Photometric investigations of alkali metals in hydrogen flame gases IV. Thermal and chemiluminescent effects produced by free radicals

Abstract

The intensity of emission of the sodium D -lines has been observed as a function of height above the burner for traces of sodium salts added to premixed hydrogen + oxygen + nitrogen flames burning at atmospheric pressure. A specially constructed Mèker burner was used, giving an almost flat reaction zone. Flames in which the final temperatures of the burnt gas ranged from 1400 to 2500°K were used. At the higher final temperatures, the intensity showed a rapid rise at the reaction zone, followed by a more steady rise to a constant value. This has been interpreted as a thermal effect, the bulk temperature of the flame gases increasing by significant amounts as a result of heat released by the recombination reactions H + H + X → H 2 + X , H + OH + X → H 2 O + X . Values of the mean velocity constants of these ternary reactions have been deduced, and are in agreement with independent estimates. At the lower final temperatures, there is a sharp peak of intensity at the reaction zone, followed by a decrease towards a steady value. This has been interpreted as a chemilumin­escent effect, the sodium being excited by ternary reaction with two free radicals: H + H + Na → H 2 + Na*, H + OH + Na → H 2 O + Na*. The decay of chemiluminescence with height in the flame gases accords with the known decreases of the concentrations of atomic hydrogen and hydroxyl. Relative and absolute velocity constants for these processes are derived and discussed.