Kinetics of gas-phase ionisation of an alkali metal, A, by the electron and proton transfer reactions: A + H₃O⁺ → A⁺·H₂O + H; AOH + H₃O⁺ → AOH₂⁺ + H₂O in fuel-rich flames at 1800–2250 K

Abstract

Fuel-rich flames of H₂ + O₂ + N₂ have been burned with trace quantities of an alkali metal, Li, Na or K. In general, these additives exist mainly as free atoms A of the metal and also molecules of the hydroxide AOH in the gas phase. Without alkali metal, the major charged species are free electrons and H₃O⁺. With an alkali metal added, the exothermic reactions:

H₃O⁺ + A → A⁺·H₂O + H

H₃O⁺ + AOH → AOH₂⁺ + H₂O

occur. Mass spectrometric measurements of ion concentrations along these well defined flat flames, in which there is plug flow, enabled the rate constants of reactions (3) and (4) to be measured at different temperatures. Reaction (4) involves proton transfer from the ion H₃O⁺ to the polar molecule AOH; the product, protonated AOH, is mass spectrometrically indistinguishable from A⁺·H₂O and it is concluded that A⁺·H₂O and AOH₂⁺ are identical. The rate constant, k₄, of the exothermic reaction (4) is found to vary with temperature as T^{−2 ± 1}; its magnitude is largest for KOH and smallest for LiOH. The rate constant, k₃, for reaction (3), also exothermic, is larger than k₄ for the same metal. k₃ does not vary from metal to metal, but its measured temperature dependence corresponds to T^{−5 ± 3}. It is concluded that reaction (3) involves electron transfer from an alkali metal atom to the ion H₃O⁺; this explains why k₃ > k₄ for each metal at a particular temperature. Detailed expressions, describing the temperature-dependence of both k₃ and k₄, are deduced.