Conventional Voltammetry

Although a variety of electtochemical methods are available, voltammetry (based on the record of the current i flowing across an electrochemical cell under the application of a given time-depending potential E) has become the most important measuring technique in pure and applied elecPochemistry [79]. 

Generally, a voltage is applied between a working electrode and an auxiliary electrode, and the potential of the working electrode is controlled versus the reference electrode possessing a constant and known potential. The current flowing through 

Domenech-Carbo et al., Electrochemical Methods in Archaeometry, Conservation and Restoration, Monographs in Electrochemistry, 

Depending on the time variation of the applied potential, several types of voltammetry can be distinguished. Among them, the most widely used are linear and cyclic voltammetries. Here, the excitation signal is a linear potential scan that is swept between two extreme values, and in cyclic voltammetry the potential is swept up and down between the two values (or switching potentials) with the same absolute scan rate (v, usually expressed in mV/s), although it has the opposite sign [79]. 

Initially, at potentials between -0.25 and about +0.1 V, a rather low current is recorded due to double-layer charging. Later, when the potential is sufficiently positive to oxidized [Fe(CN)g] to [Fe(CN)g] , the anodic current increases rapidly until the concentration of [Fe(CN)g] at the electrode surface is substantially diminished. Then, the (anodic) current increases dramatically until a maximum value is reached, thus defining a (anodic) voltammetric peak with the peak potential pa and the peak current fpa. The correct peak current must be measured in relation to the background current, which can be extrapolated from the starting region. After the peak, the current decreases slowly as the electrode is depleted of [Fe(CN)g] due to its electrochemical conversion into [Fe(CN)g] . When the (anodic) switching potential Ex is attained, the potential scan reverses its direction. In the subsequent cathodic scan, a similar cathodic peak is measured, defining a cathodic peak potential pc and a cathodic peak current Then, the current reaches a maximum and subsequently decays. 

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The use of analytical procedures based on conventional voltammetry requires the selection of a solvent, a supporting electrolyte, an electrode, an electrochemical technique, electrochemical parameters, and data treatment ... 

The advantages of a solid electrode of fixed area that functions in the voltammetric experiment with a constant diffusion-layer thickness have led to the development of the rotated-disk and ring-disk electrodes.52-54 By rotation of a disk, the electrode diffusion layer becomes fixed such that the current is constant as a function of time and does not decay [in contrast to conventional voltammetry Eq. (3.6)]. Voltammetry with such an electrode system gives a current-potential wave that is analogous to a polarogram and follows the relationship... 

If (95) is used to estimate values for the diffusion layer thickness obtained for sonovoltammetry in acetonitrile, values of the order of a few micrometres are obtained - much smaller than encountered in conventional voltammetry under silent (stationary) conditions unless either potential scan rates of hundreds of mVs, or more, are employed or alternatively steady-state measurements are made with microelectrodes with one or more dimensions of the micrometre scale (Compton et al., 1996b). 

DNA, RNA, and protein-modified electrodes can be prepared using both carbon and mercury surfaces [283]. Stability of immobilization of NAs at HMDE and graphite electrodes is very good [270]. AdTSV has been widely applied to various kinds of NA and protein studies [13, 15, 249, 270, 281, 284, 285]. Compared with conventional voltammetry, AdTSV has many advantages that are mainly due to the separation of the biomacromolecule adsorption from the electrode processes. These advantages include (1) reduction of the sample volume to 3 to 10 microliters, (2) elimination of interferences by low molecular mass substances that are washed off in AdTSV, (3) adsorption of the biomacromolecule on the electrode from media not suitable for the conventional voltammetric analysis, (4) in studies... 

Chronoamperometric studies show that the reductive desorption of adsorbed alka-nethiolates from a metal surface is an event that takes place in less than a second and, typically, in much shorter time [67]. Within this timescale, a nucleation-and-growth type treatment [67, 68, 69], and possibly, the ion penetration into the desorbed SAM [1] are needed to be considered. However, in conventional voltammetry of the scan rate that is less than 100 mV an equilibrium approach assuming an adsorption isotherm to correlate the amount of adsorbed species and its concentration in the adjacent solution phase is useful for interpreting the voltammograms [87, 70]. 

Chapter 14 presents the attributes of voltammetry of adhered microparticles in contact with ILs for determining the thermodynamics and kinetic properties of electroactive species. This technique provides an alternative and powerful electro-analytical tool, which has great versatility in ILs under conditions where conventional voltammetry with dissolved analyte is impractical. 

Responses obtained at mediator modified electrodes can be complicated if some of the analyte diffuses through to the bare electrode surface. Conventional voltammetric stripping procedures introduce another step into conventional voltammetry, namely, preconcentration of the analyte on or into the electrode surface. Similarly, with preconcentrating modified electrodes (Figure 5.7, iii) the analytical performance depends on ... 

As discussed in Problem 7.7, conventional voltammetry cannot distinguish this reaction from the case where AH reacts heterogeneously to gain an electron at the electrode surface, followed by further protonation to form AH2. The disproportionation is thermodynamically favoured if... 

Figure 7.18a shows the solid-state voltammetry of the PQT/EV-PEO memory cell when the voltage between the S and G electrodes is scanned, with the S electrode considered positive by convention. Note that unlike conventional voltammetry, there is no reference electrode, and one... 

The oxidation of di-tert-butyl nitroxide (DTBN) in acetonitrile has been shown to involve an irreversible follow-up reaction that has proven difficult to characterize by conventional voltammetry because the CVs are often distorted. Using steady-state SECM TG/SC measurements, the follow-up reaction was measured clearly and convincingly as having a rate constant of 21 s . CE and tip/substrate separation plots were analyzed using Equations 7.9 and 7.10 over a wide range of DTBN concentrations (5-50 mM) to prove that the follow-up reaction was first order in DTBN. 

This chapter has also considered the use of UMEs to measure concentration profiles at larger electrodes. This approach is powerful for understanding voltammetry and diffusion at conventional electrodes and working sensor devices the ability to probe the profiles of particular species has proven particularly useful in providing mechanistic information to which conventional voltammetry measurements may be blind. 

DPV. DPV is a pulse technique that is more sensitive towards oxidation or reduction currents (faradaic currents) than conventional voltammetry since it permits discriminating charging (capacitance) current. In this technique, the currents before the end of the pulse (7 ) and just before pulse application (Z ) are measured, using pulses superimposed on a ramp of potential changing hnearly with time (apphed with amplitudes between 10 and lOOmV, for several milliseconds). The difference between the two currents is plotted against the staircase potential and leads to a vol-... 

A word of caution may be in order at this point, however. For the very reason that controlled-potential electrolysis is capable of producing relatively high concentrations of products and intermediates while conventional voltammetry involves only momentary depletion of electro-active material and accumulation of electrolysis products in the vicinity of the electrode, the nature and extent of secondary reactions under these different circumstances may not necessarily be the same even though the electrode material, potential, and other conditions are precisely reproduced. Great care should, therefore, be exercised in employing controlled-potential electrolysis data to explain polarographic processes and vice versa. 

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