Coefficients of Diffusion of Certain Alkali Salt Vapors in the Bunsen Flame

Abstract

The method used is a modification of one suggested by H. A. Wilson. The ellipsoidal streak of luminous vapor from a salt bead held in the flame, was photographed; then the coefficient of diffusion $K$ was obtained from the equation $Kė\frac{1}{2}rv(\frac{\mathrm{dx}}{\mathrm{dr}}-1)$, where $r$ is the radial distance from the bead to a point on the boundary of the streak, $x$ is the vertical distance of the point above the bead, and $v$ is the velocity of the flame gases. A Meeker gas burner of special design was used, supplied with air under pressure and with intermittent puffs of salt spray to enable $v$ to be determined. The flame temperature, determined by Féry's method, was 1436°C. The means of about ten determinations of $K$ for each salt are: lithium chloride, 12.4; lithium sulphate, 9.7; sodium chloride, 19.0; sodium sulphate, 16.4; potassium sulphate, 11.1; rubidium sulphate, 11.7; caesium sulphate, 8.4. These are probably accurate to about 10 per cent. The chlorides of both Li and Na diffuse faster than the sulphates. Except in the case of the lithium salts, for which the observed values are relatively too low, the diffusion coefficients increase roughly as the reciprocals of the square roots of the weights of the metal atoms, also roughly as the reciprocals of the square roots of the molecular weights. If the free metal atom of the salt is the source of luminosity, as some other experimenters have concluded, it is probable that it is free only a fraction of the time, the fraction varying with the salt and the alkali.