CYCLIC VOLTAMMETRY AND LINEAR SWEEP TECHNIQUES

9 Linear potential sweep with hydrodynamic electrodes 

10 Linear potential sweep in thin-layer cells 

For the cyclic voltammetry specialist, many details of the application of cyclic voltammetry to a huge variety of electrochemical systems can be found in Ref. 4. 

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Recent studies describe the use of cyclic voltammetry in conjunction with controlled-potential coulometry to study the oxidative reaction mechanisms of benzofuran derivatives [115] and bamipine hydrochloride [116]. The use of fast-scan cyclic voltammetry and linear sweep voltammetry to study the reduction kinetic and thermodynamic parameters of cefazolin and cefmetazole has also been described [117]. Determinations of vitamins have been studied with voltammetric techniques, such as differential pulse voltammetry for vitamin D3 with a rotating glassy carbon electrode [118,119], and cyclic voltammetry and square-wave adsorptive stripping voltammetry for vitamin K3 (menadione) [120]. 

Of the electroanalytical techniques, cyclic voltammetry (or linear sweep voltammetry as it is sometimes known) is probably one of the more versatile techniques available to the electrochemist. The derivation of the various forms of cychc voltammetry can be traced to the initial studies of Matheson and Nicols and Randles." Essentially the technique appUes a linearly changing voltage (ramp voltage) to an electrode. The scan of voltage might be 2 V from an appropriate rest potential such that most electrode reactions would be encompassed. Commercially available instrumentation provides voltage scans as wide as 5 V. 

In addition to solid electrolyte potentiometry, the techniques of cyclic voltammetry" and linear potential sweep have also been used recently in solid electrolyte cells to investigate catalytic phenomena occurring on the gas-exposed electrode surfaces. The latter technique, in particular, is known in catalysis under the term potential-programmed reduction (PPR). With appropriate choice of the sweep rate and other operating parameters, both techniques can provide valuable kinetic" and thermodynamic information about catalytically active chemisorbed species and also about the NEMCA effect," as analyzed in detail in Section III. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

Electron transfer reactions are classified as reversible, quasi-reversible or irreversible depending on the ability of the reaction to respond to changes in E, which, of course, is related to the magnitude of k°. The distinction is important, in particular, for the (correct) application of linear sweep and cyclic voltammetry, and for that reason further discussion of this classification will be postponed until after the introduction of these techniques in Section 6.7.2. 

Various polarographic techniques have been successfully used to measure elemental sulfur, S , and sulfide (H2S and HS ) concentrations in laboratory solutions [116], salt water [117], saline lake [118], freshwater [119], lake porewaters [120], salt marsh sediments [109,120], and marine porewaters [121], Using stripping square wave voltammetry, researchers have been able to measure sulfide species at nanomolar concentrations [118-120, 122,123], Recently, cyclic and linear sweep voltammetries have been used to quantify elemental sulfur S , hydrogenosulfide (HS ) and polysulfides (S ) with fast scan rates (IV s ) [124] for example, in estuarine sediments (from Rehoboth Bay, an inland bay located in Delaware), the sulfur speciation was found to change throughout a core profile, with dominant in the top layers (0-6 cm), dominant... 

Basics of Cyclic Voltammetry. Electrochemical techniques such as cyclic voltammetry (CV) and linear sweep voltammetry (LSV) are most appropriate to the study of electronic processes and redox reactions. These techniques are conceptually elegant and experimentally simple thus they are popular for studying redox reactions at the electrode-solution interfaces and have been increasingly employed by electrochemists (2, 7). Several remarks regarding the cyclic voltammograms of electron-conducting BLM should be made. 

AC voltammetry is an extension of classical linear sweep techniques such as cyclic voltammetry. Its main strength lies in the quantitative characterization of electrode processes, and it can also be used for analytical purposes. A dc ramp with a comparatively slow sweep rate and an ac signal are superimposed and applied to a working electrode, and the... 

Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) are the most popular techniques in present electrochemical and bioelectrochem-ical research work. CV at static electrodes (Pt discs, mercury hanging electrodes, Pt microelectrodes and carbon fiber electrodes) is now routinely used to characterize the redox properties of compounds, to identify active intermediates and products and to qualitatively estimate their stability. In appropriate cases quantitative treatment is also possible [79]. 

Both cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques have been discussed in detail in Chapter 3 for more information, please refer to this chapter. 

The Model 384B (see Fig. 5.10) offers nine voltammetric techniques square-wave voltammetry, differential-pulse polarography (DPP), normal-pulse polar-ography (NPP), sampled DC polarography, square-wave stripping voltammetry, differential pulse stripping, DC stripping, linear sweep voltammetry (LSV) and cyclic staircase voltammetry. 

Thus, cyclic or linear sweep voltammetry can be used to indicate whether a reaction occurs, at what potential and may indicate, for reversible processes, the number of electrons taking part overall. In addition, for an irreversible reaction, the kinetic parameters na and (i can be obtained. However, LSV and CV are dynamic techniques and cannot give any information about the kinetics of a typical static electrochemical reaction at a given potential. This is possible in chronoamperometry and chronocoulometry over short periods by applying the Butler Volmer equations, i.e. while the reaction is still under diffusion control. However, after a very short time such factors as thermal... 

In linear sweep voltammetric techniques the applied electrode potential is varied from an initial value E to a final value f at a constant scan rate v (single sweep voltammetry). Once the value is reached the direction of the scan can be reversed, maintaining the same scan rate v, and the potential brought back to the initial value (cyclic voltammetry). In the two cases the form of the potential-time impulse can be represented as shown in Figure 1. 

Linear sweep and cyclic voltammetry (LSV and CV) are probably the most widely used techniques to investigate electrode reaction mechanisms. They are easy to apply experimentally, readily available in... 

Many investigators have used different techniques to study the electrochemical behavior of different sulphide mineral electrodes in solutions of different compositions. Linear potential sweep voltammetry (LPSV), and cyclic voltammetry (CV) have been perhaps, used most extensively and applied successfully to the investigation of reactions of sulphide minerals with aqueous systems. These techniques have provided valuable information on the extent of oxidation as a function of potential for various solution conditions and have allowed the identity of the surface products to be deduced. 

Figure 6.5 Potential is varied at a constant rate of dE/dt during voltammetric techniques such as polarography, linear sweep voltammetry and cyclic voltammetry. The scan rate v is always cited as a positive number.

Several techniques arising from cyclic voltammetry help the interested reader to peer into the future. Derivative polarograph (di/dV against Vt) increases the sharpness of detection of dissolved radicals and molecular fragments. Microelectrodes can be used with potential sweep circuitry. The use of varying electrical wave forms (instead of the linear potential variation) offers much to be learned in the future. Automation and the use of pattern recognition in mechanism evaluations... 

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