Complexometric determination of thallium(III) using sodium metabisulphite as a selective releasing agent

A selective complexometric method is described for the determination of Tl(III) using sodium metabisulphite as releasing agent. Thallium(III) and associated metal ions are complexed with excess EDTA and the surplus EDTA is back titrated at pH 5-6 (hexamine or acetic acid-acetate buffer) with standard zinc sulphate solution using xylenol orange as indicator. The addition of sodium metabisulphite displaces the EDTA selectively from the Tl-EDTA complex. The released EDTA, equivalent to 11(III) present, is titrated with standard zinc sulphate solution. The results obtained are reproducible and accurate in the range 4-100 mg of thallium with relative error less than 0.30% and coefficient of variation not exceeding 0.40%. The proposed method is employed in the determination of thallium in its salts, complexes and synthetic mixture of metal ions.