The ions in fuel-rich hydrogen flames with added ammonia: measurements of the proton affinity of NH3 and the enthalpy of monohydration of NH4+

Abstract

Fuel-rich flames of H₂+O₂+N₂ at 1 bar have been burnt with a flat reaction zone and sampled into a mass spectrometer. To these flames varying, but small, amounts of ammonia were added and the corresponding ion concentrations were measured along each flame. It was found that the total concentration of positive ions was increased on addition of ammonia, suggesting additional ionisation schemes were operating alongside the mechanisms of natural flame chemi-ionisation. New ions probably result from:H + H + NH₂ → NH₄⁺ + e⁻which quite likely involves electronically excited species, such as H₂*, inH₂* + NH₂ → NH₄⁺ + e⁻This production of the chemi-ion NH₄⁺ early in a flame is followed by the fast equilibria:NH₃ + H₃O⁺ ⇌ NH₄⁺ + H₂ONH₄⁺ + H₂O ⇌ NH₄⁺·H₂Oespecially downstream in the burned gas where NH₃ becomes the major NH_i species. Reaction (4) was analysed, assuming NH₄⁺ to be equilibrated with H₃O⁺. However, it is apparent that the ammonium ion is also formed by reaction (4) shifting its equilibrium position when a sample from a flame is cooled on entering the first chamber of the mass spectrometer. Nevertheless, the equilibrium constant of reaction (4) was measured in each flame. This enabled the enthalpy change of reaction (4) to be determined as ΔH°₄ = −173 ± 30 kJ mol⁻¹, together with ΔS°₄ = −5 ± 4 J mol⁻¹ K⁻¹, both at 298 K. Thus the proton affinity of NH₃ is deduced to be 864 ± 30 kJ mol⁻¹ at 298 K. Similarly, the enthalpy change for mono-hydration of NH₄⁺ in reaction (12) was measured to be −88 ± 30 kJ mol⁻¹ and ΔS°₁₂  = −92 ± 19 J mol⁻¹ K⁻¹ at 298 K. The rate constant of reaction (7) is in accord with its reverse electron–ion recombination step having a rate constant of ∼3 × 10⁻⁷ ml ions⁻¹ s⁻¹ at flame temperatures.