Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

88 EXPERIMENTAL METHODS FOR SLOW REACTIONSQuantitative estimation of a mixture may be obtained from calibrations, usingthe pure compounds that comprise the mixture. This may be carried out by peakheight or often more accurately, peak area measurements, determined by weighing,triangulation, planimetry or preferably, automatic integration. Several workershave constructed their own integrators but there are several reasonably pricedcommercial integrators available. Batt and Cr~ickshank’~~ have drawn attentionto the optimum conditions required for quantitative, reproducible calibrationusing a FID. Using these methods for calibra.tions, quantitative estimation of thereaction mixture may be made if all the products are eluted and the total numberof moles injected, either in the sample or from a PVT measurement of the contentsof the RV, is known.A method which circumvents the problem of total elution is to have an absolutemeasure of an injected standard or of one product and use this for the quantitativeestimation of the other products and undecomposed reactants. For instance,in the HCl-catalysed decomposition of di-terr.-butyl peroxide’ 3, involving thereactionsr-BuO- -+ Me. + Me,COfollowed by Me. + HCl -+ CH, + C1.or 2Me: -, C,H,the stoichiometry requires the equivalence of the total methyl and acetone amounts.A measure of CH, and C,H, gives an absolute value for Me,CO from which theother products may be estimated. This is particularly useful when the inevitableunknown compounds are present, and avoids the assumption that all products areeluted. For competitive reactions, which happens here to some extent, this problemvirtually disappears.4.4 SPECTROSCOPIC METHODS4.4.1 IntroductionOne may divide the use of spectroscopic methods into the three categories ofproduct identification, quantitative estimation of reactants or products at the endof a run, and in siru measurement of the concentrations of reactants or productsduring a reaction. Of these the last is the most important, because it presents anopportunity to follow the production and disappearance of transient species as wellas those already mentioned. This is particularly true for very fast reaction techniquessuch as flash photolysis where the concentrations of the very reactive intermediatesare likely to be high.Of the different regions of the spectrum involved, that concerning nuclear mag-