A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 74
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With either the DME or SMDE, renewal is achieved by the stirring accompanying the fall of an expended mercury drop and its replacement by a fresh drop. Atother electrodes, renewal may not be easily accomplished.Operationally, NPV is carried out at a nonpolarographic electrode, such as a Pt disk,using the waveform and sampling scheme given in Figure 7.3.5, but without any step corresponding to drop dislodgment. Thus, the electrode and its diffusion layer are takenthrough cycle after cycle of pulsing and sampling.
Progressive depletion of the electroactive species can occur and products can build up, either in the diffusion layer or on thesurface of the electrode. These effects generally cause degradation of the NPP response.By three different means, cyclic renewal can be achieved so that well-behaved voltammograms are obtained:1.Chemically reversible systems. If any electrode process carried out during thepulses can be reversed effectively at Еъ, renewal will be accomplished by electrolysis when the potential returns to the base potential after each pulse. Becausethe electrode is normally held at Еъ for a long time compared to the pulse duration, the products of the pulse can be essentially fully recollected and returned tothe initial state. It is not important that the electrode kinetics be fast enough to becalled "reversible," only that the chemistry can be efficiently reversed at the basepotential.2.Convective renewal.
When normal pulse voltammetry is carried out in a convective system, as at a rotating disk, one can rely on stirring to renew the diffusionlayer while the potential is held at Еъ. This can be true even if the chemistry cannot be reversed electrolytically, as in the case where the species created in thepulse decays to an inactive product.
The convection can also affect the currentsampled in each pulse, so that the theoretical expectation based on diffusion theory is exceeded. However the error is often either irrelevant (as in analytical applications where calibration is possible) or fairly small (because a pulse of short7.3 Pulse Voltammetry283duration creates a diffusion layer that remains largely confined to a relativelystagnant layer of solution).3.Diffusive renewal. Even without convection or electrolytic reversal, it is possibleto obtain cyclic renewal of the diffusion layer simply by waiting long enough atthe base potential for diffusion to replace the consumed electroreactant (46).If the diffusion layer can be renewed, the result is essentially as discussed earlier forNPP at the SMDE. Detection limits are typically poorer than at an SMDE because mostsolid electrodes are afflicted by background currents from slow faradaic processes associated with the electrode surface itself.9 If the diffusion layer cannot be effectively renewed,the polarographic wave will not show a plateau, but instead will pass through a peak, thendroop at more extreme potentials as cumulative depletion of the electroreactant is manifested.
The curve resembles a linear sweep voltammogram for essentially the reasonsgoverning responses in LSV (Section 6.1).(e) WaveShapesSince normal pulse polarography was historically viewed as an analytical (rather than diagnostic) tool, the shapes of waves were not a focus of interest and did not receive detailed attention. Nonetheless, the theory for them does exist (Sections 5.4 and 5.5), sinceNPP is the prototype of the sampled-current voltammetric method. The characteristictime scale of milliseconds is, of course, much shorter than the ~3-s time scale of conventional polarography. It is, therefore, possible for a chemical system to behave reversibly in a conventional polarographic experiment and quasireversibly or irreversiblyin the normal pulse mode.
Many systems that show sluggish electrode kinetics behave injust this way. Notice also that the reverse behavior can be seen, too. If a system showsfast electrode kinetics, but the product of the electrode reaction decays on a 1-s timescale, then the normal pulse experiment will show reversibility, because little productdecay will occur during the measurement; yet the conventional polarogram will show thekinds of distortion that are characteristic of homogeneous reactions following chargetransfer (see Chapter 12). At a nonpolarographic electrode, the diagnosis of wave shapesis dependent on effective renewal of the diffusion layer.
It is not practical to analyzeNPV waves in systems where renewal cannot be achieved.Reverse Pulse VoltammetryIn the normal pulse experiment, the usual practice is to select a base potential Еъ in a region where the electroactive species of interest does not react at the electrode. The scan ismade by allowing pulses in successive cycles to reach first into the potential range surrounding E0' and eventually into the diffusion-limited region. If we take the usual reversible case of О + ne ^ R, with О present in the bulk and R absent, then Еъ would beset perhaps 200 mV more positive than E°', and the pulses would be made in a negative9The surfaces of electrodes often undergo faradaic transformations, such as the formation or reduction of oxideson metals or the electrochemical conversion of oxygen-containing functional groups on the edges ofgraphite planes.
Many of these processes take place slowly and over sizable potential ranges; consequently, theygive rise to background currents that can last a long time after the potential or the medium is changed. Therecan also be a slowly decaying nonfaradaic background if the electrode is subject to potential-dependentadsorption of a species of low concentration in the electrolyte.
Background currents of this kind are often said toarise from "surface processes." In general, such currents are much larger at solid electrodes than at mercury,unless the solid electrode is held for a long time (even several minutes or an hour) at a fixed potential in anunchanging medium.284Chapter 7. Polarography and Pulse Voltammetrydirection (Figure 7.3.8a).
In the time before each pulse is applied, negligible faradaic current flows and a uniform concentration profile, extending from the bulk to the surface,prevails.In reverse pulse voltammetry or reverse pulse polarography (47), the potential waveform and sampling scheme are identical with those of the normal pulse method (Figure7.3.5). The differences (Figure 7.3.8a) are (a) that the base potential is placed in the diffusion-limited region for electrolysis of the species present in the bulk, and (b) that the0pulses are made "backward" through the region of E ' and then into range where thespecies present in the bulk is not electroactive. For the specific case mentioned above, thebase potential would be placed 200 mV or more on the negative side of E° and the pulseswould be made in a positive direction.
During the long period т\ when the potential is atЕъ, species О is electrolytically converted at the diffusion-controlled rate; hence its concentration profile is drawn down to zero at the electrode surface, while R is produced atthe electrode and a layer of it extends outward. The pulses work on this non-uniform concentration profile, which is dominated by the presence of R, not O, near the electrode. Asthe pulses reach more positive potentials, they become capable of oxidizing the R produced in the holding period at Еъ, and anodic current samples are obtained at т. When thepulses become more positive than E0' by 200 mV or more, the electrolysis of R proceedsat the diffusion-limited rate and does not change further with step potential, so that an anodic plateau is established (Figure 7.3.8b). This method is clearly a reversal experiment,because the focus is on the detection and behavior of the product from a prior, initiatingelectrolysis.The normal pulse experiment involves essentially a zero faradaic current at step potentials near Еъ (Figure 7.3.8b), because О does not react at the electrode until the pulsesreach the region of E°\ The analogous situation is quite different in RPV, where a signifi-FNPV250150RPV_L50-50Eb-150-250(a)*'d,NPу*d,RP'/r\.tz2501150150-50E - £ 1/2 /mV1(b)'d,DC-150I-250Figure 7.3.8 Reverse pulse voltammetryvs.
normal pulse voltammetry in asimple reversible one-electron system.(a) Waveforms and placement of Еъ. Thepolarographic versions of these methodsinvolve dislodgment and regrowth of themercury drop electrode at the end of eachpulse, (b) Voltammograms of i(r) vs. pulsepotential: NPV (upper), RPV (lower).7.3 Pulse Voltammetry \. 285cant cathodic current is sampled near the base potential. This current arises because О isconsumed electrolytically at the diffusion-controlled rate at potentials near Еъ. Pulses ofsmall amplitude, not reaching into the region of E°\ do not change the rate at which О iselectrolyzed; hence the same current sample is obtained for all such pulses.
The situationis as though a tast experiment were being carried out at the potential of the pulse, withcurrent sampling at time r. If semi-infinite linear diffusion applies, the cathodic plateaucurrent (at the "base" of the RPV wave), shown as / d j D C m Figure 7.3.8&, is given by theCottrell equation aszd,DC=112 1/2(7.3.6)The anodic plateau current / d ^p can be predicted from the results of Section 5.7.2,which dealt with reversal experiments involving a diffusion-controlled forward electrolysis and a diffusion-controlled collection of the product in a reversal step.
This is exactlythe situation in RPV when the steps reach the main plateau of the wave, and the currentflow is described by equation 5.7.15, which can be reexpressed in terms of the time parameters of RPV as(7.3.7)The first term of this equation is recognizable from (7.3.3) as the diffusion-limited current for the NPV experiment, / d N P , while the second term is / d D C [from (7.3.6)]; thusupon rearrangement'd,DC(7.3.8)*d,RP ~The left side of (7.3.8) is the height of the whole reverse pulse voltammogram, which isfound now to be the same as the height of the normal pulse voltammogram taken with thesame timing characteristics.These principles are valid regardless of the electrode employed, as long as semi-infinite linear diffusion can be assumed and renewal of the concentration profile can be accomplished in each cycle.
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