Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

320 DETECTION AND ESTIMATION OF INTERMEDIATESapplicable to low pressure flow systems, and have been used with great success overthe past decade. The reactions areO+NOz + Oz+NOH+NOz -+ OH+NON+NO + N,+OThe rate coefficients at room temperature arek,, = 1.5 x lo9 1 mole-'.sec-'k36 = 2.9 x 10" 1 . mole-'.sec-'k,, = 1.3 x 10" 1 . mole-'.sec-'(ref. 109)(ref. 7)(ref. 109)The reactions are so rapid under normal conditions of concentration that, with theflow velocities used in these experiments, they may be regarded as proceeding tocompletion. Thus estimation of one of the products or reactants leads to a valuefor the concentration of the atomic species. Phillips and Schiff log employed a massspectrometric technique to this end (see Section 3 for the details of the massspectrometer-flow tube apparatus). To measure atomic nitrogen concentrations, anexcess of nitric oxide over that required for complete reaction with the atomic nitrogenwas added. The height of the mass 30 peak was then determined with thedischarge on and OR the difference in peak heights represents the amount of nitricoxide consumed in (37), and thus the atomic nitrogen concentration. Atomic oxygenconcentrations were determined by similar observations on the mass 46 peakwhen nitrogen dioxide was added to the reaction system.Visual or photoelectric observations of chemiluminescent emission from reactionsinvolving atoms have, in fact, been much more widely applied than massspectrometry as methods for the study of the titration reactions (35), (36) and (37).Strutt (Lord Rayleigh)' 37 observed in 1910 a chemiluminescent emission (the "airafterglow") in air subjected to an electric discharge. It has been shown that theemission results from the reactionsO+NO+M -, NO;+M (38)No,*+ NOz+ hv (39)and the intensity of emission is proportional to the product of the nitric oxide andatomic oxygen concentrations. Measurement of intensities of the emission when atrace quantity of nitric oxideis added therefore gives relative values for atomicoxygen concentrations. Nitric oxide is not consumed in the overall reaction sinceevery molecule of NO consumed in the radiative process (38), (39), or in the equivalentnon-radiative reaction, is regenerated in the fast step (35). Indeed, the absolutequantum yield for the chemiluminescent reaction has been determined' 38,