Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

318 DETECTION AND ESTIMATION OF INTERMEDIATESThe use of other isotopically labelled compounds may be of great value in thestudy of unstable intermediates; in all cases, however, due allowance should bemade for the differing chemical reactivities of the isotopically substituted compounds.Flame studies may also make use of a number of straightforward chemical reactionswhich would proceed too slowly at lower temperatures. Thus Fenimore andJones’ 30 were able to measure atomic oxygen concentrations in hydrogen andhydrogen-carbon monoxide flames from the rate of the reactionO+NzO -+ 2N0 (27)Nitric oxide concentrations at given points within the flame were measured massspectrometrically with known small concentrations of nitrous oxide injected intothe reaction mixture. Hence the atomic oxygen concentration could be determinedusing the rate coefficient for (27), k2, = 2 x 10” exp (- 32,000/RT) 1.mole-’.sec-l. It was shown that nitric oxide itself would not decompose appreciably underthe conditions of the experiment.Sugden and his co-workers’ 31 -’ 36 h ave developed a number of special techniquesfor the estimation of intermediates in flames, with particular reference toatomic hydrogen and hydroxyl radicals. In each case the technique involves theaddition to the reaction mixture of traces of metal salts, which lead to the emissionof radiation in the flame. The basis of the first method23’ is a comparison of therelative intensities of the lithium and sodium resonance lines emitted when saltsof these metals are added in equal concentrations to the flame. Lithium hydroxideis stable at the flame temperatures, and since water is one of the combustion productsthe lithium concentration is modified by the equilibrium reactionThe corresponding reaction for sodium is negligible, so that measurement of theintensities of the resonance lines for the two metals may be combined with anestimated equilibrium constant for (28) to give the atomic hydrogen concentration.The method is satisfactory in that only trace amounts of the alkali metal salt needbe added, and no appreciable perturbation of the reaction system occurs. Valuesfor the atomic hydrogen concentrations obtained by the method described werecompared with those calculated when chlorine was added to a flame already containingtraces of sodium’ 3’. An equilibrium is again established as a result of thereactionsNa + HCl + NaCl + H (29)and the intensity of the atomic sodium emission is diminished. The concentration