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Abstract.A simple and accurate complexometric method is proposed for the determination of TI(III) using semicarbazide hydrochloride as a releasing agent. In the presence of diverse metal ions, thallium is complexed first with a known excess of EDTA, and the surplus EDTA is then titrated with standard zinc sulfate at pH 5.0-6.0 (hexamine) using xylenol orange indicator. An excess of 5% aqueous neutral solution of semicarbazide hydrochloride is then added and the released EDTA is titrated against standard zinc sulfate solution. The method works well in the range 2-50 mg of TI(III) with relative errors < 0.5%, standard deviations <~ 0.05 mg and coefficient of variation ~< 0.4%. The method is applied for the determination of thallium content in complexes and alloy compositions Key words: thallium determination, complexometry, masking, semicarbazide hydrochloride.Thallium is widely distributed on the earth's crust with very low concentrations. Thallium compounds are highly toxic, but have wide applications. Thallium forms alloys with Pd, Pt, Ti, Zr, La and Bi and these have very important uses. The most unusual alloy of T1 is the Hg-T1 alloy which forms a eutectic at 8.7% T1 and freezes at -59 ~ about 20 ~ below the freezing point of mercury. Some thallium alloys and compounds are used as catalysts [-1] and as rodenticides. Thallium has also been used for heart imaging [-2] and thallous acetate is used in medicine for the treatment of tuberculosis and skin diseases. Because of the toxic nature and wide applications of thallium and its compounds, a selective complexometric method for the determination of thallium is highly useful.

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