Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

116 EXPERIMENTAL METHODS FOR FAST REACTIONSconditions are obeyed will have different current-voltage characteristics from thereduction process of the same species in the absence of a reaction. The way in whichthese characteristics change may be used to deduce the relevant rate coefficients.EIectrochemical methods are considered in Section 6.One of the first methods for following fast chemical reactions was developed inthe early 1920’s to try to settle what was then a lively point of controversy: whetherthe human lung could pump oxygen actively into the blood from the air. Before thiscould be settled, the answer had to be found to another question: how fast does oxygencombine with haemoglobin? The experiment designed to measure this rate wasbased on thejlow method, and the events leading up to its introduction have beendescribed in a fascinating way by one of the pioneers, F. J. W. Roughton6. The useof the singular-flow method- is not, perhaps, quite valid since this technique isusually considered under three headings: continuous-flow, accelerated-flow andstopped-flow. However, all three share the same basic principles. Like so many ofthe physical methods which have been applied successfully to the study of chemicalkinetics, the flow method is beautifully simple in concept. The two reactants areallowed to flow down separate tubes leading into a mixing chamber and the chemicalbehaviour of the emerging mixture is followed. The main difference betweenthe three subdivisions concerns the nature of the time-dependence of the flow pattern;connected with this is the important question of how the reaction is monitored.Many experimental and theoretical problems. such as the design of the most efficientmixing device, are common to all three types of flow technique. Section 7discusses flow methods; it is divided into two parts. The first part deals with reactionsin solution and the second with reactions in the gas phase.Conceptually rather similar to flow systems is theflame. A flame has been definedas a combustion reaction which can propagate through space. In a controlled flame,as opposed to an explosion, this space-propagation occurs subsonically. Althoughflames have been known, of course, for many centuries, it is only in the last one ortwo decades, with the significant improvements in apparatus design, that systematicinvestigations have been undertaken of the kinetics of chemical processes involvedin combustion. The self-propagation requirement implies that chain reactions, andin turn free radical reactions, are important in flames. Generally, radical reactionshave comparatively low activation energies and so it would be predicted that moststeps in flame reactions are fast. In fact the rates covered lie in the range betweenthose studied by conventional static systems and those studied by flash photolysisor in the shock tube. Besides the possibility of working at high temperatures andunder conditions at which high concentrations of free radicals can be obtained, theuse of flames has the added advantage that wall effects are absent. Flames may beconsidered in two categories. One type may be generated in a system in which twocomponents mix in the combustion zone. This is a “diffusion” flame since the ratesof reaction are governed primarily by the speed at which mixing occurs and henceby the diffusion rates. In Section 8 discussion is limited to the other main category-