Ionisation in premixed fuel-lean flames of H2, O2 and N2. Part 1.—Naturally occurring positive ions

Abstract

Mass spectrometric measurements have been made of the axial concentration profiles of the positive ions in six fuel-lean flames of H₂, O₂ and N₂, 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 ⇌ 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, K₁, is measured to be (2.16 ± 0.80)× 10^–11× exp(–11 591/T) atm^–1. The forward step of reaction (1) determines the initial rate of formation of H₃O⁺, and has a rate coefficient, k₁, of (3.3 ± 1.6)× 10^–36 exp(–13 800/T) cm⁶ 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 k₁ and k₁ yield a rate constant for the reverse reaction (–1), of 1.9 ± 2.3 × 10^–7 cm³ molecule^–1 s^–1, approximately independent of temperature. The proton affinity of water in the gas phase H₂O + H⁺→ H₃O⁺ has also been derived to be 674 ± 14 kJ mol^–1, at 0 K.