Peak voltammetry, linear potential sweep

The simple linear-sweep voltammetry (LSV) or linear potential sweep chronoamperometry (of which polarography with a dropping Hg electrode is the earliest example) can be understood simply if one looks at just the first rise to a peak in Fig. 11.70. 

Linear potential sweep and cyclic voltammetry are at their best for qualitative studies of the reactions occurring in a certain range of potential. In Fig. 5L, for example, we see the cyclic voltammogram obtain on a mercury-drop electrode in a solution of p-nitrosophenol in acetate buffer. Starting at a potential of 0.3 V versus SCE, and sweeping in the cathodic direction, one observes the first reduction peak at about - 0.1 V. This potential corresponds to the reduction of... 

The two types of electrochemically formed chemisorbed oxygen on Pt films interfaced with YSZ are also clearly manifest via solid state linear potential sweep voltammetry (Fig. 9 bottom, Ref. [39]) The first oxygenX reduction peak corresponds to normally chemisorbed oxygen (y-state) and the second reduction peak which appears only after prolonged positive current application [39] corresponds to the 5-state of oxygen, i.e. backspillover oxidic oxygen, which is significantly less reactive than the y-state. 

Numerous excellent texts exist on the fundamentals of cyclic voltammetry. The reader is referred especially to the recent text by Bond, which provides an excellent treatment of fundamentals as well as applications. The important aspects of cyclic voltammetry are illustrated by the diagram shown in Figure 1 of a typical voltammogram of a soluble, reversible couple subjected to a linear potential sweep (and return scan) between applied voltages E and E2- The characteristic curve shown in Figure 1 provides peak potentials ( p and E° ) as well as peak currents 1° and Note that... 

A useful adjunct of linear potential sweep methods is called cyclic voltammetry. Rather than stopping an oxidative voltammogram at, say, + 0.8 V, the potential is reversed and scanned backward, i.e., a triangular wave potential is applied. The oxidation product formed is present at and close to the electrode surface. With fairly rapid potential sweeps (ca. >4 V/min) it is almost completely re-reduced back to the starting material on the reverse potential sweep. Figure 14B shows a typical cyclic voltammogram for a reversible system (solid line). The ratio of forward to reverse peak currents is unity. If, however, some rapid process removes the product(s), litde or no reverse current is obtained (dotted lines of Fig. 14B). This happens if the overall oxidation is totally irreversible, or fast chemical reactions intervene. We will also see later that a peculiar property of very small electrodes can eliminate most of the reverse current in a cyclic voltammogram. 

A more important practical problem is interpreting the apparently confusing responses obtained with linear potential sweep (peak voltammetry) methods. Although the linear potential sweep method has not been used much in vivo, it is often applied to pretest electrodes in solution, determine oxidation potentials (for example, see Table 3), etc. Depending... 

In linear sweep voltammetry, the potential is varied linearly versus time, and current peaks are registered in correspondence to oxidation or reduction (depending on the potential variation verse) of the analytes. The potential value associated to a peak is characteristic of the specie being oxidizing or reducing, while the peak height can be employed for quantitative purposes. 

Linear and cyclic sweep stationary electrode voltammetry (SEV) play preeminent diagnostic roles in molten salt electrochemistry as they do in conventional solvents. An introduction to the theory and the myriad applications of these techniques is given in Chapter 3 of this volume. Examples of the linear and cyclic sweep SEV current-potential responses expected for a reversible, uncomplicated electrode reaction are shown in Figures 3.19 and 3.22, respectively. The important equation of SEV, which relates the peak current, ip, to the potential sweep rate, v, is the Randles-Sevcik equation [67]. For a reversible system at some temperature, T, this equation is... 

Then appears linear sweep rate voltammetry in which the electrode potential is a linear function of time. The current-potential curve shows a peak whose intensity is directly proportional to the concentration of electroactive species. If the potential sweep takes place in two directions, the method is named cyclic voltammetry. This method is one of the most frequently used electrochemical methods for more than three decades. The reason is its relative simplicity and its high information content. It is very useful in elucidating the mechanisms of electrochemical reactions in the case where electron transfer is coupled... 

The potential-time relation for voltammetric measurements is presented in Figure 3.2. With linear-sweep voltammetry, the potential is linearly increased between potentials Ex and E2. Cyclic voltammetry is an extension of linear-sweep voltammetry with the voltage scan reversed after the current maximum (peak) of the reduction process has been passed. The voltage is scanned negatively beyond the peak and then reversed in a linear positive sweep. Such a... 

There is additionally the important problem involved in choosing the reduction or oxidahon potential of the electrolyte solutions from either cyclic voltammetry (CV) or linear sweep voltammetry (LSV). Since the oxidation or reduction reachon of cations or anions contained in the RTILs are electrochemically irreversible in general [8-10], the corresponding reduction or oxidation potential cannot be specifically obtained, unlike the case of the redox potential for an electrochemically reversible system. Figure 4.1 shows the typically observed voltammogram (LSV) for RTILs. Note that both the reduchon and oxidation current monotonically increase with the potential sweep in the cathodic and anodic directions, respectively. Since no peak is observed even at a high current density (10 mA cm ), a certain... 

Linear sweep voltammetry (LSV), also known as linear sweep chronoamperometry, is a potential sweep method where the applied potential (E) is ramped in a linear fashion while measming cnrrent (i). LSV is the simplest technique that uses this waveform. The potential range that is scanned begins at an initial or start potential and ends at a final potential. It is best to start the scan at rest potential, the potential of zero current. For a reversible couple, the peak potential can be calcnlated nsing equation (6). ... 

In the more advanced kinetic measurements, which were carried out by using chronopotentiometry [118], chronocoulometry [124, 139], linear [146] and convolution [18, 147] potential sweep voltammetry, or phase-sensitive ac polarography [142, 143], the ohmic drop was either numerically subtracted [118], or compensated [124, 139, 142, 143, 146, 147] with the help of the positive feedback. The feedback adjustment was based either on the assumption that the separation of the current peaks measured by the slow potential sweep voltammetry should reach the value of (59/z)mV [124, 139, 146, 147], or on the value of the solution resistance obtained by an ac bridge technique [142, 143]. However, the former adjustment is not very sensitive, whereas the estimated accuracy of 10 Q [142] in the latter case may not be... 

The solubility product of NiS was determined by linear voltage sweep voltammetry using a mercuric sulphide coated electrode (hanging mercury drop). The peak potential for the exchange reaction between the mercuric sulphide coated mercury electrode and Ni ions of a nickel perchlorate solution,... 

Separation of adjacent peaks in differential-pulse polarography, where the symmetry of the peak can also be made use of, is usually easier than that of consecutive peaks in linear-sweep voltammetry. As is the case in conventional polarography, more positive peaks that interfere can sometimes be shifted to more negative potentials and the sequence of peaks inverted by change in supporting electrolyte. 

Various electrochemical methods have been appfied for the analysis of NAs, including DPP [5, 11] and DPV[13, 269, 270], linear sweep and CV [13, 271] square wave [138] and a.c. voltammetry [272-274], and recently constant current chronopotentiometry [249, 255-257, 275, 276] and elimination voltammetry [139, 277-279]. DPP was applied for the analysis of DNA in 1966 [280], and in a short time, it replaced OP and d.c. po-larography used in the early NA studies [4, 5]. The main advantage of DPP is its better sensitivity and resolution of peaks. Calf thymus ssDNA produced a well-developed DPP peak III (Fig. 6d) at concentrations of about 10 to 20 igml while dsDNA was inactive at the same concentration. At higher concentrations (hundreds of pgml ), dsDNA produced peak II at potentials by about 70 mV more positive than peak III (Fig. 6c). For years, DPP was the most sensitive instrumental method of determination of traces of ssDNA in dsDNA samples [5]. 

As it can provide some of the most basic electrochemical information related to the reactivity of the selected analyte (peak potential and peak current) most instruments that perform amperometry can also perform some of the most basic voltammetric techniques. These techniques determine the current as a function of the potential applied to the WE (in a conventional three-electrode cell) and can be performed with relatively simple instrumentation [105,106]. As different signals can be combined in the input ports of the instrument, multiple variations of the technique have been developed including cyclic voltammetry, linear sweep voltammetry, linear sweep stripping voltammetry, stripping voltammetry [107, 108], fast-scan cyclic voltammetry [109], square-wave voltammetry [110],and sinusoidal voltammetry [111]. 

Fig. 2.13 Current versus overpotential curves showing the effect of experimental parameters in the presence of forced convection, according to the relationship = /cL lnFc. (a) Electrode size (and shape). Ideally, in the presence of a uniform current-density distribution, Deviations may be due to edge effects, non-uniformity of flow (e.g. entrance length effects) or contributions from natural convection, (b) Concentration of electroactive species in the reactor. ii should be proportional to c. It is sometimes convenient to test this by incremental increases in c . The background curve is represented by = 0. (c) Relative velocity of the electrolyte or electrode, cc where x is a constant which depends upon the geometry and flow conditions, x may vary slightly over different ranges of Reynolds number. The limiting-current plateau may shorten and tilt as velocity increases, due to the increasing importance of electron transfer to the overall reaction kinetics. The maximum on the 1 curve may arise due to unsteady-state mass transport and is akin to a peak in linear sweep voltammetry, i.e. it may arise due to an excessive rate of potential change.

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