USE OF A STOPPED-FLOW TECHNIQUE TO MEASURE THE RATE CONSTANTS AT ROOM-TEMPERATURE FOR REACTIONS BETWEEN THE NITRATE RADICAL AND VARIOUS ORGANIC-SPECIES

A stopped-flow apparatus, in which NO3 was detected by optical absorption at lambda = 662 nm, has been used to measure overall rate constants at room temperature for reaction of NO3 in systems involving ethene, simple alkanes and chlorinated methanes. Modelling of the reaction with ethene led to a rate constant for the primary step of (1.7 +/- 0.5) x 10(-16) cm3 molecule-1 s-1. However, for H-atom abstraction by NO3 from the saturated organic species, the extensive and largely unquantified secondary chemistry occurring over reaction times of 5-20 s meant that only upper limits for the primary rate constants could be accurately assessed (the stoicheiometric factor being assumed to be two or more). The values thus obtained at room temperature were (in units of 10(-17) cm3 molecule-1 s-1) 2.7 +/- 0.2, 4.8 +/- 1.7, 60 +/- 10, 0.85 +/- 0.25, 0.48 +/- 0.10 and 6.0 +/- 0.5 for ethane, propane, isobutane (2-methylpropane), acetone, dichloromethane and chloroform. For the reactions of NO3 with ethane and propane, modelling of the kinetics led to estimates of lower limits of the primary rate constants of (1.1 +/- 0.2) and (2.2 +/- 0.2) x 10(-17) cm3 molecule-1 s-1. No reaction was observed between NO3 and methane or chloromethane, suggesting upper limits (based on the noise levels) for the overall rate constants of these reactions of 8 x 10(-19) and 1 x 10(-18) cm3 molecule-1 s-1.