Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

134 EXPERIMENTAL METHODS FOR FAST REACTIONSby suddenly changing the external pressure4' on systems where the net reaction isassociated with a finite volume change; the change in conductivity is followed.Although it is not, perhaps, the most sophisticated relaxation technique available,the temperature-jump has so far proved to be the most versatile and reliable, andmany data have been collected on different types of system.3.2.1 TemperatureTjumpMost commonly the energy stored in a bank of condensers is discharged throughthe solution contained in a cell between two specially-designed electrodes (Fig. 7)41.The absorption change of the solution between the electrodes is followed with abeam of monochromatic light (passed through the solution perpendicular to theaxis of the cell) and a photomultiplier coupled to a suitably-triggered oscilloscope.A typical oscilloscope trace, which shows the relaxation behaviour directly, isshown in Fig. 8. The relaxation time may be derived from a semilogarithmic plotor comparison with standard exponential curves. One point about this apparatusis that the reactant solution must conduct sufficiently well to carry the current efficientlyand so maintain a short heating time, typically, a few microseconds. Thisdoes not necessarily mean that only ionic reactions may be studied. Often an inertelectrolyte, such as 0.1 M KNO,, is added to carry the current. A constraint whichis generally not very serious is that the change in optical absorption must be smallcompared with the total absorption-for two reasons. In the first place, the conditionfor linearizing the rate-expression is that the relative concentration change beAnalyzing -I ightFig. 7. Temperature-jump cell.