Potential step techniques chronoamperometry

Convective effects are most notable in solvents with low viscosity such as acetonitrile and dimethylfonnamide (see Chapter 3). 

For careful measurements of the diffusion coefficient, it is critical that the measured current be truly at steady state, otherwise the current will contain a planar diffusion contribution that will produce a larger apparent value of D. At a hemispherical electrode, the Cotffell equation is used to determine the relative contributions of planar and spherical diffusion  

Steady-state conditions are reached when the planar diffusion term of the Cottrell equation (equation (19.5)) is insignificant compared to the spherical diffusion term  

Disk UMEs are easier to fabricate than hemispherical UMEs, and are therefore much more conunonly used. The Shoup and Szabo equation (1) for chronoamperometry at a disk UME 

Another advantage of UME determinations of the diffusion coefficient is that measurements can be made in solutions of low conductivity, i.e. low or no supporting electrolyte (2, 3). Furthermore, double potential step techniques can be useful for determining diffusion coefficients of both Dq and (4). Although these methods are beyond the scope of this chapter, the interested reader is referred to the original literature. 

---

A variant of the potential step technique, chronoamperometry, has been applied to electrochemical junctions in the dark to estimate the charge stored in the film. The transient current resulting from an applied potential step is measured and integrated to yield a stored charge. Hoyer and Weller used this technique to estimate the charge stored in nanocrystalline ZnO... 

Step potential — is the value the potential is stepped in various - potential step techniques, including multistep versions (- chronoamperometry), as well as for the potential ramp in - voltammetry. 

Cydodextrins (CyDs) with their largely hydrophobic cavities of variable size and numerous ways of chemical modification are the subject of intensive electrochemical research induding both their behavior in homogeneous solutions and in thin films attached to the electrode surfaces [1-8]. Electroanalytical methods measuring the current response to the potential applied, linear scan, staircase, and pulse voltammetries, and potential-step techniques such as chronoamperometry and... 

A. Molina, C. Serna, and J. Gonzalez. General analytical solution for a catalytic mechanism in potential step techniques at hemispherical microelectrodes Apphcations to chronoamperometry, cyclic staircase voltammetry and cyclic linear sweep voltammetry, J. Electroanal. Chem. 454, 15-31 (1998). 

The electrochemical techniques usually utilized to obtain the spectroscopic properties of redox generated species are those based on potentiometric measurements at potential step, like chronoamperometry and chronocoulometry. Their basic principles are briefly illustrated in the following (for a more detailed discussion see [16]). 

These expressions are designed for cyclic voltammetry. The expressions appropriate for potential step chronoamperometry or impedance measurements, for example, are obtained by replacing IZT/Fv by the measurement time, tm, and the inverse of the pulsation, 1/co, respectively. Thus, fast and slow become Af and Ah I and -C 1, respectively. The outcome of the kinetic competition between electron transfer and diffusion is treated in detail in Section 1.4.3 for the case of cyclic voltammetry, including its convolutive version and a brief comparison with other electrochemical techniques. 

If the nonlinear character of the kinetic law is more pronounced, and/or if more data points than merely the peak are to be used, the following approach, illustrated in Figure 1.18, may be used. The current-time curves are first integrated so as to obtain the surface concentrations of the two reactants. The current and the surface concentrations are then combined to derive the forward and backward rate constants as functions of the electrode potential. Following this strategy, the form of the dependence of the rate constants on the potential need not be known a priori. It is rather an outcome of the cyclic voltammetric experiments and of their treatment. There is therefore no compulsory need, as often believed, to use for this purpose electrochemical techniques in which the electrode potential is independent of time, or nearly independent of time, as in potential step chronoamperometry and impedance measurements. This is another illustration of the equivalence of the various electrochemical techniques, provided that they are used in comparable time windows. 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

The simplest chronoamperometric technique is that defined as single potential step chronoamperometry. It consists of applying an appropriate potential to an electrode (under stationary conditions similar to those of cyclic voltammetry), which allows the electron transfer process under study (for instance Ox + ne — Red) to run instantaneously to completion (i.e. COx(0,0 —1 0). At the same time the decay of the generated current is monitored.20... 

Chronoamperometry is a technique in which a potential step is applied to the working electrode in a quiet solution at t = 0 (Figure 3). Initially [t < 0), the electrode attains r. For f > 0, a potential is selected, which drives the desired electrode reaction. Often, but not necessarily (see, e.g. References [23-25]) the latter is in the... 

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... 

The electroanalytical techniques chronoamperometry, chronocoulometry, and chronoabsorptometry are all based on the same excitation function of one or more potential steps that are applied to an electrode immersed in a nonstirred solution. The system response is thus identical for all three techniques. They differ only in the data domain of the monitored response. Consequently, the common excitation aspect is dealt with here, whereas each monitored response is considered individually in subsequent sections. 

In yet another variation the current is recorded at the initial potential following a series of pulses increasing in amplitude from the initial potential toward more negative values (i.e., the same excitation shown in Fig. 3.31 A). When the pulses reach sufficiently negative potentials to cause reduction of the species, the product of the reduction is oxidized at Ej, and the sampled anodic current is recorded unless the couple is totally irreversible. This is equivalent to a series of potential pulse chronoamperometry experiments with samples taken on the reverse step. Techniques of this sort have been used occasionally in reversibility studies and for identification of unstable products of electrode pro-... 

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]... 

Step and pulse techniques 10.2 Potential step chronoamperometry... 

Chronoamperometry — Chronoamperometry belongs to the family of step techniques [i-iv]. In chronoamperometry the current is measured as a function of time after application of a potential step perturbation. If the potential is stepped from Iq, where no current flows, i.e., the oxidation or reduction of the electrochemically active species does not take place, to 2 where the current... 

A double-potential-step chronoamperometry (DPSC) experiment consists of two CA experiments. The potential of the second step is normally adjusted so that the R molecules formed upon reduction of O in the first step is reoxidized to O in a diffusion-controlled process, but it might also be adjusted to other values with the purpose of detecting other species formed [7]. In contrast to the CA technique, DPSC is a reversal technique, where the intermediates/products formed during the first step are probed directly in the second step. In this sense, it corresponds to a pump/probe experiment in photochemistry. While CA, in general, provides little if any information about follow-up chemistry, DPSC is a very strong tool for distinguishing between different mechanisms such as for example E, ECj, and DIMl. It is also a good tool for the determination of the relevant rate eonstants. 

An advantage of SECM is that it is a steady-state technique that does not require the measurements of transient currents. The technique also offers the same advantage as, for instance, double potential-step chronoamperometry (Sec. IV.A) in that the potential of the tip electrode can be made sufficiently negative that the heterogeneous electron transfer proceeds at the diffusion-controlled rate. 

Of the three SECM modes that can be used to study electrode reaction mechanisms—the TG/SC, feedback, and SG/TC modes—the former is the most powerful for measuring rapid kinetics. With this approach, fast followup and sandwiched chemical reactions can be characterized under steady-state conditions, which are difficult to study even with rapid transient techniques such as fast scan cyclic voltammetry or double potential step chronoamperometry, where extensive corrections for background currents are often mandatory (44). At present, first- and second-order rate constants up to 105 s 1 and 1010 M 1 s, respectively, should be measurable with SECM. The development of smaller tip and substrate electrodes that can be placed closer together should facilitate the detection and characterization of electrogenerated species with submirosecond lifetimes. In this context, the introduction of a fabrication procedure for spherical UMEs with diameters... 

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. 

When pcrtechnctate is electrochemically reduced in aqneous alkaline solution in the presence of gelatin using the techniques of controllcd-potential coulometry, chronoamperometry, and double potential step chronoamperometry, at the mercury electrode surface the technetate ion TcO " is reported to be produced ... 

The actual biological significance of the electrode reaction scheme for oxidation of a-tocopherol and related model compounds shown in Figure 16 are not clear at this time. A perusal of the relevant literature/ however, reveals that electrochemical techniques such as cyclic voltammetry and double potential step chronoamperometry are ideally suited to unraveling the complex redox chemistry of these molecules. 

---