In the experiments for studying chemical kinetics there are three major ingredients, namely, the method for initializing the reaction, the reaction medium and thirdly the method for detecting the extent of reaction that has occurred. The initiation may consist of simple mixing of reactants, thermal activation (heating), electrical discharge, chemical activation, photoactivation (with light of wavelength 120 nm to 700 nm) and so on. The reaction medium can either be static (such as a test tube with a solvent medium, a gas bulb, living cell etc) or a flow system wherein atoms /ions /molecules are formed in a continuous discharge and the reaction occurs during the flow process. The last and the most important aspect is the technique for measuring concentration changes. The classical techniques (for times in the range of 0.1 sec to greater than an hour) include pressure measurements for gas phase reactions, conductance measurements, polarimetry, polarography, gas chromatography, chemical trapping and so on. The main criteria here is that the time to analayse the concentration changes must be much smaller than the time scale of the reaction. For shorter time scale (10-5s to 1s), the method like flash photolysis, pulse radiolysis, nuclear magnetic resonance, temperature jump, pressure jump and electrochemical methods are useful. For the time in the range of 10-5s to 10-8s, acoustical methods and electrical pulses are useful. Fluorescence decay is useful for the time span of 10-6s to 10-10s. For shorter times, pico second techniques (10-11s to 10-13s) and femtosecond methods (10-15s to 1014s) have been developed recently. In this lecture, we will consider only a few representative techniques among the methods mentioned above, which illustrate the principles involved in detecting concentrations at different time scales.
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