Pulse voltammetry normal

A related technique, reverse-pulse voltammetry, has a pulse sequence that is a mirror unage of that of nonnal-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 decreasmg amplitude is apphed. 

A waveform for normal pulse voltammetry (NPV) [4, 5] is presented in Fig. II.2.4. The electrode is subjected to a train of increasing potential pulses. During the pulses, if the electrode potential is sufficiently close to or more neg- 

The current in NPV is sampled most often near the end of the pulse in Fig. II.2.4 the current sampling is represented by fQled circles. 

At successive potential pulses the current increases accordingly until the concentration of the substrate at the electrode surface approaches zero. Then the current reaches a plateau. A typical experimental normal pulse voltammogram is presented in Fig. II.2.5. [Tris-(9,10-dioxo-l-anthryl)]tris-aminoacetyl amine (TDATAA) has been taken as an illustration. A corresponding cyclic voltammogram is added to the figure to serve as a reference. Apparently, TDATAA is re- 

If the initial conditions are renewed after each potential pulse, then the wave height in NPV, iiim,NP be described by the following formula  

Formula (II.2.5) states that the NPV current is proportional to the bulk substrate concentration, C, the number of electrons transferred, n, the square root of the substrate diffusion coefficient, D, the electrode area, A, and is inversely proportional to the pulse time, tp. In fact, the formula should contain the sampling time, ts y especially when the sampling is not performed exactly at the end of the pulse. Formula (II.2.5) is valid for electrodes of regular size (radius in the range of a few mm) and for processes where the transport of the substrate to the electrode surface is done only by diffusion. Chemical reactions involving the substrate that either precede or follow the electron transfer will also lead to different currents. The height of NPV waves does not depend on the electron transfer rate, so this technique is considered as a very reliable one for the determination of diffusion coefficients of the examined compounds. 

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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... 

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

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. 

Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep or cyclic voltammetry (CV) were employed for investigating drugs at the NB/W or DCE/W interface. A thin O-layer cell [15,16,23] was used to realize the partition equilibrium of neutral species (that is, B) at the O/W interface initially at t = 0 within a reasonably short time. All measurements were carried out at 25°C. Experimental details should be consulted in the references cited. 

A variant of double-pulse voltammetry is the DNPV or differential normal pulse voltammetry, where at the end of each pulse an additional small constant pulse is imposed63 . 

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). 

Detection of Li+ in artificial serum with a voltammetric Li-selective electrode in a flowthrough system was demonstrated [64], Lithium salts such as lithium carbonate have been extensively used for treatment of manic depressive and hyperthyroidism disorders. The therapeutic range of Li concentration is generally accepted to be 0.5-1.5mM in blood serum. The authors used normal pulse voltammetry in which a stripping potential was applied between pulses in order to renew the membrane surface and expel all of the extracted ions from the membrane, similar to galvanostatically controlled potentiometric sensors described above. Unfortunately, the insufficient selectivity... 

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).

In many respects, differential pulse voltammetry is more similar to classical polarography than to the normal pulse methods (see above). A linear potential ramp of dE/dt is applied to the working electrode (see Figure 6.24). However, in common with normal pulse voltammetry, a succession of pulses are also applied to the working electrode. (The WE is often a DME, and then we refer to differential pulse polarography .)... 

The difference between the baseline potential and the peak is AE, and is a constant. In normal pulse voltammetry, no electrolysis occurs at times between each pulse because the baseline potential Eb is constant, and is chosen to be sufficiently anodic for currents to be zero. 

In normal pulse voltammetry, the current is sampled for a short period just before the drop is dislodged. The current monitored is assumed to be constant with time. In the differential pulse method, the current is monitored twice per drop the first sample is taken just before the rise in potential when the pulse starts, while the second is taken at the end of the current pulse just before it decreases back to the baseline. The difference between these two currents is Alpuise The differential pulse voltammogram is then a plot of current difference against potential. In... 

Despite these possible drawbacks, differential pulse voltammetry is one of today s most popular electroanalytical tools. Its principal advantages over normal pulse voltammetry are twofold (i) many analytes can be sampled with a single voltammogram since the analytical peaks for each analyte are quite well resolved, and (ii) by working with a differential current, and hence obtaining a voltammetric peak, the analytical sensitivity can be improved to about 5 x 10 to mol dm. This sensitivity is clearly superior to normal pulse voltammetry. 

Alternative voltammetric methods that improve the sensitivity of voltammetry as an electroanalytical tool are normal pulse voltammetry (with a lower detection limit of 10 mol dm ), differential pulse voltammetry (with a detection limit of 10 -10 mol dm ) and square-wave pulse voltammetry (with a detection limit which is perhaps as low as 10 mol dm ). 

Concerning more general application of mercury electrode in the studies on com-plexation equilibria, one should mention the paper by Jaworski et al. [59], who have investigated oxidation of mercury microelectrode in solutions with thiocyanates without any background electrolyte added. In the experiments, normal pulse voltammetry and staircase voltammetry were used. The authors have developed a general procedure for the determination of the stability constants, based on the data taken from the voltammograms. They have applied it to the analysis of Hg(II)-SCN complexes. 

In large-amplitude pulse voltammetry (LAPV, often called normal pulse voltammetry for historical reasons) the excitation waveform consists of successive pulses of gradually changing amplitude between which a constant initial potential is applied. The initial potential is usually chosen in a region where none... 

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... 

Figure 5.12 Excitation signal for differential normal pulse voltammetry. [Adapted from Ref. 42.]...

In addition to the traditional SEV techniques discussed earlier, various pulse volt-ammetric techniques have been employed at solid electrodes in molten salts, especially in the room-temperature haloaluminate melts. Numerous pulse techniques have been devised, and some of the more common examples of this family of volt-ammetric methods are described in Chapters 3 and 5 of this volume. However, the application of these methods to molten salts is limited primarily to large amplitude pulse voltammetry (LAPV), differential-pulse voltammetry (DPV), and, more recently, reverse normal-pulse voltammetry (RNPV). The application of LAPV and... 

Variation profile of the applied potential in normal pulse voltammetry. 

Electrical-current signal profile in normal pulse voltammetry as a function of applied potential step with increasing amplitude. 

Normal pulse voltammetry is mainly used when the condition of the electrode surface should be kept constant but applying a potential would have a serious effect on this condition. In normal pulse voltammetry, the applied potential to the electrode is always the DC offset, except for those short... 

Among the double pulse techniques, DDPV is very attractive for the characterization of multi-electron transfer processes. Besides the reduction of undesirable effects, this technique gives well-resolved peak-shaped signals which are much more advantageous for the elucidation of these processes than the sigmoidal voltammograms obtained in Normal Pulse Voltammetry and discussed in Sect. 3.3. 

It is of interest at this point to compare the study of Multipulse Chronoamperometry and Staircase Voltammetry with those corresponding to Single Pulse Chronoamperometry and Normal Pulse Voltammetry (NPV) developed in Chaps. 2 and 3 in order to understand how the same perturbation (i.e., a staircase potential) leads to a sigmoidal or a peak-shaped current-potential response as the equilibrium between two consecutive potential pulses is restored, or not. This different behavior is due to the fact that in SCV the current corresponding to a given potential pulse depends on the previous potential pulses, i.e., its history. In contrast, in NPV, since the equilibrium is restored, for a reversible process the current-potential curve is similar to a stationary one, because in this last technique the current corresponding to any potential pulse is independent of its history [8]. 

Under these conditions, the CV curves are coincident with those obtained in Normal Pulse Voltammetry and the half-wave potentials corresponding to CE and EC mechanisms become independent of time and are given by Eqs. (3.239) and (3.240) by changing <5sphe by rs. 

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