Experimental monitoring techniques accumulation

In principle, any property of a reacting system which changes as the reaction proceeds may be monitored in order to accumulate the experimental data which lead to determination of the various kinetics parameters (rate law, rate constants, kinetic isotope effects, etc.). In practice, some methods are much more widely used than others, and UV-vis spectropho-tometric techniques are amongst these. Often, it is sufficient simply to record continuously the absorbance at a fixed wavelength of a reaction mixture in a thermostatted cuvette the required instrumentation is inexpensive and only a basic level of experimental skill is required. In contrast, instrumentation required to study very fast reactions spectrophotometrically is demanding both of resources and experimental skill, and likely to remain the preserve of relatively few dedicated expert users. 

Obviously, absorption and in case emission spectra of oxidized and reduced species can be also obtained by addition of suitable chemical reagents. In such a way, however, waste products resulting from the reduction or oxidation of the added reagents accumulate in solution and as a consequence it is often difficult to monitor the species of interest. This inconvenience does not happen when electrochemical techniques are employed. Indeed, these techniques, although they are slightly more complex from the experimental viewpoint, enable to identify with great accuracy a chemical system in all its oxidation states because no waste products are formed. 

The concentration of deactivator accumulated in the system is often easy to monitor using spectroscopic techniques but in some cases it may be more convenient to determine the concentration of the alkyl halide initiator at any given time (for example, by GC or NMR). An equation describing the time dependence of the initiator consumption (for the case when [Mt L Jo [RX]q) has also been derived (eqn (8.15)) and has been used to experimentally access In these cases, the values of a function F(RX) are calculated from known concentrations of reagents and are plotted against time. 

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