MASS-SPECTROMETRIC STUDY OF THE PRODUCTION AND RECOMBINATION OF BAOH-ELECTRONS FROM BARIUM ADDED TO FLAMES OF H-2+O-2+AR( IONS AND FREE)

Barium has been added in trace amounts to a variety of atmospheric-pre ssure flames of H-2 + O-2 + Ar. These are in effect tubular reactors w ithout walls and cover the temperature range 1850-2460 K. The ions pre sent were studied by continuously sampling a flame into a quadrupole m ass 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 detecte d. Chemi-ionisation via (14) BaO + H reversible arrow BaOH+ + e(-) and (15) Ba + OH reversible arrow BaOH+ + e(-) produces and removes ions and free electrons. This mechanism is accompanied by the rapid equilib rium (12) BaOH+ + H = Ba+ + H2O as well as a modest hydration of BaOH. Chemi-ionisation often initially produces more ions than required fo r thermodynamic equilibrium. Nevertheless, measurements of ionic conce ntrations for equilibrium were made and indicated that the ionisation potential of BaOH is 4.62 +/- 0.30 eV. It is found also that tiny addi tions of barium to the hottest flame are totally ionised; this fact en ables ion currents, measured with this technique, to be converted into ionic concentrations. The overall recombination coefficient for the s um of the reverse steps of reactions (14) and (15) was found to be (1. 4 +/- 0.3) x 10(-7) mi ions(-1) s(-1) at 2200 K and varied with temper ature 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 effectiv e forward and reverse rate constants for (14). Thus it is concluded th at reactions (14) and (15) are the major ones producing and removing i ons. The addition of potassium or a hydrocarbon, such as CH4, to a fla me usually only interacts with the above ionisation scheme for Ba in s o far as more free electrons are thereby produced. Thus any effects of proton transfer in e.g. H3O+ + BaO --> BaOH+ + H2O are normally negli gible, even though the rate constant is close to 10(-9) mi molecule(-1 ) s(-1). Likewise, no contribution from BaOH+ + K --> K+ + BaOH is obs erved. That reaction (12) is rapid and equilibrated enables determinat ions of [Ba+]/[BaOH+] to be used as measurements of the concentration of free hydrogen atoms in a flame. It also proved possible to assess t he perturbations of an ion spectrum due to a sample of the flame adjus ting its composition on being cooled when entering the first vacuum ch amber of the mass spectrometer; a sample is cooled by ca. 500-600 K as a result of losing heat to the sampling probe.