Voltammetry and Related Techniques

In linear potential scan (LSV) and cyclic (CV) voltammetries, a potential varying linearly with time is applied between an initial potential, usually at a value where no faradaic processes occur, and a final potential (LSV) or cycled between two extreme (or switching) potential values at a given potential scan rate v (usually expressed in mV/sec). In other techniques, such as normal and differential pulse voltammetries (NPV and DPV, respectively), or square-wave voltammetry (SQWV), the excitation signal incorporates potential pulses to a linear or staircase potential/time variation. 

In a typical CV experiment, the potential scan is initiated at the open-circuit potential and directed in the positive or negative direction. For a reversible process, when the potential approaches the formal potential of the involved couple, the current increases rapidly while the concentration of the electroactive species in the vicinity of the electrode is depleted. As a result, a maximum of current is obtained, 

For a reversible process involving species in solution, the absolute value of the peak potential separation, - EpcI, approaches 59/n (mV at 298 K), whereas the half-sum of such potentials can, in principle, be equal to the formal electrode potential of the couple. Under the above conditions, the peak current is given by the Randles-Sevcik equation (Bard et ak, 2008)  

The peak current is then proportional to the concentration of the electroactive species and the square root of the potential scan rate. A case of particular interest is when the electroactive species is confined to the electrode surface where it reaches a surface concentration, F. Here, symmetric, bell-shaped current/potential curves, described by Bard and Faulkner (2001), 

the peak current becomes proportional to the potential scan rate. It should be noted that Equations (1.7) and (1.8) are formally analogous to those obtained for species in solution diffusing in a restricted space, under the so-called thin-layer conditions (by contraposition to unrestricted space diffusion, thick-layer conditions). 

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There are several excellent articles which deal with the theory and practice of cyclic voltammetry.1-4 Foremost among these is the comprehensive treatise by Bard and Faulkner which gives a thorough description of the theory of controlled potential microelectrode techniques, including cyclic voltammetry.1 Particularly readable accounts of cyclic voltammetry and related techniques are given in Adams book, Electrochemistry at Solid Electrodes ,2 in Pletcher s review3 and in a series of articles which appeared in J. Chem. Educ.e>... 

Most of the work reported before the Second World War was carried out in aqueous electrolyte solutions. Since 1945 the focus has shifted to include the application of nonaqueous solvents. This has allowed for the detection of the primary intermediates, typically radical anions and radical cations, and for the study of their reactions. The theoretical foundations, for the analysis of kinetics and mechanisms by, for instance, cyclic voltammetry and related techniques were mostly published in the 1960s and 1970s. The application of such techniques has resulted in a steadily increasing understanding of the kinetics and mechanisms of organic electrochemical processes. The current trend is to return to water-like conditions reflecting the need to substitute organic solvents with environmentally friendlier electrolyte systems. 

W.E. Geiger and M.D. Hawley, Cyclic voltammetry, A.C. Polarography and Related Techniques. Physical Methods of Chemistry. Electrochemical Methods. A. Weissberger and B.W. Rossiter eds, Vol. 2., Chapter 1. Wiley Interscience, New York, 1986. 

Voltammetry and Related New Techniques - Methods that Electrolyze Electroactive Species Only Partially (3)... 

Most of the more advanced techniques have only rarely been used outside the laboratories where they have been developed, and for that reason it is not easy to give recommendations. Examples include normalized sweep voltammetry [34,35,157,158], linear current-potential analysis [33], and the so-called global analysis and related techniques [159-161]. However, one such technique, convolution potential sweep voltammetry, has gained some popularity, and is introduced briefly here. 

Electrochemical methods can also be used for obtaining analytical information on porous materials. Voltammetric methods and related techniques have been largely used to acquire information on reaction mechanisms for species in solution phase, whereas impedance techniques have been extensively used in corrosion and metal surface studies. In the past decades, the scope of available methods has been increased by the development of the voltammetry of microparticles (Scholz et al., 1989a,b). This methodology, conceived as the recording of the voltammetric response of a solid material mechanically transferred to the surface of an inert electrode, provides information on the chemical composition, mineralogical composition, and speciation of solids (Scholz and Lange, 1992 Scholz and Meyer, 1994, 1998 ... 

If voltammetric and related techniques are used, the ohmic drop should be either compensated (now this is usually done by the software or hardware of electrochemical devices [69]), or reduced by using, for example, a Luggin (Lu in-Gaber) capillary (see in Ref. [12]). Another important technical detail is that the components of reference redox systems (such as fer-rocene/ferrocenium) are frequently added immediately into the working compartment when voltammetry-hke techniques are applied. 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

Brown ER and Sandifer JR (1986) Cyclic voltammetry, AC polarography and related techniques. In Rossiter BW and Hamilton JF (eds.) Physical Methods of Chemistry, 2nd edn. p, pp. 273—432. New York Wiley. 

E.R. Brown, R.F. Large, Cyclic voltammetry, ac polarography, and related techniques, in Physical Methods of Chemistry, Techniques of Chemistry, ed. by A. WeissbCTgCT, W. Rossiter. Part HA Electrochemical Methods, vol. 1, 4th edn. (Wiley-Interscitaice, New York, 1971), pp. 423-530... 

E.R. BROWN and R.F. LARGE, "Cyclic Voltammetry, AC. Polarography and related techniques", in "Techniques of Chemistry", Vol. I, Part IIA, "Electrochemical Methods",... 

Stripping Voltammetry One of the most important quantitative voltammetric techniques is stripping voltammetry, which is composed of three related techniques anodic, cathodic, and adsorptive stripping voltammetry. Since anodic strip-... 

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. 

Examination of the membranes with a variety of physicochemical techniques, from related electrochemical approaches (as electrochemical impedance spectroscopy (EIS), voltammetry and chronoamperometry) to more sophisticated characterization methods (spectroscopy and microscopy), actually serves the same end as the theory and leads to a deeper understanding of the chemistry behind the functioning of these sensors [5, 6],... 

Two closely related techniques are staircase and square-wave voltammetry in the first modality, the potential is varied stepwise versus time in the second modality, a pulse square-wave is superimposed onto the staircase potential variation. In the differential acquisition mode, the current is measured immediately before each potential change, and the current difference is plotted as a fimction of potential. This way, the effect of the charging current can be decreased. 

Cyclic voltammetric methods, or other related techniques such as differential pulse polarography and AC voltammetry,3 provided a convenient method for the estimation of equilibrium constants for disproportionation or its converse, comproportionation. In this respect, the experimentally measured quantity of interest in a cyclic voltammetric experiment is E>A, the potential mid-way between the cathodic and anodic peak potentials. For a one-electron process, E,A is related to the thermodynamic standard potential Ea by equation (4).13 In practice, ,/2 = E° is usually a good approximation. 

Cyclic voltammetry and controlled-potential electrolysis are the techniques that have been used to investigate the electrochemistry of oxo-chromium and oxo-molybdenum corrolates. The data have been related to those obtained for similar porphyrin complexes. Redox potentials are reported in Table 17. 

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. 

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