Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

246 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSWeight changes, measured with a microbalance, are a convenient way offollowing the kinetics of oxidation. The use of this technique is discussed in Section2.1.4. Many metals have been s t ~ d i e d and ~ differing ~ ~ ~ behaviour ~ ~ ~ is - often ~ ~ ~observed in differing temperature ranges. Very high temperatures are required forthe more noble metals, which demand the construction of special furnaces and1 3 - 1 15.286Condit and HoltZ9' have reviewed the use of radioactive tracers in the studyof oxidation. Radioactive platinum has been used to studythe oxidation of cobaltzg1and radioactive silver to study the oxidation of molybdenumzg2. If the tracerelement remains at the surface it indicates that the oxygen is diffusing into themetal, and if it is located some distance from the surface shows diffusion of themetal through the oxide. The tracer can be detected by autoradiography and itsdistribution studied by gradual removal of the oxide layer. Its distance from thesurface can also be estimated by measurement of the energy of the emitted radiationat some point outside the surfacez9 39 z94.An important characteristic of radioactive decay is that the momentum of theemitted particle must be balanced by the momentum of the product nucleus. Insuitable cases it should be possible to detect the recoil nucleus. Thorium-228,which decays to radium-224 having an energy of 97 keV, has been used to study theoxidation of a number of metalszg0. Its advantages are its low volatility (asthorium oxide) and its relatively low rate of diffusion in lighter metals. Themaximum range of recoils in solids is of the order of 300-500 A and for thinneroxide layers, its distance from the surface can be measured. One difficulty withquantitative work is that the radium undergoes a sequence of further decay, whichcomplicates calculation of recoil ranges, and calibration may be necessary.These reactions are mainly of the typeA(so1id) + B(sol-id) + C(gas)but also a few of the typeA(solid) --f B(gas) + C(gas)2.3.2 Dissociation reactionsThe extent of reaction versus time can usually be represented by a sigmoidal curve.There is an induction period when the initial nuclei of phase B are being formed,followed by an accelerating rate which eventually decays due to the depletion ofphase A.A survey of the kinetics of dissociation reactions has been made by Garnerzg5.