Determination of Methoxyl Groups

Abstract

Modified Zeisel procedures have been used for quantitative determination of alkoxyl groups such as methoxyl (Roth 1953). In the original Zeisel procedure (Zeisel 1885, Zeisel and Fanto 1902, 1903), the treatment of methoxyl- or ethoxyl-containing compounds with concentrated hydriodic acid (HI) at refluxing temperature leads to cleavage of the alkoxyl groups with formation of methyl or ethyl iodide. The volatile iodide, when passed through a solution of silver nitrate, yields a precipitate of silver iodide which is used to determine quantitatively the methoxyl or ethoxyl content of the compound. However, any sulfur-containing groups are converted quantitatively to hydrogen sulfide during the hydriodic acid treatment. The hydrogen sulfide, in turn, reacts with silver nitrate, producing a precipitate of silver sulfide. Thus, the original Zeisel procedure is not appropriate for the determination of alkoxyl groups in sulfur-containing compounds. In order to circumvent this difficulty, Vieböck and co-workers converted the alkyl iodide to iodic acid by oxidation with bromine while simultaneously oxidizing hydrogen sulfide to sulfuric acid (Vieböck and Brecher 1930, Vieböck and Schwappach 1930). This step eliminates the interference of sulfur-containing groups and makes the Vieböck and Schwappach procedure suitable for the determination of methoxyl and ethoxyl groups in compounds containing sulfur. This is of primary importance in the characterization of technical lignins, such as kraft lignins and lignosulfonates, which contain varying amounts of sulfur-containing groups.