Determination of thiosulfate at the 10 −6 M level by its oxidation with iodine in an organic phase and spectrophometric measurement of triiodide

Abstract

A highly sensitive method is proposed for the determination of thiosulfate based on the oxidation of aqueous thiosulfate (100 or 200 ml) by iodide in 4 ml of carbon tetrachloride. The excess of iodine was extracted into 8 ml of aqueous iodide solution as triiodide to be measured spectrophotometrically; the thiosulfate could therefore be indirectly highly concentrated and determined selectively. The side-reaction of thiosulfate in a large volume of solution with the hypoiodite formed from the iodine in carbon tetrachloride could be compensated for by adding a certain amount of extra thiosulfate. A linear calibration graph with a negative slope was obtained over the concentration ranges 1.1 × 10 −7–1 × 10 −5 M (12 ppb-1.12 ppm) for 100 ml of thiosulfate solution and 6 × 10 −8 – 5 × 10 −6 M (6.7 ppb-0.56 ppm) for 200 ml of thiosulfate solution. The proposed method was successfully applied to the determination of various amounts of thiosulfate in hot-spring and lake-water samples.