J. Indian Chern. Soc., NOTE Vol. 75, April 1998, pp. 265-266 Indirect Complexometric Determination of Thallium using Thiourea as Masking Agent C. H. RAGHAVANNAMBIAR., B. NARAYANA*,B. MURALIDHARARAO and BIJUMATHEW Department of Studies in Chemistry, Mangalore University, Mangalagangothri-574 199 Manuscript received 9 Apri/1996, revised 14 November 1996, accepted 8 August 1997 A complexometric method for the determination of Tim in the presence of other metal ions is described based on the selective masking ability of thiourea towards Tim at pH 5.0--6.0 (hexamine). Thallium(m) and diverse metal ions are initially complexed with a known excess of EDTA and the surplus EDTA is titrated with standard zinc sulphate solution using xylenol orange as indicator. An excess of thiourea solution (10%) is them added, and the EDTA released from the Tlm-EDTA complex is titrated against zinc sulphate solution. The method proposed is accurate (3-92 mg Tl) with relative errors $0.13% and standard deviations $0.03 mg. This method is applied to the determination ofT! in its alloy compositions and its complexes. Thallium has been selectively determined by EDTA using masking and demasking techniques 1 . Here, selective de- composition of the Tlli1-EDTA complex by the addition of thiourea solution at pH 5.0-6.0 and at room temperature is described. This reagent can quantitatively reduce Tliii- EDTA complex, giving Tl 1 that shows little tendency for complexation in the above pH range. Results and Discussion The reaction tendency of metals which shows different oxidation states, are entirely different, hence masking can be effectively achieved by the suitable oxidation or reduc- tion of such species. Thallium is one such metal which forms a stable complex 2 with EDTA in +3 oxidation state and shows little tendency for complexation in + 1 state 3 . Even if Tliiiforms a stable complex with EDTA it may do so only under high pH condition. Thiourea forms stable s- bonded complexes with soft metal ions 4 . The +I oxidation state of thallium in the solution was confirmed by spot-test techniques 5 . The determination of thallium in thallic nitrate solution shows that accurate and consistently reproducible results are obtainable within permisible errors. Thallium is deter- mined in the range 3-92 mg in an aliquot. The effect of different metal ions and anions on the quan- titative determination of thallium(IIJ) were studied with 15.37 mg ofthallium(m) in solution. No interference was observed when the following ions were present; 100 mg of Cd 11 Co 11 NiH or Pb 1 L 40 mg of Aim Bim Crm Celli ' ' ' ' ' ' ' Lam, yiV, Ti 1 V or Zr 1 Y; 8 mg ofUVI, Pt 1 Y, Fe III, Rum, In III, Rhlll or Ag 1 ; 100 mg of acetate, nitrate, sulphate, borate, phosphate, chloride, bromide or fluoride. sniV, Hgll, Pdll and Cull interfered severely. The interference can be obvi- ated by using suitable secondary masking. Relative errors in the above studies did not exceed ±0.05%. Applications : Thallium(!) complexes were prepared from some sulfur donor ligands by conventional methods and their purity was checked from elemental analysis. The complex (0.2-0.4 g) was decomposed by evaporation to near dryness with aqua regia. The residue was then cooled, dis- solved in 2 N HN0 3 (3 ml) and made up to 100 ml with distilled water. Aliquots (I 0 ml) were used for titration as per the proposed method and the results are presented in Table 1. Various synthetic mixture ofT! with Ti, Zn, Pd or Bi metal ions were prepared corresponding to their alloy compositions and the Tl JII was determined using the pro- posed method (Table 2). TABLE I-ANALYSIS OF Tl COMPLEXES (n = 3) Compd. Tl Tl Relative present found error mg mg % Tl(C 3 H 5 N 4 S) 0 61.30 61.35 +0.08 ll(C 10 H 11 N 4 0S)b 46.51 46.59 +0.17 ll(C 11 H II N 4 S)c 46.97 46.89 -0.17 0 Thallium complex of 4-amino-5-mercapto-3-methyl-1,2,4,-triazole. bThallium complex of 3-( o-tolyloxymethyl)-4-amino-5-mercapto-1,2,4- triazole. cThallium complex of 4-benzylideneamino-3-ethyl-5-mercapto- l ,2,4-triazole. TABLE 2-ANAL YSIS OF TJIII IN ARTIFICIAL MIXTURES OF SALTS CORRESPONDING TO ALLOY COMPOSmONS (n = 4) Mixture Composition Tl found Relative % % error% Tl+Ti 20+80 19.96 -0.20 Tl+Zr 50+ 50 50.06 +0.12 TI+Pd 0 50+ 50 49.95 -O.lO Tl+Bi 75 + 25 74.91 +0.12 0 DL-Methionine used to mask Pdn The method is simple and rapid, as it does not require heating, readjustment of pH and standardization ofEDTA. The lack of effect of foreign ions on the accuracy and pre- 265