Rapid scan voltammetry

The classical dc polarography of vitamins A, B, B2, B6, BI2, and C, nicotinamide, tocopherols, and naphthoquinones has been reviewed [55]. Other studies have examined in detail the cyclic voltammetry of vitamin B12 employing rapid-scan voltammetry at the DME [90] and the HMDE [91]. Vitamin B12 is complexed with trivalent cobalt ion at the heterocyclic nitrogen atoms. As a result of the complexation, a catalytic hydrogen wave is formed for the compound. In addition to the catalytic wave, a wave corresponding to the reduction of the trivalent cobalt to the monovalent state is observed. 

We first illustrated this type of effect during rapid scan voltammetry of PVF films in concentrated NaC104 solutions, where the overall redox switching process involves ingress of counter ion, salt and solvent upon oxidation (34). Quantitative treatment of such effects is better explored using a potential step, i.e. chronoamperometry. EQCM data from such an experiment are shown in Figure 3. For comparison purposes, data for analogous experiments in 0.1 and 3 mol dm 3 electrolyte, where the polymer is / is not permselective, are superimposed. 

Bond and coworkershave developed a small-volume (0.2 ml) variable-temperature EPR spectroelectrochemical cell that enables simultaneous rapid-scan voltammetry and EPR measurements to be made. The performance of this cell is compared to that of a flow-through cell designed by Coles and Compton. The small-volume cell permits cyclic voltammetric studies at variable temperatures but has significantly lower sensitivity compared to the flow-through cell, which is not amenable to low-temperature work. 

Application of electrochemical methods can be suitable for certain oxidation-reduction reactions. Both homogeneous and heterogeneous electron transfer reactions have been investigated electrochemically. The technique and apparatus can range from reasonably standard cyclic voltammetry and rapid scan voltammetry to quite specialized arrangements.In the latter case, the method often needs to be adapted to the context of a particular reaction. An example of high-pressure electrochemistry apparatus is shown in Figure 6. 

Rapid scan voltammetry—Linear sweep voltammetry... 

Rapid scan voltammetry is the simplest technique. At the working electrode the potential is rapidly scanned linearly (20—100 mV s ) as shown in Figure 1.20(i). The scanning starts before the discharging potential and stops afterwards. The current flowing through the working electrode has two components ... 

The technique employed by Lamy and colleagues was rapid-scan cyclic voltammetry in extremely dry DMF. In order to try and increase the lifetime of the C02 species the experiments were performed in the presence of active alumina suspensions. Aylmer-Kelly et al1973 had calculated the rate constant for reaction of the radical with water as a fast 5.5dm1 niol 1 s 1 and it was also hoped that reducing the solvation of the radical by water would increase the coulombic repulsion between radicals and so reduce dimerisation). 

Wang and Dewald have investigated the possibility of using rapid scan differential pulse voltammetry for the detection of chlorpromazine in flow-injection analysis [160]. The measurement was made with the use of a flow-cell equipped with a carbon paste or a vitreous-carbon disc electrode, a carbon rod auxiliary electrode, and a silver-silver chloride reference electrode. Potential scanning was effected at 2 V/min. 

Tab. 8.9 Examples of the rate constants of electrode reactions and subsequent reactions determined by rapid-scan cyclic voltammetry [51 d, 66e]...

The electrochemistry of heteropolymolybdates parallels that of the tungstates but with the following differences the reduction potentials are more positive and the primed species (metal-metal bonded ) are much less stable. Scheme 7 applies for or-fSiMo O ]4-. Species in parentheses are detectable only by rapid scan cyclic voltammetry, and XVIII decomposes rapidly at 0°C. The reduced anions such as II and IV are easily obtained by controlled potential electrolysis or by careful chemical reduction, e.g. with ascorbate. The use of metal ion reductants generally leads to other reactions, (equation 7). The reduced anions slowly isomerize (equation 8). The isomerization can be followed polarographically (all S potentials are more positive) or by NMR spectroscopy. By this means / isomers of most Keggin and Dawson molybdates have been prepared. 

In this example, the rate of a fast cleavage reaction was determined by a combination of low-temperature and fast-scan voltammetry [24]. Upon insertion of an electron into a v/c-dinitro compound, the resulting radical anion undergoes rapid C-N bond cleavage to form nitrite ion and a p-nitroalkyl radical, which in turn undergoes further reduction. 

Nagels, L. J., Mush, G., and Massart, D. L. (1989). Rapid-scan hydrodynamic voltammetry and cyclic voltammetry of pharmaceuticals in flow injection analysis conditions. J. Pharm. Biomed. Anal. 7 1479-1483. 

The mechanism of electro-oxidation of DMA in acetonitrile was studied by rapid scan cyclic voltammetry. The reduction peak for the radical cation DMA 1 was observed at scan rates above 500 Vs1. It was found that DMA 1 " underwent a second-order radical cation—radical cation coupling with deprotonation to form V,V,V, V -tctramethyI benzidine. A rate constant of 6.3 x 105 M 1 s 1 was calculated58. No evidence of polymerization was detected. 

Another biomedical example of modern voltammetry is a rapid scan cyclic voltammetric technique that has been used to quantify dopamine in brain tissue of freely moving animals. In this application, oxidation of dopamine to a quinone species at an implanted microcarbon electrode (at approximately -i0.600 V vs. Ag/AgCl) yields peak currents proportional to dopamine levels. The electrode can be used to measure this neurotransmitter in different regions of the brain or in a frxed location. Often, pharmacological or electrical stimulation can be employed to measure the... 

We begin by considering linear-scan voltammetry (LSV). The voltage is rapidly scanned at the working electrode (Fig. 1), and the resulting current is monitored and plotted as a function of either time or (usually) potential. The experiment is performed at a small stationary electrode in an unstirred solution. The most popular working electrodes are the hanging mercury-drop electrode (hmde) and either a flat disk or small... 

That there is no peak observed in the linear-scan voltammetry (LSV) scan can be understood because under the experimental conditions (rapid feeding of a small equilibrium amount of Yq to the electrode) there will be no depletion of Y near the electrode as the reduction proceeds. Also (2) i is independent of v, and (3) Ep shifts positive by 29/n mV for each tenfold increase in v through ... 

Reproducible Hmiiing currents can be achieved rapidly when either the analyte solution or the working electrode is in continuous and reproducible motion. I, inear-scan voltammetry in which the solution or the electrode is in constant motion is called hydrodynamic voltammetry. In this chapter, we will focus much of our attention on hydrodynamic voltammetry. 

The combination of the high sensitivity of SEIRAS and a rapid-scan FT-IR spectrometer enables the spectral collection simultaneously with electrochemical measurements such as cyclic voltammetry and potential-step chronoamperome-try. The time-resolved measurement can give some information on electrode kinetics and dynamics, as has been shown in Fig. 8.24. Figures 8.25 and 8.26 represent another example of millisecond time-resolved ATR-SEIRAS study of current oscillations during potentiostatic formic acid oxidation on a Pt electrode [123]. At a constant applied potential F of 1.1 V, the current oscillates as shown in Fig. 8.25 a. Synchronizing with the current oscillations, the band intensities of linear CO and formate oscillate as shown in Fig. 8.26 (and also in Fig. 8.25 c). 

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