The mechanism of flame inhibition by sodium salts

A study has been conducted to determine whether the mode of action by the “dry chemical” flame inhibitors, sodium bicarbonate and sodium tartrate, was heterogeneous or homogeneous. The method used was the correlation of the amount of inhibitor vaporized in the flame zone with a measure of the degree of inhibition. For the first, Na atoms were determined by absorption spectroscopy at the end of the reaction zone of partially quenched premixed CH4/air flames burning at atmospheric pressure on a flat flame burner. The degree of inhibition was indicated by the extent of the temperature rise of the quenched flame on addition of inhibitor. Tests were conducted on six “siliconized” and size classified salt fractions three each of the two salts. Four of the six powder samples completely evaporated by the end of the reaction zone. The results for all six fractions can be represented by an approximately linear relationship between Na concentration at the end of the reaction zone and the temperature rise on inhibition. It is shown that this correlation is much better than one based on surface area presented to the flame. These results are interpreted as an essentially conclusive proof of the homogeneous mechanism. In addition, measurements of hydroxyl concentrations have shown that addition of inhibitor reduces peak OH concentrations and catalyzes radical recombination. Na atoms are unusually effective in this regard. While a complete mechanism has not been worked out, some discussion is given of the limitations on such a scheme. The existance of dipole-induced dipole stabilized complexes between alkali atoms and water molecules is suggested as a means by which recombination might very effectively be catalyzed.