The Square-Wave Polarographic Determination of Free Sulfur in Liquefied Petroleum Gases

A method has been developed for the polarographic determination of the free sulfur present in liquefied petroleum gases in concentrations as low as 0.01 ppm. Samples of liquefied petroleum gases are collected in a specially-fitted pressure glass bottle. After the volatilization of the sample, the free sulfur remaining in the sample container is dissolved in a methanol-methyl iodide-glacial acetic acid-sodium acetate solvent, and the resulting solution is electrolyzed polarographically. The effect of the change in solvent and in supporting electrolyte concentration, dissolved oxygen and temperature are discussed. It is found that the concentrations of 90 vol% methanol, 2 vol% glacial acetic acid, 8 vol% methyl iodide, and 1.5 m sodium acetate are necessary to obtain satisfactory analytical results. The polarographic wave of free sulfur consisted of two waves; the peak potential of the first wave was −0.57 V vs. SCE, while that of the second wave was −0.87 V vs. SCE. The limit of determination was governed by the solubility of the naphtha in the base solution.