Mass-spectrometric study of the production and recombination of BaOH+ ions and free electrons from barium added to flames of H2+ O2+ Ar

Abstract

Barium has been added in trace amounts to a variety of atmospheric-pressure flames of H2+ O2+ Ar. These are in effect tubular reactors without walls and cover the temperature range 1850–2460 K. The ions present were studied by continuously sampling a flame into a quadrupole mass spectrometer. Barium formed no negative ions and the free electron was always the dominant negatively charged species. By far the major positive ion was BaOH+, although Ba+ and BaOH+·H2O were also detected. Chemi-ionisation via BaO + H ⇄ BaOH++ e–(14) and Ba + OH ⇄ BaOH++ e–(15) produces and removes ions and free electrons. This mechanism is accompanied by the rapid equilibrium BaOH++ H = Ba++ H2O (12) as well as a modest hydration of BaOH+. Chemi-ionisation often initially produces more ions than required for thermodynamic equilibrium. Nevertheless, measurements of ionic concentrations for equilibrium were made and indicated that the ionisation potential of BaOH is 4.62 ± 0.30 eV. It is found also that tiny additions of barium to the hottest flame are totally ionised; this fact enables ion currents, measured with this technique, to be converted into ionic concentrations. The overall recombination coefficient for the sum of the reverse steps of reactions (14) and (15) was found to be (1.4 ± 0.3)× 10–7 ml ions–1 s–1 at 2200 K and varied with temperature as T–0.5 ± 1.0. The rates of ion production were also measured in these flames. Reactions (14) and (15) appear indistinguishable, but detailed balancing holds, i.e. the observed equilibrium constant of e.g. reaction (14) is found to equal the ratio of the observed effective forward and reverse rate constants for (14). Thus it is concluded that reactions (14) and (15) are the major ones producing and removing ions. The addition of potassium or a hydrocarbon, such as CH4, to a flame usually only interacts with the above ionisation scheme for Ba in so far as more free electrons are thereby produced. Thus any effects of proton transfer in e.g. H3O++ BaO → BaOH++ H2O are normally negligible, even though the rate constant is close to 10–9 ml molecule–1 s–1. Likewise, no contribution from BaOH++ K → K++ BaOH is observed. That reaction (12) is rapid and equilibrated enables determinations of [Ba+]/[BaOH+] to be used as measurements of the concentration of free hydrogen atoms in a flame. It also proved possible to assess the perturbations of an ion spectrum due to a sample of the flame adjusting its composition on being cooled when entering the first vacuum chamber of the mass spectrometer; a sample is cooled by ca. 500–600 K as a result of losing heat to the sampling probe.