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Selective spectrophotometric determination of thallium through ligand exchange at a solid surface
Analytica OElsevier
Chimica Acta, 97 (1978) 177-179 Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
Short Communication
SELECTIVE SPECTROPHOTOMETRIC DETERMINATION OF THALLIUM THROUGH LIGAND EXCHANGE AT A SOLID SURFACE
M. K. GADLA and M. C!. MJZHRA* Environmental Contaminants Research Group. Moncton, Moncton. N.B. EIA 3E9 (Canada)
Chemistry
Department,
Universitti
de
(Received 11th May 1977)
The toxicity of thallium has been rated higher than that of mercury or organomercurials [l] . Routine monitoring of possible thallium contamination is therefore of some importance, e .g. in liquid wastes from mines, or processing or coal-burning plants. Some of the existing procedures for traces cf thallium include spectrophotometry of its ion-association compounds [Z, 31, fluorimetry [ 3,4], electrometry [ 5,6] and atomic absorption spectrometry 173. Many of these are insufficiently sensitive or suffer from interferences, while others require carefully controlled conditions. The present communication describes a new spectrophotometric procedure for thallium which is based on ligand exchange at a solid surface. The exchange in aqueous samples is effected with solid bis-(2,4,6-tris(Zpyridyl)-s-triazine) iron(H) tetraphenylborate reagent. The solid reagent selectively exchanges its colored bis(2,4,6-tris(2-pyridyl)-s-triazine) iron cation for thallium ion, and the response is linear in the range’5-40 ppm thallium.
Experimental Reagents. The solid reagent was synthesized by mixing equal volumes of iron(triazine reagent (0.01 M) and sodium tetraphenylborate (0.02 M); careful slow mixing produced a bluish white precipitate. The iron(II)triaztie reagent itseif was prepared by mixing, in 1:2 proportion, equimolar iron(I1) and 2,4,6-t&(2-pyridyl)-s-triazine solutions. The precipitated complex was washed liberally with deionized distilled water after it had aged
for 24 h at room temperature. It was finally washed once with ethanol, dried under partial vacuum and stored in a dark-colored bottle. All other reagents were of analytical grade or better and were employed without further purification. Deionized distilled water was used throughout. Stock solutions of thallium and other cations were prepared by accurate weighing of nitrate or sulfate salts. Working solutions were obtained by suitable dilution. Acetate buffers were employed for pH control where necessary.
Proceclure. Dilute appropriately an aliquot of solution containing 0.252 mg of thallium arrdadjust the pH to 6.8 while making up the final volume to 50 ml in a standard flask. Add about 200 mg of the solid reagent and stir magnetically for about 1 h. Allow the precipitate to settle and carefully fiiter a 20-d aliquot through a fine filter paper (Whatman-542). Prepare a blank in a similar manner. Fill a lo-cm cylindrical quartz end-window spectrophotometric cell and record the absorbance at 596 nm against the blank. The Bausch and Lomb Spectronic-‘70 spectrophotometer used holds either rectanmar or cylindrical cells for absorption measurements. The pH was measured with a Radiometer pH meter model-26. Results und discussion
The solid reagent exchange response toward thallium was linear in the 5-40 ppm range, the corresponding absorbances being 0.03-0.335, with an average relative standard deviation of 1.2% over the range 20-40 ppm (for 5 replicate determinations at 5 concentration levels). Thallium can be determined at concentrations oi 5 ppm with a relative standard deviation of 6.6% (n = 5), but quantification remains uncertain at lower levels because of the large a.bsorbance errors. Minor changes in the acidity or alkalinity have no effect on the system; absorbances remain essentially constant in the pH range 4-8, but decrease gradually below pH 3 or above pH 9. The proposed solid reagent is reasonably selective for thallium. When a 20-ppm thallium solution was analysed in the presence of up to 200 ppm I of K+, Rb+, Cs*, NH4 ‘, Hg ‘*, Ag+, Cu”‘, Cd’+, Co’+, Fe3*, VO**, Cl-, SOG2-, COs2-, HC03-, N03-, and CH,C!OO; the absorption data were the same as in the case of a pure thaLlium solution. This selective exchange behavior for thallium is significant, since some other tetraphenylborate reagents examined have shown strong reactivity towards Hg*+ and Ag’ ions, in particular [8,9]_ The present results show that a selective analytical reagent can be tailored for a particular model ion by proper selection of the ion-association complex in the solid reagent, and offer further insight into the analytical chemistry of the tetraphenylborate reagents. The analytical response of the solid reagent must result from the iondisplacement mechanism: [Fe(triazine), (TPB)*&,) + 2Tl* * [Fe(triazine)2]f&,1
+ 2TlTPB(,,
The sensitivity of the reagent depends on rapid exchange at the solid surface, on the molar absoTptivty of the released iron triazine c2tion, on the reaction of foreign ions with either of the ions in the solid reagent, and finally on the solubility of the reagent itself. The solid reagent is remarkably insoluble in water in the pH range 4-8, even on prolonged contact with water. Surprisingly, metats such as HgZ*, Ag*, Rb*, K+ and CS* which are known to for-m insoluble tetraphenylhorates do not
179
interfere. This may be rationalized on the basis that the solubility products of these alkali metal tetraphenylborates are higher than the solubility product of the solid reagent, so that these metals cannot displace the colored iron(II)triazine cation from the solid surface. The mercury and silver cations do form rather insoluble tetraphenylborates (pK,,, = 32 and 17, respectively), but these cations compete with iron for coordination with the triazine molecule, and displacement of the colored iron(II)-triazlne cation as such is not observed. Hence thallium ion alone is appropriately suited to displace the iron -triazine cation selectively from the solid surface. Thallium tetraphenylborate is insoluble (pK,, 3 14.5), so that Tl+ combines on the solid surface with tetraphenylborate. The iron(triazine complex is known to provide 2 sensitive determination of iron(I1) in the microgram range, with maximum absorbance at 596 nm [lo]. The supematant liquid after thallium exchange shows no change in the absorption characteristics, which means that the iron(II)--triazine cation is displaced intact in the exchange reaction. These observations suggest that the lower limit of 5 ppm for thallium observed under static conditions results essentially from the rate of exchange at the solid surface. This lower limit might be improved if dynamic column operation were substituted for the static approach. The solid reagent can be converted for column operation by attaching it to an inert support [S] . When samples of drinking water, source water and sea water were spiked with lo,20 or 30 ppm thallium, the recoveries were quantitative when the recommended method was applied. This study thus offers a rapid and straightforward method for thallium determinations. The authors acknowledge gratefully the financial support provided by the National Research Council of Canada and the Universiti de Moncton. REFEREEiCES 1 V. Zitko, Sci. TotaI Environ., 4 (1975) 185. 2 A. G. Fogg, C. Bourgess and T. D. Bums, Talanta, 18 (1971) 1175; Analyst, 347. 3 H. Onishi, Bull. Chem. Sot. Jpn., 30 (1957) 567.627. 4 G. F. Kirkbright, ‘I’. S. West and C. Woodward, Talanta, 12 (1965) 517. 5 W. T. Foley and R. F. Pottie, Anal. Chem., 28 (1956) 1011. 6 I. Sinko and S. Gomiscek, Microchim. Acta, (1972) 163. 7 A. J. Curry, J. F. Reed and A. R. Knorr, Andyst, 94 (1969) 744. 8 M. C. Mehra and P. O’Brien, Micro&rem. J., 19 (1974) 387. 9 M. C. Mehra and C. Bourque, Analusis, 3 (1975) 299. 10 P. F. Collins, H. Diehl and G. F. Smith, AnaI. Chem., 32 (1960) 1862.
98 (1973)
Chimica Acta, 97 (1978) 177-179 Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
Short Communication
SELECTIVE SPECTROPHOTOMETRIC DETERMINATION OF THALLIUM THROUGH LIGAND EXCHANGE AT A SOLID SURFACE
M. K. GADLA and M. C!. MJZHRA* Environmental Contaminants Research Group. Moncton, Moncton. N.B. EIA 3E9 (Canada)
Chemistry
Department,
Universitti
de
(Received 11th May 1977)
The toxicity of thallium has been rated higher than that of mercury or organomercurials [l] . Routine monitoring of possible thallium contamination is therefore of some importance, e .g. in liquid wastes from mines, or processing or coal-burning plants. Some of the existing procedures for traces cf thallium include spectrophotometry of its ion-association compounds [Z, 31, fluorimetry [ 3,4], electrometry [ 5,6] and atomic absorption spectrometry 173. Many of these are insufficiently sensitive or suffer from interferences, while others require carefully controlled conditions. The present communication describes a new spectrophotometric procedure for thallium which is based on ligand exchange at a solid surface. The exchange in aqueous samples is effected with solid bis-(2,4,6-tris(Zpyridyl)-s-triazine) iron(H) tetraphenylborate reagent. The solid reagent selectively exchanges its colored bis(2,4,6-tris(2-pyridyl)-s-triazine) iron cation for thallium ion, and the response is linear in the range’5-40 ppm thallium.
Experimental Reagents. The solid reagent was synthesized by mixing equal volumes of iron(triazine reagent (0.01 M) and sodium tetraphenylborate (0.02 M); careful slow mixing produced a bluish white precipitate. The iron(II)triaztie reagent itseif was prepared by mixing, in 1:2 proportion, equimolar iron(I1) and 2,4,6-t&(2-pyridyl)-s-triazine solutions. The precipitated complex was washed liberally with deionized distilled water after it had aged
for 24 h at room temperature. It was finally washed once with ethanol, dried under partial vacuum and stored in a dark-colored bottle. All other reagents were of analytical grade or better and were employed without further purification. Deionized distilled water was used throughout. Stock solutions of thallium and other cations were prepared by accurate weighing of nitrate or sulfate salts. Working solutions were obtained by suitable dilution. Acetate buffers were employed for pH control where necessary.
Proceclure. Dilute appropriately an aliquot of solution containing 0.252 mg of thallium arrdadjust the pH to 6.8 while making up the final volume to 50 ml in a standard flask. Add about 200 mg of the solid reagent and stir magnetically for about 1 h. Allow the precipitate to settle and carefully fiiter a 20-d aliquot through a fine filter paper (Whatman-542). Prepare a blank in a similar manner. Fill a lo-cm cylindrical quartz end-window spectrophotometric cell and record the absorbance at 596 nm against the blank. The Bausch and Lomb Spectronic-‘70 spectrophotometer used holds either rectanmar or cylindrical cells for absorption measurements. The pH was measured with a Radiometer pH meter model-26. Results und discussion
The solid reagent exchange response toward thallium was linear in the 5-40 ppm range, the corresponding absorbances being 0.03-0.335, with an average relative standard deviation of 1.2% over the range 20-40 ppm (for 5 replicate determinations at 5 concentration levels). Thallium can be determined at concentrations oi 5 ppm with a relative standard deviation of 6.6% (n = 5), but quantification remains uncertain at lower levels because of the large a.bsorbance errors. Minor changes in the acidity or alkalinity have no effect on the system; absorbances remain essentially constant in the pH range 4-8, but decrease gradually below pH 3 or above pH 9. The proposed solid reagent is reasonably selective for thallium. When a 20-ppm thallium solution was analysed in the presence of up to 200 ppm I of K+, Rb+, Cs*, NH4 ‘, Hg ‘*, Ag+, Cu”‘, Cd’+, Co’+, Fe3*, VO**, Cl-, SOG2-, COs2-, HC03-, N03-, and CH,C!OO; the absorption data were the same as in the case of a pure thaLlium solution. This selective exchange behavior for thallium is significant, since some other tetraphenylborate reagents examined have shown strong reactivity towards Hg*+ and Ag’ ions, in particular [8,9]_ The present results show that a selective analytical reagent can be tailored for a particular model ion by proper selection of the ion-association complex in the solid reagent, and offer further insight into the analytical chemistry of the tetraphenylborate reagents. The analytical response of the solid reagent must result from the iondisplacement mechanism: [Fe(triazine), (TPB)*&,) + 2Tl* * [Fe(triazine)2]f&,1
+ 2TlTPB(,,
The sensitivity of the reagent depends on rapid exchange at the solid surface, on the molar absoTptivty of the released iron triazine c2tion, on the reaction of foreign ions with either of the ions in the solid reagent, and finally on the solubility of the reagent itself. The solid reagent is remarkably insoluble in water in the pH range 4-8, even on prolonged contact with water. Surprisingly, metats such as HgZ*, Ag*, Rb*, K+ and CS* which are known to for-m insoluble tetraphenylhorates do not
179
interfere. This may be rationalized on the basis that the solubility products of these alkali metal tetraphenylborates are higher than the solubility product of the solid reagent, so that these metals cannot displace the colored iron(II)triazine cation from the solid surface. The mercury and silver cations do form rather insoluble tetraphenylborates (pK,,, = 32 and 17, respectively), but these cations compete with iron for coordination with the triazine molecule, and displacement of the colored iron(II)-triazlne cation as such is not observed. Hence thallium ion alone is appropriately suited to displace the iron -triazine cation selectively from the solid surface. Thallium tetraphenylborate is insoluble (pK,, 3 14.5), so that Tl+ combines on the solid surface with tetraphenylborate. The iron(triazine complex is known to provide 2 sensitive determination of iron(I1) in the microgram range, with maximum absorbance at 596 nm [lo]. The supematant liquid after thallium exchange shows no change in the absorption characteristics, which means that the iron(II)--triazine cation is displaced intact in the exchange reaction. These observations suggest that the lower limit of 5 ppm for thallium observed under static conditions results essentially from the rate of exchange at the solid surface. This lower limit might be improved if dynamic column operation were substituted for the static approach. The solid reagent can be converted for column operation by attaching it to an inert support [S] . When samples of drinking water, source water and sea water were spiked with lo,20 or 30 ppm thallium, the recoveries were quantitative when the recommended method was applied. This study thus offers a rapid and straightforward method for thallium determinations. The authors acknowledge gratefully the financial support provided by the National Research Council of Canada and the Universiti de Moncton. REFEREEiCES 1 V. Zitko, Sci. TotaI Environ., 4 (1975) 185. 2 A. G. Fogg, C. Bourgess and T. D. Bums, Talanta, 18 (1971) 1175; Analyst, 347. 3 H. Onishi, Bull. Chem. Sot. Jpn., 30 (1957) 567.627. 4 G. F. Kirkbright, ‘I’. S. West and C. Woodward, Talanta, 12 (1965) 517. 5 W. T. Foley and R. F. Pottie, Anal. Chem., 28 (1956) 1011. 6 I. Sinko and S. Gomiscek, Microchim. Acta, (1972) 163. 7 A. J. Curry, J. F. Reed and A. R. Knorr, Andyst, 94 (1969) 744. 8 M. C. Mehra and P. O’Brien, Micro&rem. J., 19 (1974) 387. 9 M. C. Mehra and C. Bourque, Analusis, 3 (1975) 299. 10 P. F. Collins, H. Diehl and G. F. Smith, AnaI. Chem., 32 (1960) 1862.
98 (1973)