Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

7 FLOW METHODS 163flow or stopped-flow. The order in which they are listed here is the order in whichthey were developed, which was to some extent fortuitous and depended partlyon the availability of specialized apparatus such as fast-responding photocells andon the specific reactions which were first investigated. Although the techniques arebeing used to a greater extent to follow inorganic reactions, for whichin many casesthey are ideally suited, it is rather interesting that most of the earlier work was doneon biological systems. This is especially so in view of the complications often associatedwith the latter, such as extreme sensitivity to atmospheric oxygen, or severescattering of the analyzing light by species of large molecular weight. In general,the technique which is most useful is the stopped-flow, and it is the one used unlessa feature of the reaction makes one of the others preferable. One aspect of flowmethods which should be emphasized, however, is their versatility. The design of aparticular apparatus will depend very largely on the type of reaction to be followed.For stopped-flow, the reagents are kept in two hypodermic syringes of equalvolume, typically between 2 and 50 ml, and the plungers are pushed together soas to deliver equal amounts of solution to the mixer. The syringes are mountedon a metal block and a bar, running along parallel guide-rods and pushed eithermanually or mechanically, ensures even delivery of solution. The observation positionis ked, and the distance between it and the mixer outlet is often as short asis practicable (usually a few cm). The reactant liquids are made to flow and arethen abruptly stopped; the change in concentration in the small column of mixedliquid at the observation point P is followed with time. By the time the mixed liquidreaches P a certain fraction of the reaction, which depends on the half-life, on u,and on x, will be over and the lower limit on the half-lives of reactions which maybe followed thus depends on u and x. If the arrest of the liquid flow occurred instantaneously,the half-life of the fastest observable reaction would be about x/u sec.Of course, instantaneous arrest is not possible and the usefulness of stopping theflow depends largely on how fast it can be done. The mixed solution passes into athird syringe arranged9’ so that the plunger is forced against a block. If the mainsyringes are motor-driven, the motor must evidently be disconnected immediatelywhen the liquid flow ceases. The timing is critical, but once the apparatus is setup it is very easy to follow a eoniplete reaction with only a fraction of a millilitre ofeach reagent.In the continuous-flow technique the reaction profile is followed by keeping theflow rate constant and varying the distance between M and P. Thus, a measuredtime parameter is replaced by a measured distance, x. (In an alternative procedure,x can be kept constant and the flow rate v varied stepwise.) The reagents may becontained in large reservoirs and forced into the mixing chamber under a constanthead; the observation position can be changed at leisure. It is especially importanthere to maintain a constant flow rate and also to ensure that the bore of the observationtube is uniform. The continuowflow method is especially useful when anobservation technique with a particularly long response time is being used. Thus,References pp. 176-1 79