Pulse voltammetry defined

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

From the voltammograms of Fig. 5.12, the evolution of the response from a reversible behavior for values of K hme > 10 to a totally irreversible one (for Kplane < 0.05) can be observed. The limits of the different reversibility zones of the charge transfer process depend on the electrochemical technique considered. For Normal or Single Pulse Voltammetry, this question was analyzed in Sect. 3.2.1.4, and the relation between the heterogeneous rate constant and the mass transport coefficient, m°, defined as the ratio between the surface flux and the difference of bulk and surface concentrations evaluated at the formal potential of the charge transfer process was considered [36, 37]. The expression of m° depends on the electrochemical technique considered (see for example Sect. 1.8.4). For CV or SCV it takes the form... 

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). 

Reverse pulse voltammetry (rpv) is a very useful elect roan alyti cal technique in the cases where current measurements of direct reduction or oxidation of substrated is complicated due to poorly defined waves. It is also very useful in investigations of chemical reactions coupled with electrode processes. The potential waveform is presented in Fig.l. 

Normal pulse voltammetry is very sensitive to adsorption of both the substrate and the product at the electrode surface. If the substrate is adsorbed, a peak-shaped voltammogram is obtained. The stronger the adsorption or the lower the concentration of the substrate or the shorter the pulse time, the better defined is the peak. This is illustrated in Fig. II.2.6 by the normal pulse voltammograms of imida-zoacridinone. This compound, at -0.4 V, is strongly adsorbed at the mercury surface, therefore the two consecutive reduction steps result in the formation of two peaks instead of two waves. 

The electrochemical signals of nucleic acid bases were shown to have insufficient sensitivity for DNA analysis in the 1960s, because of the poorly developed detection devices without software systems. However, recent advancements in this field started with digital potentiostats and sophisticated baseline correction techniques in connection with differential pulse voltammetry (DPV) [9] and square wave voltammetry (SWV) [10-12]. Therefore, well-defined voltammetric peaks have been obtained from DNA or RNA at carbon electrodes in the last decade [13],... 

The adsorption of nucleic acids at the electrodes used in electroanalyti-cal chemistry has been mostly investigated with the aid of a.c. polarog-raphy, linear sweep, differential or normal pulse voltammetry, ellipsometry and surface-enhanced Raman scattering spectroscopy. The results of these studies so far obtained have, however, rather qualitative character. Up to recently the samples of nucleic acids which would contain only identical and well defined molecules have not been available in the quantities sufficient for adsorption studies. The use of oligonucleotides or plasmid DNAs appears to be a way to interpret the adsorption analysis of nucleic acids more accurately. 

One advantage of the derivative-type voltammo-gram is that individual peak maxima can be observed for substances with half-wave potentials differing by as little as 0.04 to 0.05 V in contrast, classical and normal-pulse voltammetry require a potential difference of about 0.2 V for resolving waves. More important, however, differential-pulse voltammetry increases the sensitivity of voltammetry. Typically, differential-pulse voltammetry provides well-defined peaks at a concentration level that is 2 X 10 that for the classical voltammetric wave. Note also that the current scale for A/ is in nanoamperes. Generally, detection limits with... 

Although the hybridization of single-stranded DNA to its complement results in detectable changes in electrochemical properties, particularly in support of non-Faradaic current, the DNA bases may also demonstrate redox behavior that gives rise to Faradaic currents. The electrochemical behavior of DNA has been studied over the past few decades. Differential pulse voltammograms show clearly defined peaks for the reduction of cytosine and adenosine. Electrochemical characterization of guanine by cyclic voltammetry has shown... 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting) and capillary electrophoresis (CE) as a detection technique for electroactive compounds. In this usage, the voltammetric cell has to be miniaturised (to about 1 pi) in order not to dilute the analytes after separation. A metal or carbon microelectrode has a defined potential (vs the reference electrode) depending on the substances to be detected (ions or molecules) and the mobile phase flows through the detection cell (Fig. 19.5). This method of amperometric detection in the pulsed mode is very... 

A complete comprehension of Single Pulse electrochemical techniques is fundamental for the study of more complex techniques that will be analyzed in the following chapters. Hence, the concept of half-wave potential, for example, will be defined here and then characterized in all electrochemical techniques [1, 3, 8]. Moreover, when very small electrodes are used, a stationary current-potential response is reached. This is independent of the conditions of the system prior to each potential step and even of the way the current-potential was obtained (i.e., by means of a controlled potential technique or a controlled current one) [9, 10]. So, the stationary solutions deduced in this chapter for the current-potential curves for single potential step techniques are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. Moreover, many of the functional dependences shown in this chapter for different diffusion fields are maintained in the following chapters when multipulse techniques are described if the superposition principle can be applied. 

In Staircase Voltammetry (SCV), a sequence of potential pulses of identical time length t defining a staircase of potentials is applied to the system with no recovery of the initial equilibrium at any moment of the experiment (see Scheme 5.2). In this technique, the difference between two consecutive potential pulses, IA I, is constant, and the ratio v = A /t is defined as the scan rate. 

The way the potential pulses are imposed on the electrode defines the voltam-metric technique. There are many waveforms and procedures for current measuring published in the literature. Very often they differ in details. On the other hand, the same technique may be implemented in a slightly different way in various polaro-graphs/voltammographs. The reader may also be confused by the manuals of various instrumentation makers, since the terminology is not ideally fixed. In this chapter the most popular and significant pulse techniques are discussed. Square-wave voltammetry, due to its specific character, is discussed in a separate chapter. 

A series of nanocomposites have been synthesized by varying the concentration of graphene and chloroauric acid to optimize the formulation with respect to the electrochemical activities. Out of these series of NSPANI/AuNP/GR nanocomposites, it has been foxmd that only one particular nanocomposite has the best electrochemical properties, as analyzed by cyclic voltammetry (CV) and differential pulse voltammetric (DPV) techniques (Figure 13.11) and conductivity. The CV of the best nanocomposites shows the well-defined reversible redox peaks characteristic... 

The above treatment assumes that the measured reduction potentials are thermodynamically meaningful. Although redox potentials can be measured by a variety of electrochemical techniques, cyclic voltammetry, differential pulse polarography, and more recently, square wave voltammetry have found the greatest use because of the ability of these techniques to reveal the dynamics of the associated chemical processes, and hence access the chemical and electrochemical reversibility of the couple. Chemical and electrochemical reversibility have been defined and problems associated with the distinction between these terms have been covered in Chapter 2.15 (2.15.2.2.1), however, for the purpose of this discussion it is useful to treat these behaviors separately. 

Differential pulse votammetry can be even more sensitive than cyclic voltammetry. However, due to the rapid decay of the current signal with surface controlled responses, an external resistence in series with the working electrode may be required to enable a well-defined signal to be obtained (53). More recently, square wave voltammetry has been used to investigate CME surfaces (54). 

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