Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

6 MISCELLANEOUS PHYSICAL TECHNIQUES 325hydrogenzs1, and atomic chlorine in the photochemical decomposition of molecularchlorinezs2.6.3 CALORIMETRYBonhoeffer” fist realised the possibility of detecting hydrogen atoms by measurementof the heat released when they recombine on a catalytic surface. Theatoms are destroyed, so the applications of the technique are to systems where thespecies are generated continuously. All the studies to be described refer to dischargeflowexperiments; even in these cases diffusion effects caused by the recombinationprocess can lead to errors unless the flow velocity is sufficiently great”’. The heatof recombination is known accurately, so that if the heat losses may be calculatedor measured, the atom concentration can be determined. A few flow experimentshave been described in which atomic hydrogen is measured by a determination ofthe temperature increase of a greater or smaller mass; thermocouples are normallyemployed in this kind of study to measure the temperature change. However, foratomic hydrogen, the preferred method of study is by the use of an isothermal hotwire calorimeter. The method, which was developed by Tollefson and LeRoyZS4for quantitative work, uses what is effectively a resistance thermometer in a current-passingarm of a balanced bridge circuit. The recombination of atoms on thehot wire would lead to an increase in temperature, so that to keep the bridgebalanced, the electrical power dissipated in the catalytic probe must be reduced.It is apparent that the rate of flow, @, of atomic hydrogen is given bywhere R is the resistance of the probe at the constant temperature, io and i thecurrents required to maintain the temperature without and with the atoms present,AH the heat of recombination per mole of the atoms and J the mechanical equivalentof heat. It is assumed in using the above relation that all the atoms recombineon the probe. At room temperature the recombination’efficiency of atomic hydrogenon materials such as platinum is low, but increases with temperaturezs5, andcareful design of the apparatus is necessary. Larkin and T h r ~ s h describe ~ ~ ~ an * ~ ~ ~experimental arrangement designed to eliminate uncertainties in the fraction ofatoms recombining on the probe, and their apparatus is shown in Fig. 12. Two calorimeterswere used, each consistingof 50 cm of 34 swg rhodium-platinum wire.The calorimeters were mounted in tandem about 3 cm apart, and the second, downstream,calorimeter was used to determine the efficiency of the first (which wasalways > 80 %). Kinetic measurements were effected by making the pair of calorimetersmoveable up- or down-stream in the flow tube.References pp. 336-342