High-speed voltammetry

UMEs of 10 pm in diameter and voltammetric instruments for use with such UMEs are commercially available. Electrodes of smaller dimensions can be prepared in the laboratory, although this requires considerable skill [74], In order to use UMEs successfully for high-speed voltammetry in highly resistive solutions, care must be taken concerning the effects of the ohmic drop and the capacitance of the cell system [65 b, 74, 75]. Moreover, two types of voltammograms, i.e. curves (a) and (b) in Fig. 5.23, should be used appropriately, according to the ob-... 

Microelectrodes, due to their small size in the micrometer range, allow the study of fast chemical reactions coupled with heterogeneous electron tranfer reactions. In fact, the very small RC time constant and low solution iR drop associated with microelectrodes, had allowed the achievement of high diffusion rates in fast scan techniques. This means that accuarate voltanunetric data can be obtained in high speed voltammetry. 

E Pulse Voltammetry 742 25F High-Frequency and High-Speed Voltammetry 745... 

Developments in the use of ultramicroelectrodes and instrumentation advances in high speed voltammetry have been the subject of several review articles and book chapters [38-44]. Rather than monitoring the current with concomitant problems with double-layer charging at short time scales, a rather ingenious approach is to combine the use of ultramicroelectrodes with spec-troelectrochemistry [45-47], Very fast reactions (with bimolecular rate constants up to —10 M s" ) can be thus accessed for the electrogenerated species in the diffusion layer region. 

High-speed cyclic voltammetry in the 105 V/s range has recently been applied to kinetic investigations of extremely rapid heterogeneous reactions and... 

High-speed linear-sweep voltammetry (LSV) or linear potential sweep chronoamperometry (top) potential waveform (bottom) current response. The areas between the solid lines and the dotted lines measure approximately the charge transferred in the oxidation or reduction. 

High-speed cyclic voltammetry, using ultra-microelectrodes and performed on [Ni(dmit)2] complexes, showed that the formation of [Ni(dmit)2] species follows an EC mechanism before the crystallisation of the compound occurs on the electrode... 

The use of ultramicroelectrodes for fast measurements has been recognized and developed by others and has been discussed in the literature. For the most part, the existing reports have concentrated on cyclic voltammetry. We believe that potential step methods are more appropriate to the realization of the promise of high speed kinetic measurements at ultramicroelectrodes, so we have concentrated on them. A preliminary report of our activity has appeared. ... 

N. V. Rees, J. A. Alden, R. A. W. Dryfe, B. A. Coles, and R. G. Compton. Voltammetry under high mass transport conditions. The high speed channel electrode and heterogeneous kinetics, J. Phys. Chem. 99, 14813-14818 (1995). 

Beyond the ability to probe heterogeneous electron transfer reactions that occur on the micro, or even nano timescale, high-speed cyclic voltammetry has found application in the investigation of complex reaction mechanisms. An example of this is the following chemical (EC) reaction. The oxidation or reduction (E reaction) of an electroactive species can produce a chemically unstable product that undergoes a chemical reaction (C reaction) to form an electroinactive product. Electron transfer reactions can also lead to isomerizations and ligand loss in organometallic compounds, such as... 

Prieto F, Aixill WJ, Alden JA, Coles BA, Compton RG (1997) Voltammetry under high mass transport chigh-speed channel electrode and transient measuremcaits. J Phys Chem B 101 5540-5544... 

Wightman RM, Wipf DO (1990) High-speed cyclic voltammetry. Acc Chem Res 23 64—70. doi 10.1021/ ar00171a002... 

Stamford, J. A., Kruk, Z. L., and Millar, J., 1984, Regional differences in extracellular ascorbic acid in the rat brain determined by high speed cycle voltammetry. Brain Res. 299 289-295. 

Rees NV, Klymenko OV, Maisonhaute E, Coles BA, Compton RG (2003) The application of fast scan cyclic voltammetry to the high speed channel electrode. J Electroanal Chem 542 23-32... 

Indeed, specially synthesized frans-vitamin D that immediately forms AR2 gave a reversible wave [203]. It was the first example where the cis- and frans-isomeriza-tion caused a slowing down of the preceding e stage. This stage r should be very fast in accordance with the formula ultrafast chemical reaction is the cause of slow electron transfer [204] (Sect. 9.2.2, Cathodic and Gas-Phase Reactions of the Bond Cleavage ). Thus, it could not be tracked by the high-speed cyclic voltammetry (CVA, v < 1000 V sec ) and by the HSCP [202], The major reaction product is dihydrovitamin D (III), but dihydrotachysterol (IV) is also formed, which is effective in hypercalcemia (Sect. 9.2.7, Dihydrotachysterol ). 

What determines the quality of a potentiostat Obviously, the ideal potentiostat should be fast, of low noise and of high input impedance. These three requirements rarely go together. Often the user has to choose between high speed and high stability . High stability or low noise in voltammetry is usually achieved in instruments by inserting extra capacitors in the electronic circuit. This solution leads to distorted voltammetric signals peaks are smaller and thicker, and waves are more sloped however, calibration plots constructed for analytical purposes can still be linear. For a more detailed description of the problem the reader is referred to Bard and Faulkner [2]. 

---