Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

328 DETECTION AND ESTIMATION OF INTERMEDIATESstricted, and the problem is the more acute the heavier the atom. Nevertheless,useful results have been obtained for oxygen (e.g. ref. 261), nitrogen270 and evenchlorine270” atoms.A number of workers have attempted to use multiple orifices to reduce equilibrationtime, and Beckey and Warne~k~~O~ use fritted discs, although it is possiblethat errors will result from the multiple surface reflections of atonis within thelong tubes which comprise the disc. To overcome such objections, Groth andWarne~k~~’ have developed orifices in the form of long, narrow slots, and gaugesusing this system appear to be relatively fast and accurate. Greaves and Linnett261give a useful account of the possible sources of error in effusion gauge determinationsof atom concentrations.7. Electrical methods for charged speciesThe detection and estimation of ions in the gas phase is carried out very convenientlyby the mass spectrometric methods described in Section 3. However,electrically charged species may be detected also by methods depending on theelectrical properties peculiar to these species. This section gives a very brief outlineof some of the techniques which have been employed in the study of charged speciesin chemical reactions. Flames offer the most readily investigated chemiionizationsystem, and most of the work reported here derives from flame studies.The conductivity of a gas containing charged species depends upon the numberof charge-carrying species and their mobility. Explicit mobility studies of thecharged species in flames have, in fact, been made271, although electrical conductivityis more usually the property measured. The information to be obtained fromconductivity measurements has been described by Wilsonz7’. Langmuir probemethods are well suited to flames since they can be designed to have quite goodspatial resolution. The technique is usually applied in a “single-probe” manner:that is, a probe is inserted into a flame and a large grid placed some distance abovethe probe. Electron currents to a positive probe or positive ion currents to anegative probe may then be measured. Calcote273-275 has recently put the determinationof charged species by such methods on a quantitative footing, and hediscusses274 the interpretation of the probe current curves. The use of small probesis desirable for a number of reasons: (i) it is difficult to obtain saturation electroncurrents on a large probe without saturating the grid electrode, (ii) large probesimply relatively large currents which may affect the plasma under study, (iii) betterspatial resolution will be obtained with small probes. However, most electrical insdatorsbecome either semiconductors or thermal emitters at the elevated temperaturesof flames, so that there is some difficulty in building small probes.Cal~ote”~ has overcome the difficulty by using water cooled insulators for hisprobes, and the use of boron nitride insulators, which have good high-temperature