Photometric investigations of alkali metals in hydrogen flame gases - I. A general survey of the use of resonance radiation in the measurement of atomic concentrations

Abstract

Experiments on the intensity of emitted resonance radiation in various of the first and second doublets of alkali metals have been made in the hot gases a few centimetres above the primary zone of combustion of hydrogen-oxygen flames diluted with nitrogen, using a photoelectric technique. The results confirm that the emission is thermal in origin and lead to several conclusions of interest. ( а ) Very low intensities are always obtained with lithium, on account of removal of the metal by hydroxyl radicals to form the very stable gaseous hydroxide. This effect is decreasingly important with caesium and potassium, and is negligible with sodium. ( b ) This reaction to give the metallic hydroxide appears to be thermodynamically equilibrated in the hotter flames (> 2000° K), but becomes progressively and very markedly incomplete towards lower temperatures. ( c ) In the case of caesium only, the ionization of the metal accounts for a substantial fraction of the total metal present. Estimation of this amount leads to the conclusion that negatively charged OH ¯ ions, as well as free electrons, are present. A value of the electron affinity of OH of 65 ± 5 kcal is deduced. ( d ) Consideration of the results for sodium, which are not complicated either by formation of hydroxide, or by ionization, shows that it is possible to estimate the relative concentration of unexcited atoms by a two-flame technique, and that the absolute concentration of sodium atoms in the flame gases may be measured by observation of the onset of self-absorption. ( e ) The method appears to be useful in the confirmation and determination of the f -values of resonance lines.