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

Abstract

Total positive ion concentrations N_i 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., UO_nH_m+ 3H UO_n–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, N_i 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.