Voltammetry pulse

The potential step is the basis of pulse voltammetry1314. Pulse techniques were initially developed for the dropping mercury electrode15 (Section 

The difference between the various pulse voltammetric techniques is the excitation waveform and the current sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is applied 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 mercury drop. At this point, near the end of the drop lifetime, the faradaic current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

The imposition of a potential pulse to the electrode leads in most experimental situations to a considerable improvement (increase) in the ratio of the charging and faradaic currents compared to that for linear scan voltammetry. This is because the 

Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland e-mail stojek chem.uw.edu.pl 

The following components can be distinguished in the total charging current  

In polarography, the continuous growth of the mercury drop brings about a corresponding charging current, /cap,gr- This is described as follows  

There are other components of the charging current which cannot be rigorously quantified. These are related to the changes in the electrical double-layer 

The current associated with the imposition of the potential pulse and the appropriate charging of the electrical double layer. This always exists and can be described as  

Faradaic and nonfaradaic currents flowing after the application of a potential pulse are qualitatively illustrated in Fig. II.2.2. If the aim is to obtain a current possibly free of the charging component, the current sampling should be set for a time when the charging current is negligible. On the other hand, if one wants to expose the charging current in the absence of a faradaic reaction, the sampling should be done as soon as possible after the pulse imposition. 

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 

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Osteryoung J and Murphy M M 1991 Normal and reverse pulse voltammetry at small electrodes Microelectrodes Theory and Applications (Nate ASI Series E vol 197) ed M I Montenegro, M A Queiros and J L Daschbach (Dordrecht Kluwer)... 

Selectivity Selectivity in voltammetry is determined by the difference between half-wave potentials or peak potentials, with minimum differences of+0.2-0.3 V required for a linear potential scan, and +0.04-0.05 V for differential pulse voltammetry. Selectivity can be improved by adjusting solution conditions. As we have seen, the presence of a complexing ligand can substantially shift the potential at which an analyte is oxidized or reduced. Other solution parameters, such as pH, also can be used to improve selectivity. 

Osteryoung, J. Pulse Voltammetry, /. Chem. Educ. 1983, 60, 296-298. Additional information on stripping voltammetry is available in the following text. 

Normal-pulse voltammetry consists of a series of pulses of increasing amplitude applied to successive drops at a preselected time near the end of each drop lifetime (4). Such a normal-pulse train is shown in Figure 3-4. Between the pidses, the electrode is kept at a constant (base) potential at which no reaction of the analyte occurs. The amplitude of the pulse increases linearly with each drop. The current is measured about 40 ms after the pulse is applied, at which time the contribution of the charging current is nearly zero. In addition, because of the short pulse duration, the diffusion layer is thinner than that in DC polarography (i.e., there is larger flux of... 

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... 

FIGURE 3-5 Excitation signal for differential-pulse voltammetry. 

Nikolskii-Eisenman equation, 143 Nitric oxide, 121 Nonactin, 157 Nonfaradaic processes, 21 Normal pulse voltammetry, 67 Nucleic acids, 82, 185... 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

Molecular Characterization It has been repotted that o-qulnones oxidize ascorbic acid In homogeneous solutions (25). Surface qulnones have also been reported to exist on activated carbon surfaces (16). However, cyclic voltarammetry Is not sufficiently sensitive to allow an unambiguous Identification of the reversible wave ascribed to surface qulnones (16). Therefore, differential pulse voltammetry (DPV) and square wave voltammetry were employed. 

Figure 4, Differential pulse voltammetry of a freshly polished, activated glassy carbon surface (a) and a digital simulation of the DPV (b). The pulse frequency vas2 Hz with an amplitude of 10 mV. The DC scan rate was 2 mV s...

The redox potentials of the ITO electrodes modified with CgoN -MePH clusters were measured by cyclic voltammetry and differential pulse voltammetry in the absence and presence of magnetic processing. 

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

Differential pulse voltammetry provides greater voltammetric resolution than simple linear sweep voltammetry. However, again, a longer analysis time results from the more sophisticated potential waveform. At scan rates faster than 50 mV/sec the improved resolution is lost. Because it takes longer to scan the same potential window than by linear sweep, an even longer relaxation time between scans is required for differential pulse voltammetry. 

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... 

Figure 6.15 Pulse voltammetry of a Pt(lOO) electrode in 0.5 M H2SO4 + 0.25 M HCOOH. The inset is an expansion of the region between 0.4 and 0.5 V to show the decay in the current due to the poisoning of the surface. During the pulse at high potentials (0.9 V) to remove the poison, the current was not recorded (baseline between pulses). (Adapted from Clavilier [1987].)...

To increase the mass transfer rate, Tokuda et al. [7] carried out normal and differential pulse voltammetry at micropipettes and extracted the rate constant values within the... 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

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