Chronoamperometry reversible electrode reaction

The best way to gauge the influence of convective stirring on prospective elec-troanalytical experiments such as chronoamperometry or cyclic voltammetry is to conduct preliminary experiments with a well-behaved, reversible electrode reaction employing the same experimental setup to be used for examination of the system of interest. Increases in the expected constant values of it1/2 with increasing t or ip/v1/2 with decreasing v, respectively, signal the point at which convective transport becomes important. 

In the preceding chapter, single pulse voltammetry and chronoamperometry were applied to the study of reversible electrode reactions of species in solution. Under these conditions, the surface concentrations fulfill Nemst equation and are independent of the duration of the experiment, regardless of the diffusion field... 

Mercury electrodes are usually of spherical shape. In chronoamperometry of a simple, reversible electrode reaction Ox" +ne Red, using a hanging mercury drop electrode in an unstirred solution, the current density depends on time and electrode radius [5] ... 

The motivation behind the considerable effort that was exerted in the development of DCV [42, 49, 50, 69] was based on the need to make CV and LSV quantitative tools for the study of electrode kinetics. At that time, there were three major problems that had to be overcome. These were (a) the precision in the measurement of Ep and AEp, (b) the problem with accurately defining the baseline for the reverse sweep and (c) the problem as to how to handle Rn in a practical manner. The development of DCV did indeed provide suitable solutions to all three of these problems, although the methods developed to handle the Ru problem [41, 42] only involve the derivative of the response in terms of precision necessary for the measurements. More recent work [55, 57] is indicative that the precision in Ep/2, Ep) and AEP measurements can be as high as that observed during DCV (see Sect. 3.4). Also, a recent study in which rate constants were evaluated using CV, DCV, and double potential step chronoamperometry for a particular electrode reaction showed that the precision to be expected frcm the three techniques are comparable when the CV baseline, after subtracting out the charging... 

Here, the electrode reaction is followed by a first-order irreversible chemical reaction in solution that consumes the primary product B and forms the final product C. The rate of this chemical reaction can be measured conveniently with cyclic voltammetry, double-potential-step chronoamperometry, reverse pulse voltammetry, etc. However, this is only true if the half-life of B is greater than or equal to the shortest attainable time scale of the experiment. 

This experiment, called double potential step chronoamperometry, is our first example of a reversal technique. Such methods comprise a large class of approaches, all featuring an initial generation of an electrolytic product, then a reversal of electrolysis so that the first product is examined electrolytically in a direct fashion. Reversal methods make up a powerful arsenal for studies of complex electrode reactions, and we will have much to say about them. 

This experiment, which is called cyclic voltammetry (CV), is a reversal technique and is the potential-scan equivalent of double potential step chronoamperometry (Section 5.7). Cyclic voltammetry has become a very popular technique for initial electrochemical studies of new systems and has proven very useful in obtaining information about fairly complicated electrode reactions. These will be discussed in more detail in Chapter 12. 

The most useful equation in chronoamperometry is the Cottrell equation, which describes the observed current (planar electrode of infinite size) at any time following a large forward potential step in a reversible redox reaction (or to large overpotential) as a function of t. ... 

Low amount of theoretical, and maybe even less amount of experimental work has been made on double potential step chronoamperometry that, similarly to cyclic voltammetry (see below) is classified as a reversal technique after the forward step potential, WE is polarized at a value at which the electrogenerated species is reoxidized to the starting one. Such a technique is quite effective in studies of electrode reaction mechanisms. As an example, very accurate quantitative data about the kinetics accounting for the stability of electrogenerated species can be gained. However, the issue of how the data should be treated in order to obtain similar information about different homogeneous kinetics coupled to the charge transfer is far beyond the scope of the present book. 

The most common method of electrolytes examination is electrochemical impedance spectroscopy (EIS) with blocking (usually stainless steel) and/ or reversible electrodes (most often metallic lithium, or other metal, common with the salt used Saraswat et al. 1989). It allows determination of samples conductivity (Bauerle 1969), following the evolution of complete and half cells resistance upon storage (Sannier et al. 2007 Syzdek et al. 2007) sometimes it is used for transference number measurements (Ravn Sprensen and Jacobsen 1982), as well as a complementary technique in the studies on the crystallisation of solid electrolytes (Marzantowicz et al. 2006a,b,c, 2008), where detailed equivalent circuit considerations are applied. Another very important way of studying these systems is a set of DC techniques, i.e. voltammetry (Armand et al. 1980) and DC polarisation (chronoamperometry and chronopotentiometry). They allowed studies of the electrode reactions and examination of transport properties of electrolytes, especially transference number by Bruce and Vincent and co-workers (Bruce and Vincent 1987,1990 Bruce et al. 1987 Christie et al. 1999 Evans et al. 1987 MacCaUum et aL 1986) and/or Newman method (Doyle and Newman 1995 Hafezi and Newman 2000). 

A more elaborate version of the chronoamperometry experiment is the symmetrical double-potential-step chronoamperometry technique. Here the applied potential is returned to its initial value after a period of time, t, following the application of the forward potential step. The current-time response that is observed during such an experiment is shown in Figure 3.3(B). If the product produced during a reduction reaction is stable and if the initial potential to which the working electrode is returned after t is sufficient to cause the diffusion-controlled oxidation of the reduced species, then the current obtained on application of the reverse step, ir, is given by [63]... 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Some analytical applications of chronoamperometry (e.g., in vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed time intervals. Some popular test strips for blood glucose (discussed in Chapter 6) involve potential-step measurements of an enzymatically liberated product (in connection with a preceding incubation reaction). Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first one). 

Figure 4-3. Electrochemical techniques and the redox-linked chemistries of an enzyme film on an electrode. Cyclic voltammetry provides an intuitive map of enzyme activities. A. The non-turnover signal at low scan rates (solid lines) provides thermodynamic information, while raising the scan rate leads to a peak separation (broken lines) the analysis of which gives the rate of interfacial electron exchange and additional information on how this is coupled to chemical reactions. B. Catalysis leads to a continual flow of electrons that amphfles the response and correlates activity with driving force under steady-state conditions here the catalytic current reports on the reduction of an enzyme substrate (sohd hne). Chronoamperometry ahows deconvolution of the potenhal and hme domains here an oxidoreductase is reversibly inactivated by apphcation of the most positive potential, an example is NiFe]-hydrogenase, and inhibition by agent X is shown to be essentially instantaneous.

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