Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 CHEMICAL RELAXATION METHODS 133The relaxation time associated with path @ -+@ -+ 0 is given simply by-1 k32. k21 * cMg + k12 * k23 * cHZ3,Z.l =k21 + k23 k2l +k23(Again, since cH and cMg are buffered, only terms in these species will enter therelaxation expression.)This system yields experimental values for k,,, k,, and k32.k21/(k21+k23). It isinteresting that, by changing cMg and cH, it is possible to find conditions where,in turn, each of the first three terms in equation (4) is the dominant one.3.2 STEP-FUNCTION, OR TRANSIENT METHODSThe main experimental problems are twofold: how to displace the equilibrium,e.g.AzB+C (1)and how to measure the accompanying changes of concentration of one of thespecies. The second of these is comparatively easy to solve. Concentration changesare generally measured optically, although sometimes other physical properties suchas conductivity may be used, and suitable electronic equipment is generally availablefor measuring in the required time range. One of the more serious problemsassociated with measurement at very high rates is how to obtain a sufficiently largesignal/noise ratio39. Where the signal/noise ratio is dangerously low such refinementsas “multiple jumps” in conjunction with computers of average transients(CATS) are being used to increase the resolution. If the reaction under investigationis not associated with a measurable spectral change it is often possible to coupleit with an indicator system (cf. below).The problem of how to displace the equilibrium is more difficult to solve, andmuch ingenuity has gone into finding methods of doing this. Suppose reaction (1)is associated with a finite enthalpy change. At a given temperature and total reactantconcentration, the position of the equilibrium will be uniquely defined. If thetemperature is now raised by, say, lo”, a new set of equilibrium conditions is defined.Thus if the change in temperature can be made rapidly enough the relaxationbehaviour of the system can be followed by monitoring the concentration ofthe optically-absorbing species (or indicator). A similar effect is found, for example,References pp. 176-179