Use of a rotating single probe in studies of ionization of additives to premixed flames. Part 4.—The alkaline earths and uranium

Total positive ion concentrations Ni produced from Ca, Sr, Ba and U aqueous salt spray additives in fuel-rich, premixed H2+ O2+ N2 flames have been quantitatively examined. For the alkaline earths M, the present findings are consistent with the following scheme: M(OH)2+ H ⇌ MOH + H2O, MOH + H ⇌ MO + H2, MO + H ⇌ MOH++ e–, with M(OH)2 the dominant neutral species in the region of measurement. Through a second law procedure, the ionization potentials of CaOH, SrOH and BaOH are deduced to be 5.9 ± 0.1, 5.55 ± 0.1 and 5.25 ± 0.1 eV respectively. With uranium also, the evidence suggests a gaseous scheme, equivalent overall to the above, viz., UOnHm+ 3H UOn–1H+m–1+ e–+ H2+ H2O, with M probably the uranyl species UO2, i.e., H2UO4+ 3H HUO+3+ e–+ H2+ H2O, with D°0(HO—UO3H)+I.P. (HUO3)= 950 ± 40 kJ mol–1, and I.P. (HUO3)< 3.9 eV. When any of these metals are present in the flame together with an alkali metal, Ni may be much greater than predicted if the two additives behaved independently. This effect is consistent with a catalysis of the alkali metal ionization rate by the second additive; the results reveal the possibility of a heterogeneous as well as a homogeneous route.