766 Short Papers Analyst, July, 1978 Determination of Thallium by Precipitation of Thallium( I) Chromate from Homogeneous Solution by Replacement of Thallium(1) in a Complex with EDTA K. N. Upadhyaya" Chemistry Department, University of Dar es Salaam, P.O. Box 35061, Dar es Salaam, Tanzania Keywords 1 Thallium determination ; graviwetry ; chromate precipitation ; pyecipitation fvom howogeneous solutioPz ; replacement technique Precipitation of thallium( I) chromate from weakly ammoniacal solutions has been shown to be the most suitable method for the gravimetric determination of thallium.lP2 Interferences from silver, lead and mercury(1) occur as they are also precipitated as the chromate.2 The determination of thallium by precipitation of tha.llium(1) iodide3 is one of the oldest methods, but this is not favoured as thallium(1) iodide is more soluble than thallium(1) chromate.Several other insoluble compounds of thallium(1) that have been recommended for its gravimetric determination are the hexachloroplatinate( IV) ,* the hexanitrocobaltate(II1) ,5 the 2-mercaptobenzthiazole compound,g the thionalide comp~und,~ a precipitate with diamminetetrathiocyanatochromate(II1) monohydrate,8 a dithiocarbamate compound9 and thallium(1) tetraphenylborate( 111) .lo By using EDTA PPibil and Zabrankyll precipitated thallium as thallium(1) iodide in the presence of copper, bismuth, lead and iron(II1). Precipitation from homogeneous solution has been widely accepted as a valuable technique in analysis. One of the ways in which this controlled precipitation is achieved is by replace- ment of one ion by another in an EDTA4 complex.12 Salesin and Gordon13 have reported that Ramette achieved a precipitation of nickel from homogeneous solution by replacing nickel complexed with EDTA with zinc in the presence of dimethylglyoxime.Similarly, if magnesium ions are added to a barium - EDTA complex in the presence of sulphate, barium sulphate is precipitated.l* Barium can also be determined as barium chromate by using the same prin~ip1e.l~ From a review of the literature it appears that the technique of precipitation from homo- geneous solution has not been tried with thallium. The present investigation was, therefore, undertaken with a view to determining thallium(1) by precipitation of thallium(1) chromate from a homogeneous solution by means of replacing thallium(1) in a complex with EDTA with magnesium. The determination of thallium(1) in the presence of silver, lead and mercury(1) ions, which interfere in its direct determination as the chromate, was also studied.Experimental Solutions were prepared by direct weighing and wherever necessary the concentrations were ascertained by chemical analysis. Thallium(1) solution was prepared from thallium( I) sulphate and solutions of other metal ions from their nitrates. Different sets of experiments were performed. Determinations of thallium(1) were carried out on each of the five solutions containing thallium( I) alone, thallium( I) and silver, thallium(1) and lead, thallium( I) and mercury(1) and thallium(I), silver, lead, and mercury(1). As no tendency to co-precipitate was noticed with lead and mercury(I), it was not considered necessary to include the numerical results of these determinations here.All of the chemicals used were of analytical-reagent grade. *On leave from Ramjas College, Delhi University, Delhi-7, India.SHORT PAPERS 767 Determination of thallium(I) in a solution containing thallium(1) alone Take a 50-100-ml aliquot of thallium(1) sulphate solution (50-100 mg of thallium), add the EDTA (disodium salt) solution in excess of the amount required to combine with all of the thallium [100mg of thallium(1) requires about 182 mg of EDTA], adjust the pH to 8-9 with ammonia solution, then add 4-8 ml of potassium chromate solution (0.05 M), adjust the total volume to about 200 ml and heat the solution at 60-70 "C on a water-bath.Add the magnesium solution (0.05 &I) at the rate of 1 drop in 8-10 s with constant stirring until an amount that is more than that which is equivalent to the excess of EDTA and the thallium has been added and allow the beaker containing the precipitate to cool. Filter the contents of the beaker through a weighed, sintered-glass crucible, wash the precipitate with a 1% m/V solution of potassium chromate, then twice with distilled water and finally with 50% ethanol. Dry the precipitate at 110-120 "C for 2 h and weigh it (Table I). Determination of thallium(I) in the presence of silvev A solution of silver nitrate of known concentration is mixed with the thallium(1) solution before the EDTA and potassium chromate solutions are added in excess of the amounts required for thallium and silver.Magnesium solution is added at the same rate in an amount greater than that required for the excess of EDTA, the thallium and the silver, other conditions remaining the same (Table 11). The procedure is similar to that foliowed for thallium alone (above). Determination of thallium(I) in the presence of other metal ions The amounts of EDTA, chromate and magnesium are adjusted according to the requirements for the amounts of thalliuni(1) and other metal ions. The procedures followed in other instances ai-e similar to those already described. Results and Discussion As is evident from the results (Table I) thallium can be determined by use of this method with a fair degree of accuracy.Thallium is not only precipitated quantitatively but the precipitate is finely crystalline, easily filterable and shows no tendency to adhere to the walls of the container. When present in equimolar concentrations lead and mercury(1) are not replaced in their complexes with EDTA by magnesium and hence do not interfere in the TABLE I DETERMINATION OF THALLIUM( I) BY PRECIPITATION OF THALLIUM( I) CHROMATE FROM HOMOGENEOUS SOLUTION (REPLACEMENT OF THALLIUM BY MAGNESIUM IN EDTA COMPLEX) Mass of T1 Mass of Tl,CrO, Calculated mass takenlmg obtained/mg of TI,CrO,/mg Difference Error, % 100.00* 128.1 128.30 - 0.20 - 0.234 80.0* 102.4 102.64 - 0.24 - 0.234 50.0* 64.3 64.15 +0.15 +0.248 * Average of three determinations. determination of thallium by use of this method.It is interesting that silver shows a tendency to co-precipitate with thallium( I), although when present alone with thallium(1) the precipitation from its EDTA complex is not effected by magnesium in the presence of the chromate. From the results (Table 11) it appears that the interference from silver becomes significant when it is present in an amount that is half or more than half the molar concentra- tion of thallium. The presence of the ions silver, lead and mercury(1) together in the same solution with thallium does not affect the results of the determination in any significant way. The method thus permits the determination of thallium in the presence of silver (up to a certain concentration), lead and mercury(I1). It can be argued that the separations are achieved because the EDTA complexes of lead and mercury(1) are stronger than the magnesium complex, that of silver is comparable in strength and the thalliurn(1) complex is appreciably weaker.768 SHORT P.4PERS Amdyst, Vol.103 TABLE I1 DETERMINATION O F THALLIUM(1) I N PRESENCE OF SILVER BY PRECIPITATION O F THALLIUM( I) CHROMATE FROM HOMOGENEOUS s o r 2 u m o N Mass of A&:,Cr04. Mass of co-precipi- Concentra- precitate Calculated Calculated tated (by tion of silver Mass of T1 Mass of Ag Molar ratio obtained/ mass of mass of difference)/ co-prepreci- takenlmg taken/mg T1: Ag mg TI,CrO,/mg Ag&rO,/mg mg tated, yo loo* 52.84 1: 1 139.5 128.3 81.1 11.2 13.8 loo* 26.42 1 : 0.5 130.1 128.3 40.6 1.8 4.4 1 oo* 13.21 1 : 0.25 128.0 138.3 20.3 Nil Nil * Average of three determinations. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. References Knowles, H. B., Analyt. Chem., 1949, 21, 1539. Bashilova, N. I., Khimiya Redk. Elem., 1957, 103; A~zalyt. Rbstv., 1958, 5, 3656. Mack, F., and Lepper, W., 2. Analyt. Chem., 1936, 68, 36 and 41. Cushmas, A. S., Am. Chem. J . , 1900, 24, 222. Strecker, W., and De La Pena, P., 2. Analyt. Chenz., 1926, 67, 256. Spacu, G., and Kuras, M., 2. Analyt. Chem., 1936, 104, 88. Berg, R., and Fahrenkamp, E. S., 2. Analyt. Ch~:nt., 1937, 109, 305. Bagbanly, L., and Mirzoeva, T., Dokl. Akad. Nauk Azerb. SSR, 1953, 9, 373. Bremanis, E., Schaible, L., and Bergner, K. G., .Z. Analyt. Chem., 1955, 145, 18. Wendlandt, W. W., Analytica Chim. Acta, 1957, 16, 216. Pfibil, K., and Zabranky, Z., Chemicke‘ Listy, 1951, 45, 427. Welcher, K., and Wilson, C. L., “New Methods of Analytical Chemistry,” Second Edition, Chapman Ramatte, €C. W,, personal communication, qzdoted by Salesin, E. D., and Gordon, L., Talanta, 1950, Firsching, I;. H., Analyt. Chem., 1961, 33, 1948. and Hall, London, 1964, p. 228. 2, 392. Received December 19th, 1977 Accepted January lst, 1978