Such a phase isone with mobile charge carriers, such as a metal, a semiconductor, or an electrolyte solution. When no current passes through a conducting phase, there is no net movement ofcharge carriers, so the electric field at all interior points must be zero. If it were not, thecarriers would move in response to it to eliminate the field. From equation 2.2.2, one cansee that the difference in potential between any two points in the interior of the phasemust also be zero under these conditions; thus the entire phase is an equipotential volume.We designate its potential as ф, which is known as the inner potential (or Galvani potential) of the phase.Why does the inner potential have the value that it does? A very important factor isany excess charge that might exist on the phase itself, because a test charge would have towork against the coulombic field arising from that charge.
Other components of the potential can arise from miscellaneous fields resulting from charged bodies outside the sample.As long as the charge distribution throughout the system is constant, the phase potentialwill remain constant, but alterations in charge distributions inside or outside the phasewill change the phase potential. Thus, we have our first indication that differences in potential arising from chemical interactions between phases have some sort of charge separation as their basis.An interesting question concerns the location of any excess charge on a conductingphase. The Gauss law from elementary electrostatics is extremely helpful here (22).
Itstates that if we enclose a volume with an imaginary surface (a Gaussian surface), we willfind that the net charge q inside the surface is given by an integral of the electric field overthe surface:q = s 0 <J> % • dSs a(2.2.3)7where e 0 * proportionality constant, and dS is an infinitesimal vector normal outwardfrom the surface. Now consider a Gaussian surface located within a conductor that is uniform in its interior (i.e., without voids or interior phases). If no current flows, % is zero atall points on the Gaussian surface, hence the net charge within the boundary is zero. Thesituation is depicted in Figure 2.2.1. This conclusion applies to any Gaussian surface,even one situated just inside the phase boundary; thus we must infer that the excesscharge actually resides on the surface of the conducting phase.87The parameter s0 is called the permittivity of free space or the electric constant and has the value 8.85419 X10~ 12 C 2 N " 1 ггГ1.
See the footnote in Section 13.3.1 for a fuller explanation of electrostatic conventionsfollowed in this book.8There can be a finite thickness to this surface layer. The critical aspect is the size of the excess charge withrespect to the bulk carrier concentration in the phase. If the charge is established by drawing carriers from asignificant volume, thermal processes will impede the compact accumulation of the excess strictly on thesurface. Then, the charged zone is called a space charge region, because it has three-dimensional character.
Itsthickness can range from a few angstroms to several thousand angstroms in electrolytes and semiconductiors. Inmetals, it is negligibly thick. See Chapters 13 and 18 for more detailed discussionalong this line.56 • Chapter 2. Potentials and Thermodynamics of CellsCharged conductingphaseInterior GaussiansurfaceZero included chargeFigure 2.2.1 Cross-sectionof a three-dimensionalconducting phase containing aGaussian enclosure.Illustration that the excesscharge resides on the surfaceof the phase.A view of the way in which phase potentials are established is now beginning toemerge:1.Changes in the potential of a conducting phase can be effected by altering thecharge distributions on or around the phase.2.If the phase undergoes a change in its excess charge, its charge carriers will adjust such that the excess becomes wholly distributed over an entire boundary ofthe phase.3.The surface distribution is such that the electric field strength within the phase iszero under null-current conditions.4.The interior of the phase features a constant potential, ф.The excess charge needed to change the potential of a conductor by electrochemically significant amounts is often not very large.
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