Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

Reaction Furnace CircuitI10 EXPERIMENTAL METHODS FOR SLOW REACTIONS' RV Furnace r-3Fig. 6. Circuit for heating and controlling the temperature of a furnace.possible to control at 200" C to 5 0.2" and at 600" C to & 2". Variations of thistype of control are described in ref. 22c.Temperature measurement is almost invariably made using thermocouplesz2".The latter must be constructed from fine wire30 and have a fast response3' suchthat even a very small temperature change may be measured precisely3'.3.2 THERMAL SYSTEMS3.2.1 Static method(a) Reaction vesselsReaction vessels are usually spherically or cylindrically shaped, vary in size from200-1000 cm3, and have a thermocouple well in the centre. The tip of this well isthin-walled and sometimes a drop of fluid, such as silicone oil, is used to providegood thermal contact. The material most often used is Pyrex glass (maximumtemperature 600" C) and fused silica (maximum temperature 1200" C). At roomtemperature the two materials are practically impermeable to all gases except helium.At higher temperatures hydrogen diffuses through the glasses and particularlythrough silica3 '. At still higher temperatures oxygen and nitrogen maydiff me through silica34. The mechanism appears to involve adsorption at the glasssurface followed by passage through the glass. Silica reaction vessels should notbe heated to high temperatures when in contact with metals. Iron or nickel, for instance,will diffuse into the quartz and give rise to heterogeneous reactions. Manygases are strongly adsorbed on glass surfaces and hence all glass apparatus, particularlythe RV, should be well degassed at elevated temperatures. Both silica andPyrex glass are resistant to most gases at room temperature except HF. However,HCI, HBr and probably HI are adsorbed on the surface and at high temperaturesreact with the glass35. Adsorption occursvia dissolution in a thin layer of adsorbed