Anal. Chem. 1997, 69, 3693-3696
PVC-Based Hexathia-18-crown-6-tetraone Sensor for Mercury(II) Ions Ali Reza Fakhari,† Mohammad Reza Ganjali,‡ and Mojtaba Shamsipur*,§
Departments of Chemistry, Tarbiat Modarres University, Tehran, Iran, Tehran University, Tehran, Iran, and Razi University, Kermanshah, Iran
A PVC membrane sensor for mercury(II) ions based on hexathia-18-crown-6-tetraone as membrane carrier was prepared. The sensor exhibits a Nernstian response for Hg2+ ions over a wide concentration range (1.0 × 10-34.0 × 10-6 M). It has a relatively fast response time and can be used for at least 3 months without any considerable divergence in potentials. The proposed sensor revealed very good selectivities for Hg2+ over a wide variety of other metal ions and could be used in a pH range of 0.5-2.0. It was used as an indicator electrode in potentiometric titration of mercury ions. Crown ethers1 are among the first synthetic complexing agents introduced to bind strongly and selectively to alkali metal ions,2,3 and thus they have been widely used as suitable neutral carriers for the selective and efficient transport of alkali cations through liquid membranes4-6 and for constructing membrane-selective electrodes for these cations.7-14 However, the ion-selective electrodes based on ordinary crown ethers for heavy metal ions such as Hg2+ ion have not been reported in the literature, mainly due to the strong interfering effect of alkali and alkaline earth cations. It is well known that the substitution of some oxygen atoms of crown ethers by sulfur atoms drastically decreases the formation constant of their alkali and alkaline earth complexes.3,4,15,16 On the other hand, thiacrown ethers show a considerable increase
in the stability of soft metal ions such as Ag+, Tl+, and Hg2+ ions in solution.17-21 Thus, the coordination chemistry of thiacrown ethers has received an intensive effort in recent years; complexes of macrocyclic thiacrowns with different ring size and number of sulfur atoms for coordination with a variety of transition and heavy metal ions have been reported.19-24 In recent years, we have used some aza-substituted crown ethers as neutral carriers in membrane transport and in PVC electrode membrane studies of some transition and heavy metal ions.25-30 Due to the lack of efficient commercial mercury(II) ionselective electrodes and even quite sparse literature reports on such electrodes,31 we were also interested in the preparation of a PVC-based thiacrown sensor for Hg2+ ions. In this paper, we report the use of 1,4,7,10,13,16-hexathiacyclooctadecane-2,3,11,12-tetraone (HT18C6TO, I) as an excellent neutral carrier in
†
Tarbiat Modarres University. Tehran University. § Razi University. (1) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017. (2) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J.; Sen, D. Chem. Rev. 1985, 85, 271. (3) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1991, 91, 1721. (4) Izatt, R. M.; Clark, G. A.; Bradshaw, J. S.; Lamb, J. D.; Christensen, J. J. Sep. Purif. Methods 1986, 15, 121. (5) Dozol, M. Possible Applications of Crown Ethers to Metal Extraction Using Liquid Membrane Technology. A Literature Survey. In New Separation Chemistry Techniques for Radio Active Waste and other Specific Applications; Cecille, L., Casaraci, M., Pietrelli, L., Eds.; Elsevier: Amsterdam, 1991; pp 163-172. (6) Visser, H. C.; Reinhoudt, D. N.; de Jong, F. Chem. Soc. Rev. 1994, 75. (7) Moody, G. J.; Saad, B. B.; Thomas, J. D. R. Analyst 1989, 114, 15. (8) Tuladhar, S. M.; Williams, G.; D’Silva, C. Anal. Chem. 1991, 63, 2282. (9) Lukyanenko, N. G.; Titova, N. Yu.; Karpinchik, O. S.; Melnik, O. T. Anal. Chim. Acta 1992, 259, 145. (10) Shih, J. S. J. Chin. Chem. Soc. 1992, 39, 551. (11) Suzuki, K.; Yamada, H.; Sato, K.; Watanabe, K.; Hisamoto H.; Tobe, Y.; Kobiro, K. Anal. Chem. 1993, 65, 3404. (12) Li, A.; Zhang, Z.; Wu, Y.; An, H.; Izatt, R. M.; Bradshaw, J. S. J. Membr. Sci. 1993, 15, 317. (13) Ohki, A.; Lu, J. P.; Huang, X.; Bartsch, R. A. Anal. Chem. 1994, 66, 4332. (14) Faulkner, S.; Kataky, R.; Parker, D.; Teasdale, A. J. Chem. Soc., Perkin Trans. 2 1995, 1761. (15) Frensdorff, H. K. J. Am. Chem. Soc. 1971, 93, 600. (16) Rounaghi, G.; Popov, A. I. J. Inorg. Nucl. Chem. 1981, 43, 911. ‡
S0003-2700(97)00133-9 CCC: $14.00
© 1997 American Chemical Society
construction of a mercury(II)-PVC membrane electrode. Substitution of all donating oxygens of an 18-crown-6 ring by sulfur atoms is expected to increase both the stability and the selectivity (17) Izatt, R. M.; Terry, R. E.; Hansen, L. D.; Avondet, A. G.; Bradshaw, J. S.; Dalley, N. K.; Jensen, T. E.; Christensen, J. J.; Haymore, B. L. Inorg. Chim. Acta 1978, 30, 1. (18) Lamb, J. D.; Izatt, R. M.; Swain, C. S.; Christensen, J. J. J. Am. Chem. Soc. 1980, 102, 475. (19) Craig, A. S.; Kataky, R.; Mathews, R. C.; Parker, D.; Fergusen, G.; Lough, A.; Adams, H.; Bailey, N.; Schneider, H. J. Chem. Soc., Perkin Trans. 1990, 1523. (20) Saito, K.; Murakami, S.; Muromatsu, A.; Sekido, E. Polyhedron 1993, 12, 1587. (21) Alberto, R.; Nef, W.; Smith, A.; Kaden, T. A.; Neuburger, M.; Zehnder, M.; Frey, A.; Abram, U.; August Schubiger, P. Inorg. Chem. 1996, 35, 3420. (22) Blake, A. J.; Schroder, M. Adv. Inorg. Chem. 1990, 35, 1. (23) Rawle, S. C.; Cooper, S. R. Struct. Bonding 1991, 72, 1. (24) Housecroft, C. E. Coord. Chem. Rev. 1992, 115, 141. (25) Dadfarnia, S.; Shamsipur, M. Bull. Chem. Soc. Jpn. 1992, 65, 2779. (26) Dadfarnia, S.; Shamsipur, M. J. Membr. Sci. 1992, 75, 61. (27) Akhond, M.; Shamsipur, M. J. Chin. Chem. Soc. 1996, 43, 225. (28) Akhond, M.; Shamsipur, M. J. Membr. Sci. 1996, 117, 221. (29) Tavakkoli, N.; Shamsipur, M. Anal. Lett. 1996, 29, 2269. (30) Shamsipur, M.; Akhond, M. Bull. Chem. Soc. Jpn., in press. (31) Lai, M. T.; Shih, J. S. Analyst 1986, 111, 891.
Analytical Chemistry, Vol. 69, No. 18, September 15, 1997 3693
of its mercury complex over those of other metal ions.3,4 Moreover, the presence of carbonyl oxygen atoms may not only contribute to cation selectivity but also allow the crown ether to have properties more closely resembling those of naturally occurring ionophores such as valinomycin. It is interesting to note that the thiacrown ether used is very insoluble in water and most organic solvents. It was found soluble only in hot dimethyl sulfoxide. EXPERIMENTAL SECTION Reagents. Reagent grade dibutyl phthalate (DBP), dioctyl phthalate (DOP), acetophenone (AP), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), oleic acid, and high relative molecular weight PVC (all from Merck) were used as received. HT18C6TO and the nitrate salts of the cations used (all from Merck) were of the highest purity available and used without any further purification, except for vacuum drying over P2O5. Triply distilled deionized water was used throughout. Electrode Preparation. The general procedure to prepare the PVC membrane was to mix thoroughly 66 mg of powdered PVC, 114 mg of plasticizer AP, and 10 mg of additive oleic acid in 5 mL of THF. To this mixture was added 10 mg of ionophore HT18C6TO, which was already dissolved in 5 mL of DMSO, and the solution was mixed well. The resulting mixture was transferred into a glass dish of 2 cm diameter. The solvent was evaporated slowly until an oily concentrated mixture was obtained. A Pyrex tube (8-10 mm o.d.) was dipped into the mixture for about 10 s so that a nontransparent membrane of about 0.3 mm thickness was formed. The tube was then pulled out from the mixture and kept at room temperature for about 1 h. The tube was then filled with internal filling solution (1.0 × 10-3 M Hg(NO3)2). The electrode was finally conditioned for 24 h by soaking in a 1.0 × 10-2 M solution of mercuric nitrate. A silver/silver chloride coated wire was used as an internal reference electrode. The ratio of various ingredients, concentration of equilibrating solutions, and time of contact were optimized to provide membranes which result in reproducible, noiseless, and stable potentials. Emf Measurements. All emf measurements were carried out with the following assembly:
Ag-AgCl|3 M KCl|internal solution (1.0 × 10-3 M Hg(NO3)2 + 1.0 × 10-2 M HNO3)| PVC membrane|test solution|Hg-Hg2Cl2, KCl (satd)
A Corning ion analyzer 250 pH/mV meter was used for the potential measurements at 25.0 ( 0.1 °C. The emf observations were made relative to a double-junction saturated calomel electrode (SCE, Philips) with the chamber filled with an ammonium nitrate solution. Activities were calculated according to the Debye-Hu¨ckel procedure.32 In all cases, a 0.01 M HNO3 solution was used as electrolyte medium. RESULTS AND DISCUSSION In preliminary experiments, I was used as a neutral carrier to prepare PVC membrane ion-selective electrodes for a wide variety of metal ions, including alkali, alkaline earth, transition, and heavy metal ions. The potential responses of various ion-selective (32) Kamata, S.; Bhale A.; Fukunaga, Y.; Murata, A. Anal. Chem. 1988, 60, 2464.
3694 Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
Figure 1. Potential response of various ion-selective membranes based on HT18C6TO.
electrodes based on the macrocycle used are shown in Figure 1. Among these ions, those of hard acid character (i.e., alkali, alkaline earth, and UO22+ ions)33 show negligible responses, due to their very weak interactions with the sulfur atoms of the ring as soft bases. On the other hand, the metal ions of soft acid character (i.e., Hg2+, Ag+, and Tl+ ions)33 reveal the most sensitive response in the series, among which Hg2+ ion, with the best fitting condition for the cavity of I34 and a high charge density, provides the most suitable ion-selective electrode. Finally, the transition metal ions of intermediate acid character show potential responses between the above two limiting cases, with the best response for Pb2+ ion, due to its proper size for the ligand cavity. The results thus obtained indicate that the Hg2+ ions are more easily attracted to the PVC-crown ether membrane, resulting in a Nernstian potential-concentration response in a wide range. It is well known that the sensitivity and selectivity obtained for a given ionophore depend significantly on the membrane composition and the nature of solvent mediator and additives used.29,35-37 Thus, the influences of the membrane composition, nature and amount of plasticizer, and amount of oleic acid as a lipophilic additive on the potential response of the Hg2+ sensor were investigated, and the results are summarized in Table 1. It is seen that, among three different plasticizers used, the use of 57% AP in the presence of 5% ionophore and 5% oleic acid (no. 8, Table 1) results in the best sensitivity, with a Nernstian slope of (33) Hancock, R. D.; Martell, A. E. J. Chem. Educ. 1996, 73, 654. (34) Pedersen, C. J.; Frensdorff, H. K. Angew Chem., Int. Ed. Engl. 1972, 11, 16. (35) Koryta, J. Anal. Chim. Acta 1990, 233, 1. (36) Wolfbeis, O. S. Anal. Chim. Acta 1991, 250, 181. (37) Rosatzin, T.; Bakker, E.; Suzuki, K.; Simon, W. Anal. Chim. Acta 1993, 280, 197.
Table 1. Optimization of Membrane Ingredients composition (%) no. PVC plasticizer HT18C6DO oleic acid slope (mV/decade) 1 2 3 4 5 6 7 8 9 10 11 12
33 33 33 33 33 33 33 33 33 40 50 60
DBP, 66 DBP, 64 DBP, 62 DBP, 60 DOP, 62 AP, 62 AP, 60 AP, 57 AP, 52 AP, 50 AP, 40 AP, 30
1 3 5 7 5 5 5 5 5 5 5 5
2 5 10 5 5 5
10.0 20.0 25.0 25.0 25.0 28.0 28.5 29.0 29.0 28.5 28.0 26.0
29 mV/decade. It should be noted that the presence of lipophilic anions in cation-selective membrane electrodes not only diminishes the ohmic resistance38 and enhances the response behavior and selectivity39 but also, in cases where the extraction capability is poor, increases the sensitivity of the membrane electrodes.40 The proposed sensor was also examined at different concentrations of inner reference solution. It was found that the variation of the concentration of the internal solution (in the range of 1.0 × 10-3-1.0 × 10-5 M) does not cause any significant difference in the corresponding potential response, except for an expected change in the intercept of the resulting Nernstian plots. A 1.0 × 10-3 M concentration of the reference solution is quite appropriate for smooth functioning of the electrode system. The optimum equilibration time for the membrane sensor is 24 h. It generates stable potentials when placed in contact with Hg2+ solutions. The critical response characteristic of the electrode was assessed according to IUPAC recommendations.41 The emf response of the membrane at varying concentration of Hg2+ ion (Figure 2) indicates a rectilinear range from 1.0 × 10-3 to 4.0 × 10-6 M. The slopes of the calibration curves were 29.0 ( 0.3 mV/decade of Hg2+ concentration. The limit of detection, as determined from the intersection of the two extrapolated segments of the calibration graph, was 1.3 × 10-6 M (0.26 ppm). The average time required for the Hg2+ ion sensor to reach a potential within (1 mV of the final equilibrium value after successive immersion of a series of mercury(II) ion solutions, each having a 10-fold difference in concentration, was measured. The static response time of the membrane sensor thus obtained was 45 s for concentrations e1.0 × 10-3 M, and potentials stayed constant for more than 5 min, after which only a very slow divergence within the resolution of the pH meter (i.e., 0.1 mV) was recorded. The standard deviation of 10 replicate measurements is (0.3 mV. The sensing behavior of the membrane remained unchanged when the potentials recorded either from low to high concentrations or vise versa. The membrane sensors prepared could be used for at least 3 months without any measurable divergence. (38) Ammann, D.; Pretsch, E.; Simon, W.; Lindler, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119. (39) Huser, M.; Gehrig, P. M.; Morf, W. E.; Simon, W.; Lindler, E.; Jeney, J.; Toth, K.; Pungor, E. Anal. Chem. 1991, 63, 1380. (40) Ammann, D.; Morf, W. E.; Anken, P.; Meier P. C.; Pretsch, E.; Simon, W. Ion-Sel. Electrode Rev. 1983, 5, 3. (41) IUPAC Analytical Chemistry Devision, Commission on Analytical Nommenclature. Recommendations for Nomenclature of Ion Selective Electrodes. Pure Appl. Chem. 1976, 48, 127.
Figure 2. Calibration graph for the Hg2+ sensor. Table 2. Selectivity Coefficients (kHgpot) of Various Interfering Ions (Mn+) Mn+
kHgpot
Mn+
kHgpot
Fe3+ Mg2+ Ca2+ Sr2+ Ba2+ UO22+ Co2+ Ni2+ Cu2+ Zn2+
5.9 × 10-4 1.0 × 10-5 1.0 × 10-5 1.2 × 10-5 1.3 × 10-5 2.7 × 10-4 2.1 × 10-4 2.6 × 10-4 2.2 × 10-4 2.1 × 10-4
Cd2+ Pb2+ Li+ Na+ K+ Rb+ Cs+ NH4+ Tl+ Ag+
3.1 × 10-4 5.2 × 10-4 3.9 × 10-3 4.0 × 10-3 4.1 × 10-3 3.6 × 10-3 3.1 × 10-3 2.0 × 10-3 6.9 × 10-3 7.1 × 10-3
The influence of the pH of the test solution on the potential response of the sensor was tested at 1.0 × 10-4 M Hg2+ concentration. Potentials were found to stay constant from pH 0.5 to 2.0, beyond which a gradual drift was observed. The observed drift at higher pH values could be due to the formation of some hydroxy complexes of Hg2+ ion in solution. Thus, the above range may be taken as the working pH range of the proposed sensor. Perhaps the most important characteristic of a membrane sensor is its relative response for the primary cation over other cations present in the solution, which is expressed in terms of potentiometric selectivity coefficients (kHgpot), which were evaluated graphically by the mixed solution method42 from potential measurements on solutions containing a fixed concentration of Hg2+ ion (1.0 × 10-4 M) and varying amounts of the interfering ions (Mn+), according to the equation
kHgpotaM2/n ) aHg{exp[(E2 - E1)F/RT]} - aHg
(1)
where E1 and E2 are the electrode potentials for the solution of Hg2+ ions alone and for the solution containing interfering ions and mercury(II) ions, respectively. According to eq 1, the kHgpot values for diverse ions can be evaluated from the slope of the (42) Srinivasan, K.; Rechnitz, G. A. Anal. Chem. 1969, 41, 1203.
Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
3695
Figure 3. Potentiometric titration curve of 20 mL of 5.0 × 10-2 M Hg2+ solution with 0.4 M KI, using the proposed sensor as an indicator electrode.
graph of aHg{exp[(E2 - E1)F/RT]} - aHg vs aM2/n. The resulting values of the selectivity coefficients are summarized in Table 2. From the data given in Table 2, it is immediately obvious that, for polyvalent cations, the selectivity coefficients are in the order
3696
Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
of 10-4 or smaller, which seems to indicate that these metal ions have negligible disturbance of the functioning of the Hg2+ membrane sensor. On the other hand, the selectivity coefficients seem to be somehow larger in the case of univalent cations (in the order of 10-3). However, it should be noted that such deceptively larger coefficients arise from the term a2/n in eq 1; the smaller the charge of interfering ion, n, the smaller the selectivity coefficient, kHgpot. Thus, despite their larger selectivity coefficients, the univalent cations used would not disturb the functioning of the sensor significantly. The proposed Hg2+ membrane sensor was found to work well under laboratory conditions. It was successfully applied to the titration of a Hg2+ ion solution with potassium iodide, and the resulting titration curve is shown in Figure 3. As shown, the amount of Hg2+ ions in solution can be accurately determined with the electrode. It is interesting to note that the resulting titration curve is unsymmetrical, as it was noticed before.31 Before the titration’s end-point, the measured potential shows an usual logarithmic change with amount (mL) of titrant added, while the potential response after the end-point will remain almost constant, due to too low of a concentration of free Hg2+ ion in solution. Received for review February 5, 1997. Accepted May 19, 1997.X AC970133B X
Abstract published in Advance ACS Abstracts, July 1, 1997.
