ANALYST, DECEMBER 1986, VOL. 111 1359 Coated-wire Ion-selective Electrode for the Determination of Thal I iu m( 111) Concepcion Sanchez-Pedrefio," Joaquin A. Ortufio and Maria C. Torrecillas Department of Analytical Chemistry, Faculty of Sciences, University of Murcia, 3007?-Murcia, Spain The construction, performance characteristics and applications of a coated-wire thallium(lll) ion-selective electrode, based on the ion pair between TIC14- and the 1,2,4,6-tetraphenylpyridinium cation in a poly(viny1 chloride) matrix, are described. The influence of membrane composition, hydrochloric acid concentration and foreign ions was investigated. The electrode shows a near-Nernstian response over the thallium(lll) concentration range IO-5-lO-* M, with good selectivity and precision. Applications to the potentiometric titration of 0.255.0 mg of thallium(lll) and to the direct potentiometric determination of thallium in sphalerites and zinc concentrates are reported.Keywords: Thallium ion-selective electrode; coated-wire electrode; 1,2,4,6-tetraphenylpyridinium tetrachlorothallate(lIl); potentiometric titration; thallium(111) determination An exciting advance was made in ion-selective electrodes by Catrall and Freiserl when they developed coated-wire ion- selective electrodes (CWE). CWEs have become popular owing mainly to their simple and cheap construction. Coated-wire electrodes, with ion-association compounds formed between negatively charged halide complexes of the metal ions to be determined and strong hydrophobic cations as the electroactive materials, have been reported previously.2-* Iron,2 mercury,3 copper,4 zinc,5 bismuth6 and gold7 have been determined as their chloro complexes and cobalt as its tetrathiocyanato cobaltate(I1) complex.8 Aliquat 336s has been frequently used as a counter ion.2-6 The formation and extraction of an ion-association com- pound of tetrachlorothallate(II1) and the 1,2,4,6-tetra- phenylpyridinium cation have been reported.9 The high degree of extraction and good selectivity of the ion pair of thallium(II1) led us to investigate this compound as the electroactive material in a thallium(II1) CWE.The behaviour of electrodes based on organic ion exchangers depends on the extraction characteristics of the compounds involved. 10 In this paper we report the construction and applications of an electrode for the selective determination of thallium.Experimental Apparatus Potentials were measured using a Philips PW9415 ion-selec- tive meter and an R44/2-SDl calomel double-junction refer- ence electrode, containing 1 M KCl solution in the outer compartment. All solutions were stirred continuously with a magnetic stirrer in a thermostatically controlled 100-ml beaker (25 k 0.1 "C). Automatic titrations were performed with a Radiometer ABU12b autoburette and an OmniScribe D5000 chart recorder. Reagents All inorganic chemicals used were of analytical-reagent grade. Doubly distilled water was used throughout. Tetrahydrofuran (THF) and dibutyl phthalate (DBP) were supplied by Merck. The poly(viny1 chloride) (PVC) of high relative molecular mass used for the ion-selective electrode was from Fluka.1,2,4,6- Tetraphenylpyridinium acetate (TPPA) solution, 0.1 M. Prepared by the method of Chadwickll and standardised * To whom correspondence should be addressed. gravimetrically with perchlorate. Working solutions were prepared by dilution with doubly distilled water. Thallium(III) standard solution, 0.01 M. Prepared by dissolving thallium(II1) chloride in 0.1 M hydrochloric acid and standardised by titration with EDTA. l2 Electroactive Material This was prepared by adding slowly a slight excess of 1,2,4,6-tetraphenylpyridinium acetate (10.5 ml of 0.02 M solution) to 20 ml of 0.01 M thallium(II1) solution in 1 M hydrochloric acid. The mixture was stirred for 30 min and the resulting white precipitate was filtered on a sintered-glass crucible (porosity 4), washed with doubly distilled water and dried at 100 "C.Construction of Electrodes The coated-wire electrodes were constructed as described elsewhere? Powdered PVC, dibutyl phthalate (plasticiser) and the electroactive material were dissolved in tetrahydro- furan; coating solutions and membrane compositions are shown in Table 1. A platinum wire, about 2 cm long and 1.0 mm in diameter, sealed into the end of a glass tube and soldered on to a shielded cable, was dipped into this solution 20 times and the solvent was evaporated with an air gun each time. A membrane was formed on the platinum surface and the electrode was allowed to set overnight. Conditioning and Direct Potentiometric Measurements These electrodes were conditioned by soaking with constant stirring in a 10-3 M thallium(II1) solution with the same hydrochloric acid concentration as the standards, until the electrodes gave a constant potential.The same procedure was then followed with a solution containing only hydrochloric acid of the same concentration. The standard thallium(II1) solutions were then determined in ascending order of concen- tration. The electrodes were stored dry and conditioned as above before a series of determinations. The first conditioning time was about 90 min, and then only 30-40 min for successive uses. Potentiometric Titrations An aliquot of the sample solution containing 0.25-5.0 mg of thallium(II1) was pipetted into the 100-ml beaker, 5 ml of 5 M HCl were added and the solution was diluted to 25 ml with doubly distilled water.This was titrated automatically with1360 ANALYST, DECEMBER 1986, VOL. 111 Table 1. Preparation of the coating solutions and compositions of the membranes Coating solution* Membrane composition, % m/m Electroactive Electroactive Membrane PVC/mg DBP/mg material/mg PVC DBP material A . . . . . . 69.4 136.9 9.8 32.1 63.3 4.5 B . . . . . . 103.1 107.4 10.0 46.8 48.7 4.5 c . . . . . . 134.4 67.4 9.4 63.6 31.9 4.5 * Dissolved in 3 ml of tetrahydrofuran. 1 X 10-3-2 x 10-2 M TPPA solution. Titration rates were held constant at 0.3 ml min-1 for samples containing 0.25-1.25 mg of thallium and at 0.15 ml min-1 for larger amounts. Potentials were then monitored with the thallium(II1) CWE. Determination of Thallium in Sphalerites and Zinc Concen- trates The samples were dissolved in aqua regia and boiled nearly to dryness three times with doubly distilled water, to reduce the acidity.The sample solution was transferred into a calibrated flask, diluted and the hydrochloric acid concentration adjus- ted to 1 M. Thallium(II1) was determined by direct poten- tiometry, measuring the potentials of both the standards and the sample. Results and Discussion Composition of the Membrane Fiedler and RfiiiCka13 have suggested several suitable plasti- ciser - polymer combinations. Three membrane compositions were investigated by varying the ratio DBP to PVC (Table 1). The responses for several thallium(II1) concentrations in 1 M hydrochloric acid solution are shown in Fig. 1. As can be seen, only membrane B, corresponding to a 1 : 1 m/m DBP to PVC ratio, shows a fast and stable response.Membrane A (2: 1 ratio) shows a severe drift, as has been observed in some CWE by other workers at high plasticiser contents. 14 Membrane C (1 : 2 ratio) shows unstable responses, and therefore all subsequent investigations were carried out with membrane B. Time -+ Fig. 1. Responses of membranes A, B and C to different thall- ium(II1) concentrations in 1 M hydrochloric acid. The numbers above the lines indicate pT1I11 Table 2. Potentiometric selectivity coefficients, KE,:, for the coated-wire thallium(II1)-selective electrode Ion @?,; * Cu", Ni", CdII, Mn", Zn", Pb", FeIII, CrIII, InIII, BPI, SnIV, AsV, AgI, nitrate, perchlorate, sulphate . . . . . . . . . .. . . . . . <2 x lO-4a mrr . . <5 x 10-4a PdI', GaIII, PtIV . . . . . . . . . . . . . . <2 x 10-3b AU"I 1.oc Sb\' 15.8= * Concentrations of interferent: a 10-2 M; b 10-3 M; c 10-4 M . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Response Range and Effect of Hydrochloric Acid Concentra- tion The response of the electrode in the thallium(II1) concentra- tion range 10-6-10-2 M was studied at various hydrochloric acid concentrations ranging from 0.01 to 1 M. The electrode gave a near-Nernstian response in the range 10-5-10-2 M thallium(II1) at 0.1-1 M hydrochloric acid concentrations, corresponding to the response to the T1C14- anion. The slopes (S) and correlation coefficients ( r ) obtained were as follows: 1 M HC1, S = -57.4, r = 0.9999; 0.5 M HC1, S = -55.9, r = 0.9999; 0.1 M HC1, S = -55.8, r = 0.9997; 0.01 M HC1, S = -48.9, r = 0.9987.The highest slope and the best correlation coefficient were obtained for 1 M HCl. This strongly acidic medium is also more convenient for the direct analysis of thallium in real samples. Taking all these results into account, a 1 M hydrochloric acid medium was selected for further work. Response Time and Detection Limit Response times of the electrode were tssyO within 10 s for 10-2-10-4 M thallium(II1) and within 1 min for 10-'-10-6 M and tgsyO within 1.5 min for 10-2-10-4 M and within 3-4 min for lo-5-10-6 M thallium(II1). The detection limit of the electrode, considered as the thallium(II1) concentration at which the potential deviates by 18 mV from the extrapolation of the linear portion, is 2 x 10-6M.Reproducibility and Stability The average change in potential for consecutive measure- ments of the 10-4 M standard thallium(II1) solution (five determinations) is kO.1 mV. The reproducibility and stability of the electrode were evaluated by determining replicate calibration graphs (n = 10) over a period of 2 weeks. The electrode was stored dry and conditioned each time. Although the absolute potential of the electrode changed, the calibration slope remained constant over this period (mean k standard deviation, 57.28 k 0.18). Selectivity The interference of various ions was studied by the mixed solution method.15 The concentration of the interfering ion was generally fixed at 10-2 or 10-3 or 10-3 M while the concentration of thallium(II1) was varied between 10-2 and M solution of the interfering ion was used.The selectivity coefficients, Kr;;.), presented in Table 2 show very good selectivity with respect to many common ions. Gold(II1) and antimony(V) cause the largest interferences. M. In examples where the interference was large, aANALYST, DECEMBER 1986, VOL. 111 1361 Table 3. Determination of trace amounts of thallium in materials Thallium obtained*/mg g- Sample Potentiometry Atomic absorption Sphalerite . . . . . . 0.0743 0.0749 Zincconcentrate . . . . 0.334 0.339 * Average of three determinations. Potentiometric Titrations The use of the coated-wire thallium(II1) electrode in the potentiometric titration of thallium(II1) solutions containing 1 M hydrochloric acid with 1,2,4,6-tetraphenylpyridinium acet- ate solutions was investigated.The method relies on the decrease of the concentration of the tetrachlorothallate(II1) anion by precipitation with the 1,2,4,6-tetraphenyIpyridinium cation. In the titrations of 0.5 and 5.0 mg of thallium(II1) with 2 x 10-3 and 2 x M TPPA solutions, respectively, following the recommended procedure, typical potentiometric titration curves with steepness near the end-point (12 and 36 mV per 0.05 ml, respectively) were obtained. The coefficients of variation for the titration of different amounts of thallium in the recommended range (five titrations each) vary between 1.1 and 3.0% and the relative errors are between +0.7 and -2.5%. Applications The method has been applied satisfactorily to the determina- tion of thallium in sphalerites and zinc concentrates. The results of the direct potentiometric analysis shown in Table 3 are compared with those obtained by atomic absorption spectrometry.Good agreement was found between the two methods. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. References Catrall, R. W., and Freiser, H., Anal. Chem., 1971, 43, 1905. Catrall, R. W., and Chin-Poh Pui, Anal. Chem., 1975, 47, 93. Catrall, R. W., and Chin-Poh Pui, Anal. Chem., 1976,48,552. Catrall, R. W., and Chin-Poh Pui, Anal. Chim. Acta, 1976,83, 355. Catrall, R. W., and Chin-Poh Pui, Anal. Chim. Acta, 1976,87, 419. Alexander, P. W., and Joseph, J. P., Talanta, 1981, 28, 931. Ortuno, J. A., PCrez Ruiz, T., and Sanchez-Pedrefio, C., Anal. Chim. Acta, in the press. Burger, K., and Petho, G., Anal. Chim. Acta, 1979, 107, 113. PCrez Ruiz, T., Sanchez-Pedreiio, C., and Ortufio, J. A., Analyst, 1982, 107, 185. Koryta, J., and Stulik, K., “Ion-selective Electrodes,” Second Edition, Cambridge University Press, Cambridge, 1983, p. 30. Chadwick, T. C., Anal. Chem., 1976, 48, 1201. Kinnunen, J., and Wennerstrand, B., Chemist Analyst, 1957, 46, 92. Fiedler, U., and Rfiiitka, J., Anal. Chim. Acta, 1973, 67, 179. Catrall, R. W., Drew, D. M., and Hamilton, I. C., Anal. Chim. Acta, 1975, 76, 269. Bailey, P. L., “Analysis with Ion-Selective Electrodes,” Heyden, London, 1976, p. 48.