Voltammetric techniques cyclic voltammetry

Linear sweep voltammetry (LSV), with the corresponding reversal technique, cyclic voltammetry (CV), are definitely the most frequently used voltammetric techniques. This is true when a qualitative approach to the study of the redox characteristics of a solution is pursued, as well as when a quantitative study of the electrode mechanism and even the evaluation of the relevant thermodynamic and kinetic parameters are faced, and also when electroanalytical quantitative information are sought. The potential waveforms for LSV and for CV, with the relevant equations expressing the time dependence of E, are shown in Fig. 10.12. 

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. 

Another important bioanalytical application of voltammetric ISEs is the detection of polyions (see also above). A technique using cyclic voltammetry on micropipette electrodes filled with the organic electrolyte solutions in 1,2-dichloroethane was successfully applied for the detection of protamine [65] in saline solution and heparin in undiluted sheep plasma samples [66]. Protamine transport was facilitated with dino-nylnaphthalenesulfonic acid (DNNS). As a heparin-selective component the tetrakis-(4-chlorophenyl)borate salt of trimethyloctadecyl ammonium was used. 

This is a dynamic electrochemical technique, which can be used to study electron transfer reactions with solid electrodes. A voltammo-gram is the electrical current response that is due to applied excitation potential. Chapter 18b describes the origin of the current in steady-state voltammetry, chronoamperometry, cyclic voltammetry, and square wave voltammetry and other pulse voltammetric techniques. 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

Microelectrolytic techniques such as cyclic voltammetry are very well suited to observation of the electrochemical triggering of SrnI reactions and detailed investigation of their mechanism. A typical example of the evolution of the cyclic voltammetric responses of an Srn 1 substrate upon addition of increasing amounts of a nucleophile is shown in Figure 2.39. 

The most popular voltammetric technique is probably cyclic voltammetry (CV), partly because of its early development in theory and the availability of the corresponding commercial equipment. In this technique, the electrode potential is first scanned linearly with time from a starting potential, where no reaction occurs, passing E°, towards another potential, and then reversed back to the starting potential. In this case, the time variable can be conveniently represented by the scan rate, v. 

Without any doubt, cyclic voltammetry is the most popular voltam-metric technique used in the field of inorganic chemistry. Unfortunately, the power of the technique is frequently overestimated in that simple cyclic voltammetric measurements rarely allow one to gain complete electrochemical information. As we will discuss, it must be always coupled with complementary techniques. 

Cyclic voltammetry belongs to the category of voltammetric techniques based on a linear potential sweep chronoamperometric technique. It certainly constitutes the most useful technique for a preliminary determination of the redox properties of a given species. 

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. 

It is conceivable that the presence of such complications must affect the shape of the cyclic voltammograms, and hence perturb to some extent the diagnostic criteria for the above-mentioned fundamental electron transfer processes. As these reactions proceed at their own rates, cyclic voltammetry will be able to detect them only if their rates fall within the time scale of the voltammetric technique (which ranges from a few tens of seconds to a few milliseconds). 

Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of normal voltammetric experiments until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method calledjhst scan voltammetry [27]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MV s are possible, and species with lifetimes in the nanosecond scale can be observed. Using this technique, P. Hapiot et al. [28] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation and substituted derivatives. The resulting rate constants for the dimerization of such monomers lie in the order of 10 s . The same... 

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.

Two voltammetric techniques, stationary-electrode voltammetry (SEV) and cyclic voltammetry (CV), are among the most effective electroanalytical methods available for the mechanistic probing of redox systems. In part, the basis for their effectiveness is the capability for rapidly observing redox behavior over the entire potential range available. Since CV is an extension of SEV, many points pertinent to CV are discussed in the SEV section. 

When using microelectrodes to obviate resistance problems, it is convenient to develop a procedure to determine what conditions are required to reduce the error to an acceptable level. The results of such a procedure applied to disk electrodes are shown in Figure 16.6 [45]. In this and the remaining discussion, the technique of cyclic voltammetry is considered, as it is one of the most widely used voltammetric methods. The region of practical working conditions of electrode radius and scan rate is defined by the area set off by lines A, B, and C. 

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]. 

In this section, we will show that the stationary responses obtained at microelectrodes are independent of whether the electrochemical technique employed was under controlled potential conditions or under controlled current conditions, and therefore, they show a universal behavior. In other words, the time independence of the I/E curves yields unique responses independently of whether they were obtained from a voltammetric experiment (by applying any variable on time potential), or from chronopotentiometry (by applying any variable on time current). Hence, the equations presented in this section are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. 

Equation (6.96) can be applied to any sequence of constant potential pulses and so to any voltammetric technique. In the particular case of cyclic voltammetry, the waveform is given by Eq. (5.1) and the current takes the form... 

The kind of voltammetry described in Sect. 4.2. is of the single-sweep type, ie., only one current-potential sweep is recorded, normally at a fairly low scan rate (0.1-0.5 V/min), or by taking points manually. Cyclic voltammetry is a very useful extension of the voltammetric technique. In this method, the potential is varied in a cyclic fashion, in most cases by a linear increase in electrode potential with time in either direction, followed by a reversal of the scan direction and a linear decrease of potential with time at the same scan rate (triangular wave voltammetry). The resulting current-voltage curve is recorded on an XY-recorder,... 

For chemists, the second important application of electrochemistry (beyond potentiometry) is the measurement of species-specific [e.g., iron(III) and iron(II)] concentrations. This is accomplished by an experiment in which the electrolysis current for a specific species is independent of applied potential (within narrow limits) and controlled by mass transfer across a concentration gradient, such that it is directly proportional to concentration (/ = kC). Although the contemporary methodology of choice is cyclic voltammetry, the foundation for all voltammetric techniques is polarography (discovered in 1922 by Professor Jaroslov Heyrovsky awarded the Nobel Prize for Chemistry in 1959). Hence, we have adopted a historical approach with a recognition that cyclic voltammetry will be the primary methodology for most chemists. 

A very useful extension of the voltammetric technique is cyclic voltammetry (Adams, 1969 Cauquis and Parker, 1973) in which one scans the potential of the working electrode in an unstirred electrolyte solution in the anodic (cathodic) direction and records one or several peaks due to oxidation (reduction) of the substrate. At some suitable potential, the direction of the scan is reversed and peaks due to reduction (oxidation) of intermediates and/or products formed during the forward scan are observed. In the simplest case a linear increase (decrease) of the potential with time is employed (triangular cyclic voltammetry) with scan rates in the range 0 01-1000 V s 1. It should be noted that cyclic voltammetry at scan rates above 1 Vs"1 requires the use of a differential cell to reduce the residual current due to charging of the electrified interface (see, for example, Peover and White, 1967). The theory of cyclic voltammetry has been... 

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