Use of a rotating single probe in studies of ionization of metal additives to premixed flames. Part 2.—Observations on the role of involatile transition metal oxides

Abstract

Total positive ion concentrations N_i from various transition metal salts introduced into premixed H₂+ O₂+ N₂ flames as fine sprays of their aqueous solutions have been measured as a function of additive concentration, form of additive salt, flame temperature and height up the flame. The observed behaviour is more complicated than that from alkali metals or alkaline earths, and is bound up with the presence of involatile oxide particles. Transition metals in groups 3, 4, 5 and 6 (excepting Cr) show large effects, in size of N_i, stability of oxide and persistence of oxide particles, whereas the opposites are true for Cr and the transition metals examined in groups 7 and 8. With uranium (at least), the O/U atomic ratio in the involatile oxide particles seems also to be important. Explanations in terms of (A) slow thermal ionization of solid particles, to give solid particle ions, and (B) production of gaseous ions as a consequence of the presence of solid particles, are considered; the observations favour mechanism B at the low particle densities examined. When alkali metal salt + transition metal salt mixed solutions are sprayed into the flame, N_i is much greater than would be expected if the two salts had no mutual effect. Associated results suggest that the solid oxide particles promote equilibrium ionization of alkali metal via a “solid state” reaction. The possibility that the alkali metal lowers the work function of solid oxide is not favoured.