Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

142 EXPERIMENTAL METHODS FOR FAST REACTIONSbeam and the photomultiplier signal, after amplification and rectification, is recordedby a logarithmic pen recorder. To avoid difficulties associated with the continuousillumination of a photomultiplier, it is well to have a pulsed-light signal.This can be achieved elegantly ty using a periodically pulsed excitation of the quartzcrystal.3.3.3 Ion-pair formation of metal saltsAn interesting application of ultrasonic measurements was to aqueous solutionsof certain metal salts. The sound-absorption characteristics of several 2:2 electrolytesof the type MX have been measured, and found to have two maxima (cf.Fig. 12) from which two relaxation times can be determi~ied~~. It was not obviouswith which relaxation processes these times should be associated, especially sincethe (diffusion-controlled) rate coefficients for “ion-pair’’ formation (which is knownto occur at the concentrations used, 10-2-10-’ M) give relaxation times too shortto be observed. It seemed, then, that when the two ions M2+,q and X2-,q haddiffused together to within a certain distance two other, slower, processes occurredwithin the ion pair. The faster of these two effects was more or less independentof the metal ion, whereas the slower was almost independent of the anion but dependedstrongly on the nature of the cation. The former was attributed to the lossof a water molecule from the hydration sheath of the anion, to give an ion pairin which the M2+ and X2- are separated by a single water molecule, while the latterwas attributed to the loss of this “bridging” water molecule from the coordinationshell of the metal. Such a sequence is represented in Fig. 14.4. Spectral line-broadening4.1 NUCLEAR MAGNETIC RESONANCEIn order to appreciate its application to kinetics, a brief outline of the nuclearmagnetic resonance phenomenon will be given. This treatment will be rather pic-[Mz+* OH2lOq tFig. 14. Ion-pair formation between Mg:,‘ and Xf, (cf. Fig. 12). k,*, k21: too fast to detect;k,,, k3,: the shorter relaxation time (+ pre-equilibrium k,,, kzl); k,,, k4a: the longer relaxationtime (+ pre-equilibria k,,, kz,; k,,, k32).