Voltammetry fast-sweep cyclic

DC polarography of three aryl-substituted cyclopropenyl cations (40) revealed that a reversible potential could not be obtained due to a rapid dimerization of the intermediate radical formed to give 41. When fast sweep cyclic voltammetry was employed both cathodic and anodic waves were observed, indicating that the radical can be reoxidized before dimerization. 

The kinetics of c,e-type processes are also conveniently studied by means of fast-sweep cyclic voltammetry where the current measured depends on sweep-rate due to the time required for the electroactive species Ox to be produced from the initial reactant, C. Information on ks in relation to the magnitude ot k and k2 can be obtained from suitably designed experiments. 

Andrienx, C.P., Garrean, D., Hapiot, R, Pinson, J. and Saveant, J.M. (1988) Fast sweep cyclic voltammetry at nltra-microelectrodes. Evaluation of the method for fast electron-transfer kinetic measnrements. Journal of Electroanalytical Chemistry, 243, 321-335. 

Even at very low applied potentials oxidations of 5-HT leads to formation of some tryptamine-4,5-dione (B, Figure 8). The way in which this can be formed is by electrochemical oxidation (le, 1H+) of primary radical 1 to quinone imine 6 (Figure 8). This electron-deficient molecule cannot be detected by fast-sweep cyclic voltammetry at low pH and so must be rapidly attacked by water to give 4,5-DHT or by HCl to give 4-chloro-5-hydroxy-... 

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]. 

Multiple-sweep cyclic voltammetry is a useful method for the qualitative investigation of complex reaction phenomena at electrodes. In this method, the potential is swept repetitively in the forward and reverse directions at a relatively high rate, typically from one to several volts per second. A high scan rate permits to distinguish phenomena with fast time constants. Furthermore, it improves the... 

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]. 

The delocalised radical formed by protonation of the radical-anion is more easily reduced than the starting arene. For some polycyclic aromatic hydrocarbons, the redox potential for this radical species can be determined using a cyclic voltammetry technique [10]. Reduction in dimethylformamide is carried out to the potential for formation of the dianion. The dianion undergoes rapid monoprotonation and on the reverse sweep at a fast scan rate, oxidation of the monoanion to the radical can be observed. The radical intermediate from pyrene has E° = -1.15 V vs. see in dimethylformamide compared to E° = -2.13 V vs. see for pyrene,... 

Fig. 7.35. Development of diffusion concentration profiles in ensembles of microelectrodes. Concentration distortions at very short times during chronoamperometry or fast sweep rates during (a) cyclic voltammetry, (b) intermediate times or sweep rates, and (c) long times or slow sweep rates. Voltam-metric responses are shown schematically. (Reprinted from B. R. Scharifker, Microelectrode Techniques in Electrochemistry, in Modem Aspects of Electrochemistry, Vd. 22, J. O M. Bockris, B. E. Conway, and R. E. White, eds., Plenum, 1992, p. 505.)...

Historically, the potential sweep technique and cyclic voltammetry were developed for analysis (as successors to polarography) and much of the theoretical development is concerned with the situation under conditions of diffusion control, for that is where the analytical applications are most readily made. In many of these approaches, the underlying assumption is that the electron transfer that must necessarily occur at the interface is a fast process and plays little part in determining the dependence of the observed current upon potential or upon the concentration of the reactant. However, these assumptions may not always apply. 

After a description of how to control the sweep experiment and its two forms, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) (where the sweep direction is inverted at a certain, chosen potential), the voltammetric waveshape obtained for slow and fast electrode reactions is analysed. Recent advances in these topics are considered. Finally, the type of curve obtained from linear sweep in a thin-layer cell is presented thin-layer cells are important because they permit almost 100 per cent conversion of the electroactive species, and show differences in relation to electrochemical behaviour in a normal-sized cell. 

In linear sweep or cyclic voltammetry, high scan rates can be used to shorten the time-scale below the steady-state time-scale arising from diffusion or convective diffusion, lypically scan rates of 10-3000 Vs are used, corresponding to time-scales of 1-10 p,s, although in fast-scan CV they may reach... 

Cyclic voltammetry studies of 1,5-dithiocane (DTC) show its unusual electrochemical behavior, as in Eq. (40) [72-74]. An increase of current during the reverse scan occurs with decreasing sweep rates, which can be explained by assuming that the cation radical, DTC "-, exists in equilibrium with a dimer, (DTC) , whose rate of formation is higher than that of its dissociation. Therefore, at high sweep rates, the direct reduction of the dimer to DTC is observed, whereas at low sweep rates the dissociation is sufficiently fast for the observation of the reduction current of the cation radical DTC " . 

In many preparative applications of EGBs the rate-determining step in product formation is the proton transfer. This is often the case when the deprotonated substrate is removed in a fast product-forming reaction (cf. Sec. II.B). For EGBs formed in situ, electrochemical methods such as cyclic voltammetry (CV), derivative cyclic voltammetry (DCV), linear sweep voltammetry (LSV), double-potential-step chronoamperometry (DPSC), and other electroanalytical methods can often be used to estimate the kinetics of proton transfer from the substrate to the EGB. When the EGBs are formed ex situ (because the acidic... 

In the type of linear-sweep voltammetry discussed thus far, the potential is changed slowly enough and mass transfer is rapid enough that a steady state is reached at the electrode surface. Hence, the mass transport rate of analyte A to the electrode just balances its reduction rate at the electrode. Likewise, the mass transport of P away from the electrode is just equal to its production rate at the electrode surface. There is another type of linear-sweep voltammetry in which fast scan rates (1 V/s or greater) are used with unstirred solutions. In this type of voltammetry, a peak-shaped current-time signal is obtained because of depletion of the analyte in the solution near the electrode. Cyclic voltammetry (see Section 23D) is an example of a process in which forward and reverse linear scans are applied. With cyclic voltammetry, products formed on the forward scan can be detected on the reverse scan if they have not moved away from the electrode or been altered by a chemical reaction. 

The transition from typical peak-shaped voltammograms at fast sweep rates in the linear diffusion region to steady-state voltammograms at small v is shown for cyclic voltammetry in Figure 6.2.2. In the steady-state region, the voltammograms are... 

In cyclic voltammetry, which usually works with an unstirred electrode, the potential is varied with a typical sweep rate of 0.5-100 V/s. A triangular sweep is applied and the currents are in the range from femto- to milliamperes (lO -lO A). An oscilloscope or a fast X-Y recorder can sweep a cyclic voltam-... 

This fast linear sweep voltammetry with periodic reversal of scan direction is better known as Cyclic Voltammetry. Cyclic voltammetry is a vital tool to the physical electrochemist but has little or no direct electroanalytical application. 

Cyclic voltammetry is not primarily a quantitative analytical technique. The references at the end of this chapter provide additional guidance to its applications and interpretation. Its real value lies in the ability to establish the nature of the electron transfer reactions—for example, fast and reversible at one extreme, slow and irreversible at the other—and to explore the subsequent reactivity of unstable products formed by the forward sweep. Suffice it to say that such studies are valuable for learning the fate and degradation of such compounds as drugs, insecticides, herbicides, foodstuff contaminants or additives, and pollutants. 

---