Voltammetry voltammetric indicator electrodes

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

The functional grouping is as follows. The electrode which is under study is called working electrode in voltammetry or indicator electrode in potentiometry. The electrode the potential of which is practically constant and used to make comparison of electrode potentials, i.e., to define the value of the potential of the electrode on the scale based on standard hydrogen electrode, is called a reference electrode. The electrode that serves to maintain the current in the circuit formed with the working electrode in voltammetric experiments in three-compartment cells is the auxiliary (or counter) electrode. 

Voltammetry and DD. It is evident that DD processes are analogous to voltammetric analysis, first of all to stripping voltammetry. Such similarity strongly facilitated the development of theory, methodology and instrumentation of the DD method. For example, construction of the stoichiograph resembles that of J. Heyrovsky polarograph. Nevertheless, voltammetry can determine only the substances that are capable of redox transformations and only in a relatively narrow range of the indicator electrode polarization, whereas the DD... 

The qualitative voltammetric behavior of methanol oxidation on Pt is very similar to that of formic acid. The voltammetry for the oxidation of methanol on Pt single crystals shows a clear hysteresis between the positive- and negative-going scans due to the accumulation of the poisoning intermediate at low potentials and its oxidation above 0.7 V (vs. RHE) [Lamy et al., 1982]. Additionally, the reaction is also very sensitive to the surface stmcture. The order in the activity of the different low index planes of Pt follows the same order than that observed for formic acid. Thus, the Pt(l 11) electrode has the lowest catalytic activity and the smallest hysteresis, indicating that both paths of the reaction are slow, whereas the Pt( 100) electrode displays a much higher catalytic activity and a fast poisoning reaction. As before, the activity of the Pt(l 10) electrode depends on the pretreatment of the surface (Fig. 6.17). 

Tpiectwanalytical methods that depend on the measurement of current as afunc-iZ/tion of applied potential are called voltammetric methods. They employ conditions that encourage polarization of the indicator or working electrode. Generally, to enhance polarization, the working electrodes in voltammetry are relatively small, with surface areas of a few square millimeters at the most and, in some applications, only a few square micrometers. 

Conventional cyclic voltammetry s greatest utility is in the diagnosis of electrode reactions involving chemical complications, and the ac variant is also useful in meeting this kind of problem. The ratio /p,r//p,f is a sensitive indicator of product stability, just as the dc voltammetric ratio l/p,r/ p,f I is. However, the ac ratio is easier to measure precisely, and it lends itself well to quantitative evaluation of homogeneous rate constants. 

The voltammetric behavior of compounds I" " and 2+ in aqueous media is characterized, as expected, by the reversible oxidation of the ferrocene group. All three unmodified CD hosts cause a substantial decrease of the currents associated with the ferrocene couple, indicating that complexation is taking place in all cases, fhe decrease in current levels results from the lower diffusion coefficient of the CD-bound ferrocene derivative as compared to the free compound. We have used the CD concentration dependence of the apparent diffusion coefficients (determined by rotating electrode voltammetry) to obtain the binding constants of all three CDs with compounds and 2" [8]. The values obtained from this kind of analysis are shown in Table 1. 

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