Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

14 EXPERIMENTAL METHODS FOR SLOW REACTIONSOf the two gauges the spiral is the more robust, but is less sensitive. The sensitivitymay be improved by etching the spiral in 10 HF solution, which does not drasticallyaffect the robust nature of the gauge56. This gauge may be mounted horizontallyor vertically. If the latter, any vibrations may be eliminated by attaching afloat to the spindle and immersing this in silicone oil. All three of the gauges arediscussed fully in refs. 22c and 56b. All-metal diaphragm gauges have also beencon~tructed~~ (Fig. 8). Once again, a movement of the diaphragm, correspondingto a change in pressure, is detected by capacitance measurement. An alternativeto this gauge is a pressure transducer of which there are several commercial versions.In one set-up a pressure difference across the two sides of the transducerproduces an off-balance on a Wheatstone bridge network consisting of two straingauge windings and two temperature-compensating windings. The voltage outputmay be fed to a pen-re~order’~. Should corrosive gases be used, the resistancespirals of the transducer may be protected with silicone oils8 or diaphragms. Thelatter usually introduce vacuum problems and a better solution is to seal the spiralsin glass or to coat with teflon or a silicone in the manner described by Eggerand Ben~on~~.Provided that there is a change in the number of moles upon reaction and thestoichiometry of the process is known, pressure measurements may be used todetermine the order of the reaction according to equation (A). Thus Letort3 foundthat the order for the decomposition of AcH was 3 with respect to initial concentrationand 2 with respect to time (see p. 2). Such direct conclusions cannot usuallybe drawn from pressure measurements with oxidation reactions. However, directinformation may be obtained from a very neat differential system devised by Dugleuxand Frehling” (Fig. 9). V1 and V, are two RV’s, of different size connectedto the inside and outside of the Bourdon gauge J. R, allows simultaneous introductionof mixtures into V, and V2. Any fluctuation in temperature of the furnaceis thus compensated for. Rapid reactions and the direct effect of promoters andinhibitors on an oxidation may be studied. This apparatus may well be usefulwith other systems.L-I ‘I- -ElectrodeT--28 __c4 2A ----DiaphragmFig. 8. All-metal diaphragm gauge. From ref. 22c.