Potential step methods voltammetry

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

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. 

The use of ultramicroelectrodes for fast measurements has been recognized and developed by others and has been discussed in the literature. For the most part, the existing reports have concentrated on cyclic voltammetry. We believe that potential step methods are more appropriate to the realization of the promise of high speed kinetic measurements at ultramicroelectrodes, so we have concentrated on them. A preliminary report of our activity has appeared. ... 

In this chapter the synthetic aspects of the earlier mentioned [M(bipy)2 (PVPjnCl]" polymers (where M = Os,Ru) are discussed. The main part of the chapter is devoted to the effect of electrolyte and polymer loading on the electrochemistry observed at electrodes modified with these materials. Interaction between the polymer layer and the electrolyte is investigated using electrochemical techniques such as cyclic voltammetry, potential step methods, and the electrochemical quartz crystal microbalance. Attention is also paid to mediation reactions using such modified electrodes. Finally, the implications of these observations for analytical applications of these materials are discussed. 

The double potential step method (6) is included in this chapter as it underpins many other reversal techniques designed to study the stability of the product. Its principle is rather simple and it is a good stepping stone before tackling cyclic voltammetry. The potential is stepped to a value E- where species R is formed at a diffusion-controlled rate, and then, after a time t, to a more anodic value 2 where R is oxidised (Figure 11.2a). 

In potential sweep methods, the current is recorded while the electrode potential is changed linearly with time between two values chosen as for potential step methods. The initial potential, E, is normally the one where there is no electrochemical activity and the final potential, 2, is the one where the reaction is mass transport controlled. In linear sweep voltammetry, the scan stops at E2, whereas in cyclic voltammetry, the sweep direction is reversed when the potential reaches 2 and the potential remmed to j. This constitutes one cycle of the cyclic voltammogram. Multiple cycles may be recorded, for example, to study film formation. Other waveforms are used to study the formation and kinetics of intermediates when studying coupled chemical reactions (Figure 11.4c). 

Cyclic voltammetry is a powerful technique because the CV waveshape is sensitive to all the parameters of the electrochemical mechanism. For the same reasons, however, a full quantitative analysis with CV can be difficult. It is usually helpful if qualitative or quantitative information can be obtained from other sources. Potential step methods can play a complementary role to CV in the analysis of electrochemical mechanisms. This is because they can be performed under conditions of a fast and irreversible forward heterogeneous rate constant. 

Double-potential-step chronoamperometry does not require precise control of potential. It is only necessary that the potential during each step be at a value for which the desired reaction occurs at the mass-transport-limited rate. Thus, it was not necessary to correct for solution iR drop in these experiments as it would have been if cyclic voltammetry had been selected as the method for quantitative evaluation of the rate constant. 

The term voltammetry refers to measurements of the current as a function of the potential. In linear sweep and cyclic voltammetry, the potential steps used in CA and DPSCA are replaced by linear potential sweeps between the potential values. A triangular potentialtime waveform with equal positive and negative slopes is most often used (Fig. 6.8). If only the first half-cycle of the potential-time program is used, the method is referred to as linear sweep voltammetry (LSV) when both half-cycles are used, it is cyclic voltammetry (CV). The rate by which the potential varies with time is called the voltage sweep (or scan) rate, v, and the potential at which the direction of the voltage sweep is reversed is usually referred to... 

Kakutani et al. described an ion-transfer voltammetry and potentiometry method for the determination of acetylcholine with the interface between polymer-nitrobenzene gel and water [13]. The PVC-nitrobenzene gel electrode was prepared as described by Osakai et al. [14]. The transfer of acetylcholine ions across the interface between the gel electrode and water was studied by cyclic voltammetry, potential-step chronoamperometry, and potentiometry. The interface between the two immiscible electrolyte solutions acted as the ion-selective electrode surface for the determination of acetylcholine ions. 

One of the main uses of digital simulation - for some workers, the only application - is for linear sweep (LSV) or cyclic voltammetry (CV). This is more demanding than simulation of step methods, for which the simulation usually spans one observation time unit, whereas in LSV or CV, the characteristic time r used to normalise time with is the time taken to sweep through one dimensionless potential unit (see Sect. 2.4.3) and typically, a sweep traverses around 24 of these units and a cyclic voltammogram twice that many. Thus, the explicit method is not very suitable, requiring rather many steps per unit, but will serve as a simple introduction. Also, the groundwork for the handling of boundary conditions for multispecies simulations is laid here. 

The use of a potential-step technique such as cyclic staircase voltammetry represents a simple alternative to Ichise s method (j0 of obtaining information on both adsorption and electron transfer kinetics. The current decay immediately after a step is primarily capacitive while current at later times is almost totally due to electron transfer reactions. Thus, by measuring the current at several times during each step and by changing the scan rate, information on both the kinetics of the electrode process and the differential capacity can be obtained with a single sweep. 

From the experimental standpoint, the use of a.c. techniques offers many advantages. Sensitivity is much higher than in d.c. measurements, since phase-sensitive detection can be used and very small probe signals can be employed ( 5mV). The technique is therefore a truly equilibrium one, unlike cyclic voltammetry. An alternative approach to the commonly used sinusoidal signal superimposed on the selected d.c. potential is to use a potential step and to employ Laplace transform methods. Instrumentally, this is rather more demanding and the advantages are not clear [51]. Fourier transform methods have also been considered and their use will have advantages in terms of the time-scale for an experiment, especially at very low frequencies. 

Normal Pulse Voltammetry (NPV). Normal pulse voltammetry (NPV) has the waveform shown in Figure 10(b), and uses a stephke waveform to minimize the capacitive current. Again it is best to start at rest potential. The pulse is comprised of an initial potential step up, followed by holding the potential steady for a length of time which defines the pulsed width. The current measurement takes place at the end of the pulse, followed by the potential stepped back down to the initial potential, usually zero or the rest potential. This is the simplest waveform of the pulsed methods. An example of a current signal response is shown in Figure 10(a). 

In many preparative applications of EGBs the rate-determining step in product formation is the proton transfer. This is often the case when the deprotonated substrate is removed in a fast product-forming reaction (cf. Sec. II.B). For EGBs formed in situ, electrochemical methods such as cyclic voltammetry (CV), derivative cyclic voltammetry (DCV), linear sweep voltammetry (LSV), double-potential-step chronoamperometry (DPSC), and other electroanalytical methods can often be used to estimate the kinetics of proton transfer from the substrate to the EGB. When the EGBs are formed ex situ (because the acidic... 

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