Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 ANALYSIS 99by measuring the heat liberated at a coated thermometer bulb, a glowing wire or athermocouple junction327. More fundamentally, Elias et al. 328 have used the methodof isothermal calorimetry, to measure the concentrations of several atomicspecies. The apparatus is shown in Fig. 68. It consists of a conventional flow systemwhich incorporates a discharge tube for the production of atoms. Two resistancewire detectors D, and D,, are used, the former being moveable. The platinumresistance wires of the detectors, are electroplated with several metals whichare, or their oxides, capable of catalysing the recombination of the atcas. Thedetector forms one arm of a Wheatstone bridge. The surface of the RV is coatedwith phosphoric acid or boric oxide, which are very efficient poisons for surfaceatom recombination. The thermal conductivity of the system must be the samein the presence and absence of atoms, while combination at the detector must becomplete. Also, no other species capable of heating the detector must be present.A known current i, is passed through the wire which has a measured resistanceR. To counteract the heat liberated by atom combination, the current is reducedto i, such that the resistance of the wire again has the value of R. The differencein energy W required to keep the wire isothermal is given by equation (P’)AW = R(ii-i2)( P‘)and the rate of flow of atoms F = A WIAH, where AH is the heat of atomic combination.Callear and R ~bb~’~ measured the extent of heating caused by Hg( 3P1) atoms. The heating effect was modified according to the nature of any quench-Fig. 69. Diaphragm manometer for measuring very small temperature changes. From ref. 330.References pp. I04411