Pulse voltammetry technique

Pulse voltammetry techniques are characterized by a succession of potential steps. During the sequential potential steps, the rates of current decay of the capacitive (If.) and the faradaic currents (7p) are essentially different (specifically, while 4 in Equation 2.1 decays exponentially with time. Ip decreases as a function of t 2, characteristic of a diffusion-controlled electrochemical reaction). In this way, the rate of decay of If. is significantly faster than that of Ip, and thus I. is negligible at a time of 57 uQ after the potential step is imposed (where 7 uQ is the time constant, Tj-gy, for the electrochemical cell having values from microseconds to milliseconds, and R is the uncompensated resistance between reference and working electrodes). Consequently, Ip is the main contribution to the measured current I when its value is measured at the end of a potential step. The detection limits of these techniques therefore fall around 10 M making them suitable for quantitative analysis. 

The most important parameters for pulse voltammetric techniques are defined as follows. Pulse amplitude is the height of the potential pulse, which may or may not be constant depending on the technique. Pulse width is the duration of the potential pulse. Sample period is the time of the pulse at whieh the current is measured. A number of different pulse techniques are available in commercial potentiostats, which essentially differ in their potential step wave-forms and the number of sampling points [1]. 

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Large-amplitude ( normal ) pulse voltammetry techniques were introduced in Chapter 3. The differential normal pulse (DNP) method combines several features of both the small- and large-amplitude pulse techniques. This technique is normally performed at a DME and is actually a form of polarography. The... 

This section deals with the solution corresponding to an EC mechanism (see reaction scheme 4.IVc) in Reverse Pulse Voltammetry technique under conditions of kinetic steady state (i.e., the perturbation of the chemical equilibrium is independent of time see Sect. 3.4.3). In this technique, the product is electrogenerated under diffusion-limited conditions in the first period (0 < t < ) and then exam-... 

Baranowska, I., Markowski, P., Gerle, A. Determination of selected drugs in human urine by differential pulse voltammetry technique. Bioelectrochemistry 73, 5-10 (2008)... 

The difference between the various pulse voltammetrie techniques is the excitation waveform and the ciuTent sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is apphed for each drop of mercury when the DME is used. (Both techniques can also be used at solid electrodes.) By controlling the drop time (with a mechanical knocker), the pulse is synchronized with the maximum growth of the merciuy drop. At this point, near the end of the drop lifetime, the faradaie current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

Panke et al. [69] performed a different approach related to a competitive binding protocol for the determination of DNA single base mismatches by using methylene blue in combination with differential pulse voltammetry technique. Duwensee et al. [70] reported a strategy for sequence-specific DNA detection by means of a competitive hybridization assay with osmium tetroxide-labeled signaling probes. 

Figure 1. Potential-time waveforms for pulse voltammetry techniques. In each case the time at which current is sampled is indicated by a filled circle. The initial potential is E, potential increment in each cycle is AE, pulse amplitude AE, period x, and pulse width t. (Reproduced with permission from Ref. 1. Copyright 1988 CRC Press, Inc.)...

A related technique, reverse-pulse voltammetry, has a pulse sequence that is a mirror image of that of normal-pulse voltammetry (5). hi this case, the initial potential is on the plateau of the wave (i.e., where reduction occurs), and a series of positive-going pulses of decreasing amplitude is applied. 

Differential-pulse voltammetry is an extremely useful technique for measuring trace levels of organic and inorganic species, hi differential-pulse voltammetry, fixed-magnitude pulses—superimposed on a linear potential ramp—are applied to the working electrode at a time just before the end of the drop (Figure 3-5). The current... 

Differential pulse voltammetry has been widely used for in vivo electrochemical analysis This technique combines the linear sweep and pulsed potential... 

When the poisoning reaction is analyzed under potential control, the formation rate is dependent on the electrode potential. The hrst experiments that clearly showed that the poison formation reaction was potential-dependent were performed by Clavilier using pulsed voltammetry [Clavilier, 1987] (Fig. 6.15). In this technique, a short pulse at high potential is superimposed on a normal voltammetric potential... 

Tacussel and their application by Gonon et al.148 to differential pulse voltammetry (DPV) and differential normal pulse voltammetry (DNPV) in vivo, also called the biopulse technique the microelectrodes are implanted in the living animal brain and variations in the concentrations of some molecules can be followed via the Tacussel PRG 5 and BIPAD instruments (see also the selection of commercial models in Table 3.4). 

Phase-sensitive detection is not at all specihc for EPR spectroscopy but is used in many different types of experiments. Some readers may be familiar with the electrochemical technique of differential-pulse voltammetry. Here, the potential over the working and reference electrode, E, is varied slowly enough to be considered as essentially static on a short time scale. The disturbance is a pulse of small potential difference, AE, and the in-phase, in-frequency detection of the current affords a very low noise differential of the i-E characteristic of a redox couple. 

Other techniques that have been used include subtractive differential pulse voltammetry at twin gold electrodes [492], anodic stripping voltammetry using glassy-carbon electrodes [495,496], X-ray fluorescence analysis [493], and neutron activation analysis [494],... 

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 ... 

This is a dynamic electrochemical technique, which can be used to study electron transfer reactions with solid electrodes. A voltammo-gram is the electrical current response that is due to applied excitation potential. Chapter 18b describes the origin of the current in steady-state voltammetry, chronoamperometry, cyclic voltammetry, and square wave voltammetry and other pulse voltammetric techniques. 

The realization that current sampling on a step pulse can increase the detection sensitivity by increasing the faradaic/charging ratio is the basis for the development of various pulse voltammetric (or polarographic) techniques. Also, the pulses can be applied when it is necessary and can reduce the effect of diffusion on the analyte. Figure 18b. 11 shows the waveform and response for three commonly used pulse voltammetric techniques normal pulse voltammetry (NPY), differential pulse voltammetry (DPV), and square-wave voltammetry (SWV). 

Fig. 18b.11. Figures show the pulse waveform and response for three techniques (a) normal pulse voltammetry (NPV), (b) differential pulse voltammetry (DPV), and (c) square-wave voltammetry (SWV).

An alternative and more recent electroanalytical tool is square-wave voltammetry (which is probably now employed more often than normal or differential pulse voltammetry). In this technique, a potential waveform (see Figure 6.26) is applied to the working electrode. Pairs of current measurements are then made (depicted on the figure as t and f2) these measurements are made for each wave period ( cycle ), which is why they are drawn as times after to (when the cycle started). The current associated with the forward part of the pulse is called /forward, while the current associated with the reverse part is /reverse- A square-wave voltammogram is then just a graph of the difference between these two... 

The voltammetric sensitivity can be improved further by analyte preconcentration in conjunction with stripping analyses (cf. Chapter 5). Anodic stripping voltammetry (ASV) (Section 6.5) is the best known of the stripping techniques, and is capable of detecting concentrations as low as 10 " mol dm . Differential pulse voltammetry, when applied to stripping, can further improve the accuracy of electroanalytical measurement and, in principle, further improve the sensitivity of the technique. 

Labrador et al. (2009) developed a technique based on pulse voltammetry, used to predict concentrations of bisulfites, ascorbic acid, and histamine in wine samples, by means of PLS models evaluated via cross-validation. The best prediction results have been obtained for bisulfites. 

Investigation of thiol- and disulfide-modified oligonucleotides with either 25 or 10 bases, or base pairs immobilized on polycrystaUine and Au(lll) electrodes has also been carried out [171]. In these studies, several techniques were employed, including X-ray photoelectron spectroscopy, cyclic and differential pulse voltammetry, interfacial capacitance data, and in situ STM. 

Constant potential techniques at steady state Pulse voltammetry Stripping voltammetry Coulometry... 

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