Application of Special Techniques for Fast Reactions

The methods discussed above are suitable for simple and direct reactions where rates are simple power function of concentrations. These methods would not be helpful when the reactions are of complex nature and are occurring in multiple steps. The methods are also not applicable for the study of fast reactions where some special technique are required to be employed. 

The stopped-flow technique is the most commonly used for studying and implementing application of fast reactions with half-lives between a few milHsec-onds and a few seconds. The special features of this technique, in which reactants are driven at a high rate into a mixing and/or observation cell, the flow is abruptly stopped, and the extent of reaction monitored (Figure 5), have facilitated studies on the kinetics and mechanism of fast reactions and enabled the development of reaction rate-based determinations. 

It may be surprising to find the most extensive application of collinear laser fast-beam spectroscopy in a field that a priori has little connection with the special features of this technique. Neither the Doppler shift nor the accessibility of ionic spectra plays a decisive role for the on-line experiments on radioactive isotopes from nuclear reactions. However, most of the problems encountered in the preparation of a sample of free atoms (cf. Part B, Chapter 17 by H.-J. Kluge) are solved by a combination of the fast-beam technique with the well-established concept of on-line isotope separation. The isotope separators (with ISOLDE at CERN as an outstanding example) provide the unstable species in the form of ion beams whose phase-space volume is well matched to the requirements of collinear spectroscopy. 

A special and very important case of EC-type processes is denoted as the catalytic or EC (or ErgyC y) electrode reaction. In this reaction sequence (see Eq. II. 1.24), the heterogeneous electron transfer produces the reactive intermediate B, which upon reaction with C regenerates the starting material A. The redox system A/B may therefore be regarded as redox mediator or catalyst, and numerous applications of this scheme in electroorganic chemistry are known [97]. Furthermore, a redox mediator technique based on the EC process has been proposed by Sav ant et al. [98] allowing the voltammetric time scale for the study of very fast ECj ev processes to be pushed to the extreme. 

If one is interested in the kinetics of reactions that occur at very fast rates, having half-lives on the order of a fraction of a second or less, the methods that we have discussed previously for the determination of reaction rates are no longer applicable. Instead, measurements of the response of an equilibrium system to a perturbation are used to determine its relaxation time. The rate at which the system approaches its new equilibrium condition is observed using special electronic techniques. From an analysis of the system behavior and the equilibrium conditions, the form of the reaction rate expression can be determined. 

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