Indirect Spectrophotometric Determination of Microamounts of Thiosulfate

Abstract

A spectrophotometric method for microdetermination of S₂O₃^2- has been developed. The method is based on the oxidation of thiosulfate with chlorine (Cl₂) and the residual chlorine is determined by the color reactions with o-tolidine (3, 3′-dimethylbenzidine). A linear calibration graph (0-8×10^-6M S₂O₃^2-) is obtained under optimum reaction conditions at room temperature and pH 11. The stoichiometric mole ratio of chlorine, Cl₂, to thiosulfate is 4. The effective molar absorptivity for thiosulfate with o-tolidine is 2.15×10⁵M^-1 cm^-1 at pH 1.6. The lower limit of determination is 4.65×10^-8 or 5.21ppb (0.026μg thiosulfate in 5ml).