Potential step double

The potential is altered between two values, perhaps repeatedly (Fig. 10.2). The second step inverts the electrode reaction. We consider an initial step from a potential where there is no electrode reaction to a value corresponding to the limiting reduction current (only O initially present in solution) at t = x the potential reverts to its initial value and there is oxidation of R that was produced. The equations for a planar 

Expression (10.34) shows that a conventional Cottrell response for an oxidation is obtained, superimposed on the continuation of the reduction reaction profile. Expressions for kinetic control in one of the two steps and in both steps have been derived6. 

There are various applications. If, for example, the product of the initial reaction is consumed in solution by homogeneous reaction, analysis of the reoxidation current will show its extent, and perhaps its kinetics. In the case of O being reduced to R and also to other species, the reoxidation of R (the potential would have to be very carefully chosen) gives information about the couple O R. As a final example, if R is unstable but with a lifetime significantly greater than r, then the generation of R in situ and the study of its reoxidation can lead to the calculation of the rate of decay of R. 

In this last case the use of a double hydrodynamic electrode, generating R on the upstream electrode and detecting it on the downstream electrode, may be easier and more sensitive. The rotating disc electrode has also been used with success to distinguish similar mechanisms with coupled homogeneous reactions (ECE, DISP1, and DISP2)5. 

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Double potential steps are usefiil to investigate the kinetics of homogeneous chemical reactions following electron transfer. In this case, after the first step—raising to a potential where the reduction of O to occurs under diffrision control—the potential is stepped back after a period i, to a value where tlie reduction of O is mass-transport controlled. The two transients can then be compared and tlie kinetic infomiation obtained by lookmg at the ratio of... 

FIG. 2 Principal methods for inducing and monitoring interfacial processes with SECM (a) feedback mode, (b) induced transfer, and (c) double potential step chronoamperometry. 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

FIGURE 2.7. Double potential step chronoamperometry for an EC mechanism with an irreversible follow-up reaction, a Potential program with a cyclic voltammogram showing the location of the starting and inversion potentials to avoid interference of the charge transfer kinetics, b Example of chronoamperometric response, c Variation of the normalized anodic-to-cathodic current ratio, R, with the dimensionless kinetic parameter X. 

FIGURE 2.12. Double potential step chronoamperometry for an ECE (dashed line) and a DISP (solid line) mechanism. Variation of the normalized anodic-to-cathodic current ratio, RDps = [—ia(2tR)/ic(tR)]/(l — l/y/2), with the dimesionless kinetic parameter X — ktR. 

Calculation stability implies that At/Ay2 <0.5. The fulfillment of this condition may become a problem when fast reactions, or more precisely, large values of the kinetic parameter, are involved since most of the variation of C then occurs within a reaction layer much thinner than the diffusion layer. Making Ay sufficiently small for having enough points inside this layer thus implies diminishing At, and thus increasing the number of calculation lines, to an extent that may rapidly become prohibitive. This is, however, not much of a difficulty in a number of cases since the pure kinetic conditions are reached before the problem arises. This is, for example, the case with the calculation alluded to in Section 2.2.5, where application of double potential step chronoamperometry to various dimerizations mechanisms was depicted. In this case the current ratio becomes nil when the pure kinetic conditions are reached. 

Potential Step and Double Potential Step Chronoamperometry of Nernstian Systems... 

Overlapping of Double-Layer Charging and Faradaic Currents in Potential Step and Double Potential Step Chronoamperometry. Oscillating and Nonoscillating Behavior... 

Double Potential Step Responses for Processes Involving First- or Second-Order Follow-up Reactions... 

A more useful chronoamperometric method involves a double potential step (somewhat reminiscent of the previously discussed reversal electrolysis). In this case, for instance for a reduction process, a first cathodic potential step is applied according to the preceding criteria (such that instantaneously COx(0,0 — 0), followed at a time x by the application of a second anodic potential step which causes the species Red previously generated at times < x to be instantaneously reoxidised (such that CRed(0,0 — 0). 

Figure 48 Chronoamperometric double potential step experiment for the process Ox + ne Red inversion time t = 0.2 s. The top part shows the perturbation of the potential applied to the working electrode with time...

For times t> t, where the reoxidation of the previously generated species Red takes place, the diagram represents the double potential step response. Under these conditions, the anodic current follows the equation ... 

Figure 49 The variation of the current ratio in a double potential step experiment for a simple reduction process (t > t)...

The kinetics of following chemical reactions cannot be studied by the single potential step technique in that the response would simply obey the Cottrell equation. In contrast, the double potential step technique, that measures the response exhibited by either the reagent Ox or the product Red, is sensitive to the chemical fate of Red. The cathodic response before the inversion of the applied potential (t < x) is expressed by the Cottrell equation ... 

Scheme 4 Schematic of scanning electrochemical microscopy-double potential step chromoamperometry (SECM-DPSC) measurements, (a) In the forward step, Bt2 is produced via the oxidation of Br . (b) At tswitch. the potential is reversed to collect Br2 by reduction to Br. ...

Determinations of rate constants for the catalytic process were carried out by double potential step chronocoulometry... 

Reconstruction of Au(lll) is observed in STM images as double rows separated from each other by 6.3 nm [335]. Some model calculations have been performed [362] to show that the energy difference of the reconstructed and unreconstructed Au(lll) is small. The effect of Triton X-100 on the reconstruction process of Au(lll) surface has been studied in chloride media [363] applying CV and double potential-step chronocoulometry. It has been found that adsorption of Triton X-100 stabilizes the reconstructed face of Au(lll). Hobara etal. [364] have used in situ STM to study reconstruction of Au(lll), following reductive desorption of 2-mercaptoethanesulfonic acid SAMs. 

Indicators and Dyes Abdel-Hamid [154] has studied adsorption of phe-nolphthalein at a HMDE in aqueous buffer solutions containing 10% v/v ethanol, applying cychc voltammetry and double potential-step chronocoulometry. At pH =... 

Several detailed studies of electrochemiluminescence emission under controlled potentials have been conducted.11 13,63"67 These have been double potential step experiments where one of the ion radicals was first generated at a potential slightly more negative (or positive, for cation generation) than its half-wave potential for a short time period, and then the potential at the electrode was switched to some... 

Double potential step experiments have provided useful information on some details of the electrochemiluminescence phenomenon. As expected, they have shown that emission is not generated on potential reversal when the initially generated ion is not sufficiently stable to be detected with cyclic voltammetry. Furthermore, current reversal with... 

Feldberg68,69 has made a valuable analysis of the relationship of the light produced in a double potential step electrochemiluminescence experiment to the current, time, and kinetic parameters involved. The analysis presumes that the reaction which produces excited states is cation-anion radical annihilation which occurs when the radical ions, separately produced, diffuse together in the solution near the electrode. The processes that Feldberg initially considered were eqs. (7)—(13). The assumptions involved are that decay of the excited state... 

In a suggested alternative mechanism, lower energy triplets produced by the cation-anion annihilation produce fluorescent singlets by triplet-triplet annihilation.73 This possible pathway cannot be overlooked on two accounts. First, it is known from phosphorescence emission that triplets are produced in these systems13,15,60 and second, the double potential step data analyzed by the Feldberg method seems to be in agreement with such a double annihilation mechanism.65 If triplets are initially produced in high local concentrations by the redox reaction in... 

Both cyclic voltammetry and double potential step chronoamperometry have been... 

Such electrochemical experiments have been used to generate and study the reactivity of anionic species [(R4)- = NCCH2 ]112. In another application, electrogenerated extra radical anions or dianions were used in the determination of pKA values for common phosphonium ions, via double potential step chronoamperometry at a platinum cathode180. 

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