IONIZATION IN PREMIXED FUEL-LEAN FLAMES OF H-2, O-2 AND N-2 .1. NATURALLY-OCCURRING POSITIVE-IONS

Mass spectrometric measurements have been made of the axial concentration profiles of the positive ions in six fuel-lean flames of H-2, O-2 and N-2, incorporating no additives. The principal positively charged species is the hydronium ion, H3O+, whilst the sole negative species is the free electron, e(-). The reaction H + H + OH reversible arrow H3O+ + e(-) (1) is found to be responsible for establishing H3O+ and e(-) concentrations close to their values for equilibrium well downstream in the burnt gases. The equilibrium constant, K-1,, is measured to be (2.16 +/- 0.80) x 10(-11) x exp(-11591/T) atm(-1). The forward step of reaction (1) determines the initial rate of formation of H3O+, and has a rate coefficient, k(1), of (3.3 +/- 1.6) x 10(-36) exp(-13 800/T) cm(6) molecule(-2) s(-1). The activation energy for the forward step of reaction (1) is equal to the endothermicity of reaction (1), namely 97 +/- 10 kJ mol(-1). These expressions for K-1 and k(1) yield a rate constant for the reverse reaction (-1), of 1.9 +/- 2.3 x 10(-7) cm(3) molecule(-1) s(-1), approximately independent of temperature. The proton affinity of water in the gas phase has also been derived to be 674 +/- 14 kJ mol(-1), at 0 K.