Some observations of the NO + ion and neutral no in atmospheric pressure flames of acetylene and hydrogen

Abstract

Mass spectrometric measurements of the positive and negative ions in flames of either C 2 H 2 or H 2 burning in O 2 with N 2 as diluent have shown that the reaction NO + H 3 O + ⇌ N O + + H 2 O + H (I) is balanced fairly quickly in the burnt gases. In fact, only in the coolest C 2 H 2 flames (T<2400 K) is an approach of (I) to equilibrium observable. This has enabled the rate constant of its forward step to be estimated. The indications are that the reaction proceeds according to the two-step scheme: NO + H 3 O + → N O + ⋅ H 2 O + H N O + ⋅ H 2 O + = N O + + H 2 O with the first reaction being rate-determining. Reaction, (I) is apparently always at equilibrium in H 2 flames with NO added to them. In this case, measurements of [NO + ]/[H 3 O + ] confirm that H atoms are disappearing from the burnt gases by a second order process, which is catalysed by NO. In hot (T≥2400 K) H 2 /O 2 /N 2 flames, the relaxation time of (I) becomes small enough (≤5 μs) for the reaction to proceed somewhat as the flame is sampled into the mass spectrometer. Otherwise, the reaction is not affected by sampling. The fact that reaction (I) is balanced has enabled measurements of [NO] to be made in C 2 H 2 /O 2 /N 2 flames from the known equilibrium constant, [H], [H 2 O] and the measured [NO + ]/[H 3 O + ]. In addition [NO] was arrived at directly in the cooler C 2 H 2 flames using a chemiluminescent analyser. The observations indicate a rate coefficient of 2.6×10 −10 exp(−38,000/T) ml molecule −1 s −1 for O + N 2 → N O + N , which is the rate-determining step in the Zel'dovich mechanism. In addition, it is clear that NO is produced in these C 2 H 2 flames by the Fenimore ‘prompt’ process and in fact the resulting amounts of NO can be up to roughly 300 p.p.m. On occasions the appearance of NO by this mechanism is not prompt, but is prolonged for some time (by up to about 1 ms) into the burnt gases. Also, the amount of Fenimore NO varies with both temperature and flame composition.