Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

6 MISCELLANEOUS PHYSICAL TECHNIQUES 327molecular oxygen. Arnold et ~ 1 describe . ~ a similar ~ kinetic ~ study using a nickelcoated calorimetric detector.Vibrationally excited species also may give up their energy to a suitable catalyticprobe. Morgan ei al.' report calorimetric studies of vibrationally excited molecularnitrogen, and compare the results obtained with those from mass spectrometricinvestigations.6.4 WREDE-HARTECK GAUGESWrede267 and HarteckZ6' first applied the property of effusion to the measurementof atom concentrations. A catalyst, effective for the atom recombinationreaction, is placed within an enclosed volume which is connected to the reactionsystem by one or more small orifices. Molecules and atoms effuse into the enclosedvolume, while only molecules effuse out. Thus an equilibrium is set up, and massbalance requires a pressure difference, Ap, between the two sides of the orifice,given byAp=ap 1--( 9wherep is the total gas pressure and 01 the fraction of atomic species. Hence, if thechemical identity of the atomic species is known, measurement of Ap and p givesits concentration. Neither foreign reactant gases nor excited species affect the valueof Ap, so that the method commends itself for use in circumstances where suchspecies may be present. There are, however, certain limitations to the empIoymentof the technique. First, the technique cannot distinguish between different atomicspecies. Secondly, the accuracy and sensitivity limit depends on the sensitivitywith which Ap is measured. Groth and Warne~k~~' have described recently theuse of a diaphragm manometer capable of measuring a pressure difference oftorr; at p = 1 torr, this corresponds to an atom concentration of 0.03 %.Greaves and Linnett261 use a special Pirani gauge which enables them to measureatom concentrations to kO.1 % at p = 0.1 torr. A third factor which is of h-portance is the rate at which equilibration occurs between the enclosed volume andreaction system. In order to minimise mass transfer through the orifice (whichwould lead to erroneous, low, values of Ap), the size of the orifice must be keptsmall compared with the mean free path. Thus the equilibration rate may beslow, and it is essential to use as small an enclosed volume as possible. It has beenshown261 that with a volume as small as 3 cm3, the time required for 99 % equilibrationwith a single orifice of diameter equal to one-tenth of the mean free pathis 4 sec in oxygen at p = 0.1 torr, but more than 7 min at p = 1 torr. The pressurerange within which concentration measurements may be made is therefore re-References pp. 336-342