Other voltammetric methods

In this chapter, some commonly known electrochemical methods are described to complete the set of methods described in Chapter 1. In contrast to the methods discussed in Chapter 3, the methods described here are not used in the further work considered and evaluated in this book, though they have been included for general information. 

In fact this method is identical to cyclic voltammetry, except that the potential is swept from a certain starting potential to an end potential without returning to the initial starting potential. In other words, only the forward sweep is recorded in linear sweep voltammetry. However, relationships for the measured current and its relation to applied potential, and 

In potential step voltammetry, a DC potential window is covered by a DC ramp (linearly increasing applied DC potential) with discrete and symmetrical pulses superponated to this ramp. As an alternative, the pulse amplitude can increase with each pulse, superponated on a constant DC offset. Methods where pulses are involved superponated on a DC signal are generally called potential step methods. 

10 Variation profile of the applied potential in normal pulse voltammetry. 

11 Electrical-current signal profile in normal pulse voltammetry as a function of applied potential step with increasing amplitude. 

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T Riley and A Watson, Polarography and Other Voltammetric Methods... 

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... 

Riley, T. Watson, A., Polarography and Other Voltammetric Methods, Wiley, Chichester, 1987. Svehla, G., Automatic Potentiometrie Titrations, Pergamon, Oxford, 1977. 

Riley, T. and Watson, A., Polarography and Other Voltammetric Methods, ACOL Series, Wiley, Chichester, 1987. This is the best of the three ACOL electroanalysis texts (although again out of print). Its structure and examples are well chosen and well executed. Unfortunately, as with the other ACOL texts, the lack of an index and the irritating format may prove annoying to many readers. 

Electrochemical methods include potentiometry, cyclic voltammetry and chronoamperometry. These methods as well as other voltammetric methods and the impedance of electrochemical systems are discussed in this chapter. 

A differential pulse CSV method for the determination of traces of butyltin species in water was compared with two other voltammetric methods, namely differential pulse polarography and ASV (Schwartz et at., 1995). The butyltin species were accumulated on the mercury drop electrode as their tropolone complexes. Detection limits were 5 mg 1 1 for tributyltin (TBT), 0.5 mgl-1 for dibutyltin (DBT) and 0.5 mgl-1 for monobutyltin (MBT). These detection limits were better than the corresponding values obtained in the other analytical methods. 

Since this closing section is devoted to a criticism of dc polarography we will start by reminding ourselves of the advantages of the DME, which is the feature that sets polarography apart from other voltammetric methods. These advantages are ... 

What are the advantages of mercury electrodes for electrochemical measurements What are the advantages of the DME versus a Pt microelectrode for polarography What are the disadvantages of the DME Describe the method of anodic stripping voltammetry. What analytes can it be used to determine Why is stripping voltammetry more sensitive than other voltammetric methods ... 

The problems discussed in this section have been restricted to reversible electron transfer processes coupled with first-order chemical reactions (for the most part). The current responses are usually expressed as functions of the dimensionless kinetic parameters (cf. Table 2) involving the life-time of mercury drop, For the estimation of the chemical rate constants of reversible reactions the equilibrium constants K should be known. As in other voltammetric methods (see below), the experimental data are transformed into normalized quantities. Kinetic... 

Modification of the mass transfer equations generally respects the same paths used in other voltammetric methods. Appropriate dimensionless parameters have been listed in Table 2. Four cases will be presented here ... 

They are impractical for the analysis of experimental data. A computational procedure has been worked out [129] to allow the comparison of experimental and simulated voltammograms. The analogous effects of kinetic and thermodynamic parameters on SW peaks and peak potentials can be derived for various electrode mechanisms as has been described for other voltammetric methods. 

This method operating with the time window of about 100 to 3000 s is suitable to study reactions with rate constants, k, of the order of 10" to 10"" s" This voltammetric technique enables the determination of reaction products by the electroanalysis of the solution following the electrolysis and hence the interpretation of the overall reaction scheme. Moreover, the experiments are carried out at potentials of the limiting current this is why the electron transfer kinetics does not disturb the estimation of the results. The interpretation of the coulometric responses is usually much simpler than those of other voltammetric methods. The experimental parameters to be evaluated are as follows ... 

From the difficulties encountered with interpretation of CVs which the discussions above amply show, it would appear that other voltammetric methods, especially differential methods, would have found wider application to CPs. This has unfortunately not been the case. The results in Figs. 4-17-a.b.c and 4-18 represent some of the few studies of this nature. In Fig. 4-17. the results of CV and of Differential Pulse Voltammetry (DPV) are compared. The latter is a technique in which a small potential pulse is superimposed on a staircase potential function with the difference between the post-pulse and pre-pulse current measured (inset in Fig. 4-171. The differential method yields peak-shaped curves unencumbered by residual current tails, as in CVs, and thus a clearer identification of peaks and their widths. Fig. 4-19 then shows DPV of Poly(phenylene vinylene) used to compute the bandgap, as described earlier. Normal Pulse Voltammetry (NPV), in which a sort of digital pulse-ramp is applied in place of the analog ramp of CV and the current sampled at the end of the pulse [50], has been applied to poly(l-amino pyrene) [48], yielding redox potentials as well as diffusion coefficients (Fig. 4-181. Other differential methods such as Square Wave Voltammetry have been applied to poly(aromatic amines) in the author s laboratories. There is however little other extant work with pulse voltammetry of CPs, although the very brief results above clearly provide a strong indication for it. 

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