Studies on the ionization produced by metallic salt in flames - III. Ionic equilibria in hydrogen/air flames containing alkali metal salts

Abstract

An examination has been made of the electron concentration produced when alkali metal salts are added to various hydrogen/air flames containing excess hydrogen. The results have shown that although the variation of the measured electron concentration with the concentration of added alkali metal shows the thermodynamically predicted behaviour, the variation with respect to flame temperature and ionization potential of alkali metal is very different from that predicted by theory, Coupled with this, there is a serious discrepancy between the measured and predicted levels of electron concentration, the former being much too low. The electron concentrations were measured by the attenuation of centimetric radio waves by the flame, and the temperatures by the sodium D -line reversal method. Since the flames contain a fair proportion of hydroxyl radicals (of the order of 01 %) in the regions studied, i.e. the zone of hot gases succeeding the cones of primary combustion in the burner, the formation of negative hydroxyl ions in considerable quantity appeared to be a plausible explanation of some of the observed discrepancies. An experimental method has been devised for the estimation of the atomic and molecular ions (other than electrons) in flames by making the flame form a portion of the dielectric of a simple parallel-plate condenser. The relative effects of the electrical conductivity, due mainly to the electrons, and the dielectric constant, which is affected to a large extent by the other ionic species at frequencies of the order of 100 Mc/s, on the resonant frequency and selectivity of a simple circuit containing this flame capacity, allow of an analysis for the electrons and massive ions respectively. In the absence of the massive ions, the resonant frequency shift and selectivity change induced by the flame should show parallel effects, but if massive ions are present in sufficient concentration, they show themselves by a marked effect on the resonant frequency and little effect on the selectivity. From the observations made the presence of such massive ions in these flames was inferred. An important preliminary step was to ensure that the concentrations of electrons which could be deduced from both the attenuation and capacity methods were in agreement. It has been shown that this will not be so unless the flames used are uniform in the sense of their cross-sectional temperatures. The required agreement was obtained when such flames were used, entrainment of air with production of temperature inhomogeneity being prevented by sheathing the flame with a stream of nitrogen. Measurement of the temperature distribution in unsheathed flames, and application of a simple mathematical analysis, given in the appendix, to these distributions, also brought about agreement for electron concentration. These sheathed flames gave a variation of electron concentration more consistent with the predicted relative behaviour for different alkali metals, although the temperatures deduced were several hundred degrees above the measured ones. Analysis of the heavier ion concentrations from the results indicated in general about twenty times as many as there were electrons. These were divided, to give charge balance, between positive alkali metal ions and OH- , these being the only reasonable ionic species for these flames. Incorporation of this hydroxyl ion effect largely removes the discrepancy between the measured and predicted absolute electron concentration. The results lead to a reasonable value of the collision frequency of a heavy ion with molecules, and, combined with the theoretical hydroxyl concentrations for the flame, suggest a value for the electron affinity of hydroxyl of 62 ± 6 kcal/mole. This value is reasonable by comparison with those for the halogens.