Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

8 FLAMES 171namics for measuring the flow velocity may be used to estimate the accelerationand hence the extent of expansion and the temperature of the gases as they passthrough the flame. Very small, inert particles are suspended in the reactant gasesand their movement is photographed under stroboscopic light. A modification ofthis method traces a temperature contour by using particles of a substance whichevaporates at a known temperature. Yet a third method for determining the temperatureuses the variation with temperature in the vibrational or rotational finestructure of the spectrum of a species present in the flame. Probably the most commonway of estimating the flow rate is to use an orifice or capillary upstream of theflame and measure the pressure above and below the restriction. The standard equationgoverning critical flow through the restriction is then appIiedio3.(io) Concentration measurement. The determination of the concentration profilesof (n - 1) species (n is defined above) completely defines the flame system chemically.It is convenient to consider the measurement of these concentrations in twocategories, depending on whether the species is table'^ or not. The stable speciesare molecular, such as H,, H,O, and CO, : species which may be kept for longperiods without readily undergoing further change. They are determined most convenientlyby using a probe. Thus a sample is removed from a known part of theflame through a small tube which is designed so that the reaction is frozen throughrapid decompression and also, often, by water cooling. The concentrations maythen be determined with a time-of-flight mass spectrometer, which can perform acomplete analysis within the millisecond time range. Because molecular speciestend not to have convenient, strong absorption bands it is usually necessary toemploy specially modified techniques, such as multiple-pass optics, if absorptionmethods are to be used for analysis.Unstable species fall into two categories: free radicals, including reactive atoms,and ions. The latter are present in such small concentrations that their measurementis not normally required. Sampling coupled with mass spectrometry may be usedfor free radicals as for molecules. Absorption and ESR spectrometry have alsofound limited application, although they are potentially very useful. The techniquewhich has been most widely used depends on the addition to the flame of an indicatorwhich reacts at a known rate or to a known extent with one of the radicalsor atoms present. The rate of this reaction is followed either by sampling or spectroscopically.The details of this type of technique depend to a large extent on thenature of the species concerned, and the methods which have been developed naturallyinvolve those radicals, such as H, OH and 0, which tend to be present inhigh concentration in flamesio4.Two methods which have been used to estimate the concentration of hydrogenatoms in hydrogen-oxygen flames diluted with nitrogen will illustrate the idea.Sugden and co-workers"* added very small amounts of lithium and sodium salts,in the form of a fine spray from a calibrated atomizer, to the reactant gases. In theflame, complete decomposition takes place and the emission spectra of Li and NaReferences pp. 176-179