Chronoamperometric techniques

The chronoamperometric technique illustrates the principle that analytically useful current responses depend critically on the efficiency of analyte mass transport within the solution. The analyte mass transport in turn depends on the efficiency with which an appHed voltage can maintain the surface concentrations of oxidized and reduced species at values specified by the Nemst equation. It is generally the case in chronoamperometry that the bulk concentration of one of the species is zero whereas the surface concentration of the other species is forced to zero by the appHed potential, but this is not always so. 

Cyclic voltammetry belongs to the category of voltammetric techniques based on a linear potential sweep chronoamperometric technique. It certainly constitutes the most useful technique for a preliminary determination of the redox properties of a given species. 

The simplest chronoamperometric technique is that defined as single potential step chronoamperometry. It consists of applying an appropriate potential to an electrode (under stationary conditions similar to those of cyclic voltammetry), which allows the electron transfer process under study (for instance Ox + ne — Red) to run instantaneously to completion (i.e. COx(0,0 —1 0). At the same time the decay of the generated current is monitored.20... 

The single-potential-step chronoamperometric technique was selected here for the study of diffusion versus kinetic control for the formation of benzonitrile. 

A chronoamperometric technique employing a double potential step was applied to the EHD reaction in an attempt to distinguish between five possible mechanisms. These included mechanisms corre-... 

Fig. 22. Current-time curves for the catalytic reduction of hydrogen ions (chronoamperometric technique). The reaction is catalyzed by hydrogenase, the reduction process is mediated by viologen medium Tris buffer (1) the diffusion-controlled reaction (only the mediator is present), (2-4) the enzyme (hydrogenase) is added, eg (2) 0.15, (3) 0.31, (4) 0.43.

Voltanunetric and chronoamperometric techniques are widely used recently to determine the methanol permeability through proton exchange membranes intended for nse in direct methanol fuel cell [36]. These techniques can be used with either a real fuel cell or a simulated one (diffusion cell). It is expected that the only electron transfer reaction that can occur at the working electrode surface, within the potential range or potential value under consideration, is the methanol oxidation. 

Demoustier-Champagne et al. further investigated the formation of these polypyrrole tubules in polycarbonate membranes. They used a pulsed chronoamperometric technique and found the tubular morphology was preserved with similar wall thickness as compared to nonpulsed techniques. They also found that monomer and dopant concentration did not affect the nanotubular morphology however, they did find that by using a NaPSS instead of a LiC104 supporting electrolyte, the polypyrrole wall thickness was increased. 

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

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) ... 

Chronoamperometry is a useful technique in those cases where cyclic voltammetry does not succeed in identifying the electrode mechanisms underlying certain redox changes. It is important to state that chronoamperometric measurements can be performed using the same equipment of cyclic voltammetry. 

The conclusions drawn from analysis of the chronoamperometric response of sphere and disk electrodes apply equally to other electrochemical techniques, such as cyclic voltammetry. The characteristic time, tc, of a cyclic voltammetry experiment can be conveniently expressed by the reciprocal of the scan rate RT/nFv. When rc (Dtc),/ , the voltammogram will appear as predicted for a macroplanar electrode (Chap. 3), and when rc (Dtc) A, the voltammogram will take on a sigmoidal shape given by ... 

Voltammetry is an electroanalytical technique most widely used for transduction as the anodic and cathodic peak potential is characteristic for a particular target redox analyte. In this transduction, current is measured vs potential swept. Similarly to chronoamperometric detection, the analyte diffuses through the MIP film towards and, in some cases, away from the electrode surface. Linear sweep voltammetry... 

The latest contribution to the theory of the EC processes in SECM was the modeling of the SG/TC situation by Martin and Unwin [86]. Both the tip and substrate chronoamperometric responses to the potential step applied to the substrate were calculated. From the tip current transient one can extract the value of the first-order homogeneous rate constant and (if necessary) determine the tip-substrate distance. However, according to the authors, this technique is unlikely to match the TG/SC mode with its high collection efficiency under steady-state conditions. 

The technique employs a triple-step potential waveform, typically as illustrated schematically in Fig. 21(a), although others are possible [81]. The potential Ex is such that the platinum electrode surface is rapidly oxidized, any contaminants being desorbed. The electrode is then stepped to a potential E2, which is large and negative enough to reduce the oxidized surface and allow adsorption of the analyte. Stepping the potential to E3 oxidises the species on the electrode surface. The analyte is detected in the chronoam-perometric response of the electrode to this last step. Figure 21(b) illustrates schematically the chronoamperometric behaviour in the absence of adsor-... 

Since the forward reaction for a potential step to the limiting current region is unperturbed by the irreversible following reaction, no kinetic information can be obtained from the po-larographic diffusion current or the limiting chronoamperometric i-t curve. Some kinetic information is contained in the rising portion of the i-E wave and the shift of 1/2 with Wx- Since this behavior is similar to that found in linear potential sweep methods, these results will not be described separately. The reaction rate constant k can be obtained by reversal techniques (see Section 5.7) (32, 33). A convenient approach is the potential step method, where at = 0 the potential is stepped to a potential where Cq(x = 0) = 0, and at t = T it is stepped to a potential where Cr(x = 0) = 0. The equation for the ratio of (measured at time j.) to (measured at time Figure 5.7.3) is... 

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