Potential sweep techniques

Linear scan voltammetry (LSV) and cyclic voltammetry (CV) (see Chapter 11) are among the most common electrochemical techniques employed in the laboratory. Despite their utility, however, they are not particularly well suited to careful measurements of diffusion coefficients when using electrodes of conventional size. We will briefly discuss techniques for measuring D with LSV and CV, but the reader should be cautioned that these measurements under conditions of planar diffusion (i.e., at conventional electrodes) are probably useful to only one significant digit, and then only for nemstian systems with no coupled homogeneous reactions and with no adsorption. For more reliable results with LSV and CV, UMEs should be used. 

The most straightforward method for determining the diffusion coefficient is to measure the peak current (ip) at several scan rates (v) and then plot ip as a function of The slope of the best-fit line can then be used to calculate D  

It is important to re-emphasize that this equation applies only to reversible reactions. If one wishes to use this method, then steps should be taken to ensure that the system is in fact reversible and has no coupled homogeneous reactions. CV is useful for this purpose (see Chapters 11 and 15). 

For irreversible systems (0+e— R), D could also (in theory) be calculated from an analogous equation (10)  

This equation requires knowledge of the transfer coefficient a, which can be estimated from 

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Potential Sweep Method, In the transient techniques described above, a set of measurements of the potential for a given current or the current for a given potential is measured in order to construct the current-potential function, i = f(E). For example, the Tafel lines shown in Figure 6.20 were constructed from a set of galvanostatic transients of the type shown in Figure 6.18. In the potential sweep technique, i = f(E), curves are recorded directly in a single experiment. This is achieved by sweeping the potential with time. In linear sweep voltammetry, the potential of the test electrode is varied linearly with time (Fig. 6.23a). If the sweep rate is... 

Historically, the potential sweep technique and cyclic voltammetry were developed for analysis (as successors to polarography) and much of the theoretical development is concerned with the situation under conditions of diffusion control, for that is where the analytical applications are most readily made. In many of these approaches, the underlying assumption is that the electron transfer that must necessarily occur at the interface is a fast process and plays little part in determining the dependence of the observed current upon potential or upon the concentration of the reactant. However, these assumptions may not always apply. 

Thus, potential sweep techniques and cyclic voltammetry are excellent tools for... 

The nature and structure of surface intermediates in hydrocarbon adsorption has been investigated using galvanostatic (constant current) and potential sweep techniques (7, 10, 172-174 or radiotracer methods (175. Niedrach s (172, 173 galvanostatic results with C1-C4 alkanes and with ethylene indicate the existence of common, partially oxidized surface species, despite differences in the initially adsorbed hydrocarbons. Methane adsorption is very slow, but higher saturated hydrocarbons adsorb faster and at similar rates. Potentiostatic adsorption followed by an anodic potential sweep gives two peaks [Fig. 14 (174 corresponding to different adsorbed species. The intermediate responsible for the peak at low potentials (0.7-... 

Figure 1. Current-potential profiles at some noble metals obtained by the potential sweep technique. Potential h is referred to the reversible hydrogen electrode potential, (a) Pt, Rh, and Pd black electrodes deposited on Au in H2SO4 (pH = 1) (Chevillot et (b) Smooth Pt...

The development of the ultrasensitive potential sweep technique, capable of detecting submonolayer amounts of substance on electrode surfaces and its application to metal deposition studies, resulted in detailed investigations of the phenomenon of deposition of metals on foreign substrates at potentials more positive than the thermodynamic reversible potential for the given conditions.This phenomenon has been termed underpotential deposition (UPD). 

In a comparative study of potentiostatic anodic polarization methods, Greene and Leonard found that the anodic polarization characteristics were strongly controlled by the experimental procedure employed.They concluded that while potential sweep techniques yielded the most reproducible results, they were not necessarily the most accurate and that measurements should be conducted as slowly as possible, even though only qualitative data are required. 

In corrosion and in electrochemistry, the potential sweep technique is commonly used to measure polarization curves, and the result is referred to as potentiodynamic polarization curves. Equation (5.98) offers a criterion for the selection of the maximum sweep rate while still working under steady state conditions with respect to mass transport. As a rule of thumb, for a value of > 20 the error in the measured steady-state limiting current is less than 1% [7]. Equation (5.98) shows that to attain a steady state, the sweep rate must be the slower the larger the diffusion layer thickness, in other words, the weaker the convection. If, for example, D = 10 m s, = 2 and 5= 10 pm, the sweep rate must not exceed 15 mV s ... 

Currents measured in the potential step or potential sweep techniques consists not only of faradaic current, Ip, due to electrolysis, but also of charging current (non-faradaic current), Ic, which is required to charge the electric double layer. Thus total current is given by ... 

As mentioned above, the measured current in the potential sweep technique includes a contribution of charging current as background and an appropriate subtraction of background current is necessary. This may be carried out either by using data obtained on a base solution which contains all ingredients except the electroactive species of interest or by extrapolating the baseline current that precedes the faradaic current [2]. 

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