Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

326 DETECTION AND ESTIMATION OF INTERMEDIATESInlet jet forrpact an tCalorimeter leadsto bridges IFromdischarge'AalorirnetercoilsTo pumpsFig. 12. Double isothermal wire calorimeter probe for low pressure flow system.Atomic oxygen concentrations may be determined by the calorimetric method ifthe surface of the catalytic probe is chosen suitably. Linnett et a1.259*260 havestudied the recombination of oxygen atoms under various conditions using a thermocouplecoated with silver. In the presence of atomic oxygen, the surface of thesilver is oxidised, and the oxide layer appears to be a particularly efficient catalystfor the 0-atom recombination. The calorimetric determinations were shownz6' tobe related directly to atom concentrations measured with the Wrede-Harteckgauge to be described in Section 6.4.Absolute atomic oxygen concentrations are best measured with the isothermalresistance probe. Elias et aLZ6' describe investigations of atomic oxygen reactionsusing a silver coated platinum wire coil.Atomic oxygen and hydrogen have been the species most widely studied by thecalorimetric technique, although relative concentrations of a few other atomicspecies have also been measured. For example, Schwab used the heating of a thermocoupleto measure relative chlorinez6 and bromine264 atom concentrations.However, one of the most interesting developments of the method has been discoverythat excited, energy-rich, species may be estimated calorimetrically. Eliaset af.26z reported that the apparent concentration of atomic oxygen measuredwith the catalytic probe was about 10 % higher than that measured with a Wrede-Harteck gauge (Section 6.4) and 25 % higher than the value obtained by NOz titration(Section 5). The effect was ascribed to appreciable concentrations of metastable02('A,) present in the products leaving the electric discharge, and, indeed,some heat is released even after the atomic oxygen has all been removed by reactionwith ethylene. So far as measurements of atom concentrations are concerned, it isdesirable to find a surface which is an efficient catalyst for the 0-atom recombination,but which does not deactivate the metastable molecules. A fresh silver oxidesurface has this property, although in use it becomes progressively more sensitiveto excited molecular oxygen. On the other hand, where it is themetastable specieswhich are of direct interest, an oxidised cobalt or nickel surface is found to beespecially suitable. Atomic oxygen may be removed from the products of theelectric discharge by passing the effluent gases over a mercury mirror. Bader andOgryzl~~~~ studied calorimetrically the reactions of both 'A, and 'Z: metastable