Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

164 EXPERIMENTAL METHODS FOR FAST REACTIONSfor example, it can be used in conjunction with an ESR spectrometer or a commercialspectrophotometer such as a Beckman DU. The main disadvantage is the vastamount of reactant solution required.The third method, accelerated-flow, is similar to stopped-flow in many respects.It, also, has the advantage of requiring only small quantities of solution, and thereaction is followed at only one position, x. The reactants are driven from syringes,and the concentration changes at P are followed from the start of the drive untilthe maximum flow-velocity has been achieved. The motion of the syringe driverbaris also followed electronically and, by making use of the fact that the time betweenmixing and observation is inversely proportional to the flow rate, the conventionalconcentration-time reaction profile can be constructed. When it was firstintroduced, the accelerated-flow method had the advantage over the stopped-flowin that reactions with shorter half-times could be followed. Since more efficientstopping devices are now used this is barely true, and the rather more complicatedelectronic circuitry required in the former has led to its eclipse by the stopped-flowmethod. Both techniques require detecting systems with very fast response times.It has already been mentioned that the greatest single difficulty to be overcomein applying “conventional” techniques to the investigation of the kinetics of fastreactions is the problem of mixing the reagents in a time short compared with thehalf-life of the reaction. In fact, it is true to say that the crux of any fast kinetictechnique is the way in which this mixing problem is either overcome or avoided.Much thought and experimentation has gone into the design of the mixing chambersfor flow systems and although much of the work was done on continuous-flow,the same considerations apply to the other flow methods. A simple capillary T-junction mixer is quite adequate for reactions of half-life greater than about 10millisecondses, but to increase the time range below this, special designs are necessary.It is important for the motion in the outlet tube to be as near to “mass flow”as possible-a (hypothetical) condition typified by zero relative motion along theaxis of flow between liquid elements at the centre and at the periphery of the column-fromthe point of view of following the concentration changes at P since itis assumed that the concentrations are uniform across the tube. If a velocity greaterthan a certain empirically-derived critical velocity (which depends on the diameterof the tube and the viscosity and density of the liquid) is achieved, the flow willbecome “turbulent”. Associated with turbulent flow is a rotational movement whichhelps to give efficient mixing. This feature is especially important for stopped-flowworkg3. Velocities of the order of 10 metre.sec.-’ are quite common; a distance of1 cm from the mixing chamber would then correspond to a reaction time of abouta millisecond. The mixer design most commonly used in such work has at leasttwo pairs of inlets arranged semi-tangentially with respect to the central outlet. Afurther important point about mixer design is that efforts must be made to avoidcavitation (the local “boiling” of the solution generally associated with a suddendecrease in pressure). Rough edges and sudden decreases in overall cross-sectional