Within quite recent years, benzidine (p-diaminodiphenyl) has come into use as a precipitant for the sulfate ion. It was first applied as a quantitative reagent for the determination of sulfate by Raschig in 1903. Other investigators subsequently introduced modifications in the original method of Raschig and succeeded in obtaining very satisfactory analyses with this reagent. For example, in the analysis of water samples which contain iron salts, hydroxylamine hydrochloride is added to prevent oxidation of the benzidine. The precipitated benzidine sulfate is collected in the usual way and is either weighed direct or titrated with sodium hydroxide, using phenolphthalein as an indicator. Titration is rendered possible by the very weak basic properties of benzidine. Bruckmiller states that the benzidine method for sulfates in water compares favorably with the time-honored barium chloride method in point of accuracy, and has the advantage of being more rapid
