Studies in the analytical chemistry of selenium: absorptiometric determination with 2-mercaptobenzoic acid
Selenium(IV) forms a 1:3 complex with 2-mercaptobenzoic acid that shows maximal absorption at 268 nm in the pH range 0·5 to 2·5. The formation of the complex provides the basis of a rapid and sensitive method for the determination of selenium in the presence of a wide range of elements. When the complex is extracted into ethyl acetate, 22 of 36 other ions investigated do not interfere, even at 200-fold excess. The interference of the others, including tellurium, is readily overcome by a simple technique of total-ion exchange, by masking or by solvent extraction. Of the ions examined only mercury(II) still interferes.
The molar absorptivity of the complex at 268 nm is 15,600 and the sensitivity index 5 × 10–3µg per cm2. The complex is formed instantaneously and maintains an unchanging absorbance for over 2 hours. The method is suitable for the determination of selenium in the range 0·008 to 1·5 p.p.m. in aqueous solution.