Photometric investigations of alkali metals in hydrogen flame gases - II. The study of excess concentrations of hydrogen atoms in burnt gas mixtures

Abstract

Photometric measurements on alkali metals in hydrogen-oxygen flame gases, diluted with various proportions of nitrogen, are interpreted as giving a measure of the concentration of free hydrogen atoms, which persist in these gases for several milliseconds after primary combustion. These concentrations are well in excess of those expected from thermodynamic equilibrium, especially towards the lower end of the range of temperatures studied (2400 to 1600°K). Two kinds of measurement have been made. (i) Comparison of the intensities of the Na D lines and the Li resonance doublets, for equal traces of the two elements present in the gases. The amount of free lithium is modified by the balanced process, Li + H 2 O ⇌ LiOH + H, whereas corresponding reactions for sodium are negligible. Using estimated equilibrium constants for these reactions, [H] can be obtained. (ii) Measurements of the change in intensity of the Na D lines when 0∙01 to 0∙5% of chlorine or its compounds (a large excess over the sodium) are added to the flame gases. NaCl is considered to be formed by the balanced reactions Na + HCl ⇌ NaCl + H. The concentration of HCl, the most important chlorine compound in the hydrogen-rich flame gases, may be obtained from the total chlorine added. Using estimated equilibrium constants for the above reaction, [H] can again be obtained. The agreement between the [H] values obtained by these two independent methods is good. The decrease of [H] with height in the gases is consistent with ternary recombination towards full equilibrium. A general discussion of excess radical concentrations in hydrogen flame gases is given, for hydroxyl radicals and oxygen atoms as well as hydrogen atoms. The conclusions reached are supported by experimental evidence. A further discussion of the way in which the amounts of chlorine used in the experiments may affect the other radical concentrations is given.