Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

280 DETECTION AND ESTIMATION OF INTERMEDIATESCases arise where the method of investigation uses techniques of more thanone category. This is particularly true of the “chemical” methods described inSection 5, in which the effect of the active intermediate on foreign reactants isstudied, for example, by mass spectrometry or optical absorption or emissionspectroscopy. The principle process involved in the estimation of the intermediateis, however, a chemical one, and this determines the heading under which themethod is described.Certain general observations may, perhaps, be made at this stage. If a reactionis in a steady state, then it is usually valid to assume that the concentration ofreaction intermediates is also stationary. The high reactivity of many intermediatespecies results in low stationary concentrations unless the rate of intermediateformation is unusually high (e.g. as it is in flames). In order, therefore, to studyreactive intermediates, a number of special experimental techniques have beendeveloped. Flash photolysis may be used isothermally to produce high intermediateconcentrations photochemically, or adiabatically to produce the intermediatesthermally, and the rise and decay of intermediate species may be followedby one or other of the techniques available (in this case spectroscopy is normallyused). Again, shock waves may be used to produce large concentrations of reactivespecies by thermal decomposition of reactants. Such studies necessitate analyticalmethods for the intermediates which have a response comparable with the lifetimeof the intermediate. The non-stationary techniques have been complementedby steady-state methods, such as flow techniques, designed to follow fast reactions.The active species -no longer truly “intermediates”-may be produced in highconcentration within the apparatus, and used as the reactants in suitable chemicalprocesses. Isolation of individual steps has proved a powerful aid to the understandingof complex chemical reactions, but the problem remains of the determinationof the active species involved.It is often desirable to investigate the intermediates without appreciably disturbingthe reaction system. Methods such as optical spectroscopy, or, to a lesserdegree, mass spectrometry, which affect only a small fraction of the total numberof species present, satisfy this requirement. On the other hand, the chemicalmethods of Section 5 rely directly on the participation of intermediates in processesnot concerned with the original reaction. In general, then, allowance must be madefor the perturbation caused by the foreign substance, or the experiment designedso that the results are applicable to the system free from additive.1.1 STABLE INTERMEDIATESA number of reactions proceed via intermediates which would be recognised asstable species outside the reaction system. Typical of these are the aldehydes andperoxides formed in the combustion of hydrocarbons. If during the course of the