The determination of sulphate: A study of the conditions necessary for the precipitation of benzidine sulphate, with special reference to the estimation of sulphates in urine.

THE chief difficulties in the benzidine method for determination of sulphate arise in obtaining the conditions requisite for maximum precipitation of benzidine sulphate. Most investigators have drawn attention to the importance of adjusting the acidity of the solution, while some of them have studied the interference known to be caused by certain ions: chlorides, for example, may increase the solubility of benzidine sulphate [Fiske, 1921; Kahn and Lieboff, 1928]. Undoubtedly the phosphate ion has been the source of most error. The earlier workers, Raschig [1903; 1906], Friedheim and Nydegger [1907], Jarvinen [1912], Gauvin and Skarzynski [1913], Rosenheim and Drummond [1914], Raiziss and Dubin [1914] and Drummond [1915], considered that approximate adjustment of the reaction of the solution to PH 2 by addition of hydrochloric acid would eliminate precipitation of benzidine phosphate. However Fiske [1921], Yoshimatsu [1926], Hubbard [1927; 1930], Pohorecka-Leslesz [1927], Chatron [1931, 1, 2], Power and Wakefield [1931], Letonoff and Reinhold [1934], and Ollgaard [1934] found it advisable to remove phosphate. Quite recently however Kahn and Lieboff [1928] and Friedrich and Bauer [1934] reported that they found it unnecessary to remove phosphate. There is also some difference of opinion as to the best method of washing a benzidine sulphate precipitate. The present work consists of an attempt to standardise the method of analysis, the chief points studied being: (a) the influence of reaction on the precipitation of benzidine sulphate and phosphate; (b) the influence of chlorides in affecting the solubility of benzidine sulphate; (c) the influence of the concentration of benzidine hydrochloride; (d) the influence of different methods of washing the precipitate.