Determination of thiosulphate at trace levels using an iodide ion-selective electrode

Aqueous thiosulphate was shaken with a solution of iodine in carbon tetrachloride. The organic phase was separated, the excess of iodine extracted into an aqueous sulphite solution and the iodide liberated was measured with an iodide-selective electrode. It was possible to determine thiosulphate in various concentration ranges by changing only the concentration of the standard iodine in carbon tetrachloride solution. A linear calibration graph was obtained over the concentration range 4 × 10–7–8 × 10–6M thiosulphate and the relative standard deviation was 0.92% at the 6 × 10–6M thiosulphate level. Good recoveries of thiosulphate from natural water samples were achieved using the proposed method.