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Kinetic Spectrophotometric Determination Of Thiosulfate Based On Its Inhibitory Effect On The Oxidation Of Methyl Red By Bromate

Jahanbakhsh Ghasemi, Mahsan Miladi

A simple and versatile kinetic spectrophotometric method for determination of thiosulfate is described. The proposed method is based on the inhibitory effect of thiosulfate ion on the oxidation of methyl red (MR) by bromate in the presence of nitrite, which was monitored at 520 nm. The absorbance of the reaction mixture decreases with time and reaches to a minimum in the absence of thiosulfate. The variables affecting the rate of the reaction were investigated and the optimum conditions were established. Under the optimum experimental conditions (0.03 M sulfuric acid, 2 × 10-5 M MR, 2.4 × 10-4 M bromate, 0.45 M nitrate, 1 μg ml-1 nitrite and 20°C ), thiosulfate can be measured in the range of 0.1 - 9 μg ml-1 with a detection limit of 0.095 μg ml-1. This method has been used to determine the trace amounts of thiosulfate ion in tap water samples.