Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

7 FLOW METHODS 165area should be avoided and wherever possible it is advantageous to de-gas the solutions.The reaction half-life limits measurable by flow techniques are a few tenthsof a millisecond at one end and seconds or even minutes at the other. The upperlimit is often set by the diffusion of unreacted solution from the delivery tubes towardsthe observation position. Such a difficulty been has overcome94 by closinga tap between M and P after the mixture has been delivered.A few interesting modiiications to the general experimental arrangement have beenmade. Of particular interest for biological problems, where the amounts of the reactantsavailable are liable to be very small, is an apparatus of the stopped-flow typewhich uses an optical microcell as the observation chamber. In this variantg5 thevolume of each solution is as little as 15 pl. A slight modification to the continuousflowmethod, in which each experiment yields just one point on the reaction profileplot, uses quenching of the reaction followed by chemical or physical analysisover an unrestricted timeg6. Thus the reagents are mixed as normal (Fig. 21), butat P they are mixed further with a substance which quenches the main reaction,and a sample is removed for analysis. A reaction which has been studied in thisway9’ is the radiochloride exchange in acetone of organosilicon chlorides withC1-, a reaction which could be quenched by precipitating the free chloride ions assodium chloridefollowed byR,SiCl+ *C1- a R,Si*Cl+ C1-Counting the washed precipitate enabled the fraction of radiochloride to be determined.The variables in such a technique are again o and x. A system which combinesflow and relaxation (Section 3) techniques46 will be of great value for studyingthe reactions of unstable intermediates, for example in enzyme reactionswhere several fast steps may precede the rate-determining step. In a flow tube it isoften possible to observe these intermediates, and at a given distance from the mixera steady state will be set up. If this steady state is perturbed by a sudden increasein temperature a relaxation process will result, and the rate coefficients of all thesteps before the rate-determining one may in principle be deduced from the spectrumof relaxation times. One such technique incorporates a mixer in close proximityto one of the electrodes of an electricaldischarge temperature-jump apparatus and allowsthe mixture to flow towards the other electrode98. The change in optical absorptionof the solution is observed perpendicular to the direction of flow.One of the inorganic systems studied by flow methods” takes advantage of thefact that reactions with half-lives in the millisecond range may be followed but fasterReferences pp. I76179