Studies on the ionization produced by metallic salts in flames. IV. The stability of gaseous alkali hydroxides in flames

Abstract

The electron concentration produced by the addition of alkali metal salts to a wide range of hydrogen/air flames extending to the air-rich side of stoichiometric composition has been measured by the method of attenuation of centimetric radio waves. Considerable deviations from the predicted behaviour on the basis of the known ionization potentials of the metals and the measured temperatures have been observed. The main effect noted is that for two flames of the same temperature, but on opposite sides of stoichiometric composition, the air-rich one always shows a markedly lower attenuation, an effect which is by far the most pronounced in the case of caesium. The electron concentration throughout is considerably below the predicted level. Simple statistical calculations have shown that the formation of gaseous ion pair molecules of the alkali hydroxides by combination with the hydroxyl radicals in the flame is plausible, and an analysis of the electron concentrations on this basis has shown that the results are compatible with removal of a large amount of caesium as hydroxide, but not of potassium and sodium. This agrees with the expected difference in energy of formation of caesium and potassium hydroxides, from the gaseous metal and hydroxyl, of about 5 kcal/mole. It has not proved possible to explain the whole of the deviations of the electron concentration in this way, but by incorporation of a further effect, of production of hydroxyl ions, which is independent of the metal added, a complete resolution may be obtained. The concentrations of hydroxyl ion predicted by the analysis show agreement both with the observed discrepancy with the theoretical electron concentration, and with the order of magnitude of hydroxyl ion concentration obtained earlier by an independent and more direct method (Smith & Sugden 1952). The hydroxyl radical concentrations implied by this analysis show excellent agreement with those obtained from thermodynamic calculation of the burned gas composition at equilibrium. Values for the heat of formation of gaseous potassium hydroxide and gaseous caesium hydroxide, from the gaseous metal and hydroxyl, which the data require are 87.5 ± 3 and 92.5 ± 3 kcal/mole respectively. The analysis of the results further implies an electron affinity for gaseous OH of 69 ± 5 kcal/mole, which agrees reasonably with the value obtained previously from flame measurements of 62 ± 6 kcal/mole.