Voltammetric Diagnostics

The peak current of a voltammogram is a readily measurable quantity for diagnostic purposes. Specifically, the Randles-Sevcik equations as defined in Problem 4.3 are used to parametrically define this measurement. Explain why in the following cases these equations are not applicable, and peak current is a misleading parameter  

The Randles-Sevdk equations are derived from the assumption that the concentration of the species at the electrode surface at the beginning of the linear voltammetric scan is equal to that in bulk. This is correct for a forward scan, but is not correct for the two cases outlined above because electron transfer has aheady taken place. In this case, a diffusion layer has aheady been developed and so the system is perturbed. 

The observed voltammetry is then a superposition of currents from different Faradaic processes and so care must be taken in discussing peak currents . These are not usually meaningful when compared to a baseline of zero current. Hence, the use of the Randles-Sevcik equations to analyse peak currents in either of the above cases will result in significant errors. As a further point, it should be noted that the size and potential of the reverse peak vary as a function of the switching potential, as highlighted in Fig. 4.3 (unless, for a reversible redox couple, the switching potential is beyond a certain threshold). 

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Table 8.2 lists some characteristic electrode reactions. The linear sweep and cyclic voltammetric diagnostic features of these are described below. They have been selected as examples of the chemical systems most likely to be incorporated in chemical sensors. Further discussion of a wider range of mechanisms can be found in (21). Cyclic voltammetry of surface-confined species is discussed in Chapter 5. 

The cyclic voltammogram is characterized by several important parameters. Four of these observables, the two peak currents and two peak potentials, provide the basis for the diagnostics developed by Nicholson and Shain (1) for analyzing the cyclic voltammetric response. 

Scanning Electron Microscopy and X-Ray Microanalysis Principles of Electroanalytical Methods Potentiometry and Ion Selective Electrodes Polarography and Other Voltammetric Methods Radiochemical Methods Clinical Specimens Diagnostic Enzymology Quantitative Bioassay... 

Diagnostic Criteria to Identify an Irreversible Process. In order to characterize an irreversible process it would be necessary to be able to calculate either the thermodynamic parameter E° or the kinetic parameters a. and k°. Unfortunately, we will see below that k° can only be calculated if E0/ is known, and E0 cannot be calculated by voltammetric techniques. Thus, either one knows Eel (for example, by using potentiometric techniques in solutions containing both Ox and Red), or one is limited to give simply the peak potential of the electrode process at a certain rate (usually at 0.1 V s-1 or at 0.2 V s-1). 

It is conceivable that the presence of such complications must affect the shape of the cyclic voltammograms, and hence perturb to some extent the diagnostic criteria for the above-mentioned fundamental electron transfer processes. As these reactions proceed at their own rates, cyclic voltammetry will be able to detect them only if their rates fall within the time scale of the voltammetric technique (which ranges from a few tens of seconds to a few milliseconds). 

Diagnostic criteria to identify an irreversible dimerization reaction following a reversible electron transfer. In the presence of a chemical reaction following an electron transfer, the dependence of the cyclic voltammetric parameters from the concentration of the redox active species are sufficient by themselves to reveal preliminarily a second-order complication (a ten-fold change in concentration from = 2 10-4 mol dm-3 to 2 10-3 mol dm-3 represents a typical path). 

Diagnostic criteria to identify an irreversible disproportionation reaction following a reversible electron transfer. Once again the dependence of the parameters of the cyclic voltammetric response from the concentration of the species Ox preliminarily reveals the second-order complication. 

Diagnostic criteria for two-electron transfers with different extents of reversibility. The voltammetric responses of EE processes can be qualitatively confused with processes complicated by coupled chemical reactions. The only distinctive criterium to define these processes is the constancy of the parameter ip/v112 relative to each step with the scan rate. 

The voltammograms are unique for each basal plane and are often used as a diagnostic tool for the identification of primary Pt-surface sites [2-6,54]. The structure-sensitivity of the voltammogram serves as a means to characterize the preferential orientation of a given sample [55]. The voltammogram for platinum nanoparticles obtained by different preparation procedures is shown in Figure 6.13. The difference in voltammetric behavior displays the influence of preparation procedures on the fraction of (100), (110) and (111) atomic sites on the surface of the nanoparticles. 

To this point, we have considered only the electron transfer reactions that occur between the electrode and soluble substrates, that is, heterogeneous processes. In most cases, heterogeneous electron transfer reactions are sufficient to account completely for the shapes and diagnostic responses of voltammetric curves. It is well known, however, that in the solution layer adjacent to the electrode, second-order electron transfer reactions occur between electrolysis products and reactants. There is a growing body of information showing that under some circumstances these homogeneous electron transfer reactions present a more facile electron transfer pathway than do the heterogeneous reactions and... 

The diagnostic criteria for the first voltammetric signal are identical to those for an ErevQrrev reaction ErevQrrev diagnostics, while for the second voltammetric criteria the diagnostic criteria are those given for... 

Randles-5>evcik equation — An equation introduced by - Randles [i] and - Sevcik [ii] describing the magnitude of the voltammetric peak current /p (in - linear scan voltammetry or in - cyclic voltammetry) for a reversible electron transfer ( rev mechanism -> Erev diagnostics in cyclic voltammetry). 

Similar equations can be derived for the peak current for voltammetric responses for irreversible electron transfer ( irrev mechanism - Eirrev diagnostics in cyclic voltammetry) or for other mechanistic cases. 

Diagnostic Electrochemical Measurements 12.3.2.1. Voltammetric Methods 12.3.2.1.3. Alternating-Current Polarography... 

The data below were recorded from a series of cyclic voltammetric experiments designed to elucidate the mechanism of the electrode reaction involving reduction of a certain compound. Formulate a mechanism to explain the behavior of the diagnostic functions, then briefly rationalize as many of the trends in the data as you can in terms of your mechanism. The switching potential was held constant at —1.400 V V5-. SCE. 

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