Anal. Chem. 1999, 71, 1350-1353
Triiodide PVC Membrane Electrode Based on a Charge-Transfer Complex of Iodine with 2,4,6,8Tetraphenyl-2,4,6,8-tetraazabicyclo[3.3.0]octane Ahmad Rouhollahi† and Mojtaba Shamsipur*,‡
Departments of Chemistry, Tehran University, Tehran, Iran, and Razi University, Kermanshah, Iran
A novel triiodide ion-selective electrode based on a chargetransfer complex of iodine with 2,4,6,8-tetraphenyl2,4,6,8-tetraazabicyclo[3.3.0]octane as membrane carrier was prepared. The electrode has a linear dynamic range between 5.0 × 10-2 and 3.5 × 10-6 M, with a nearNernstian slope of 54.7 ( 0.8 mV decade-1 and a detection limit of 2.0 × 10-6 M. The potentiometric response is independent of the pH of the solution in the pH range 4.0-10.5. The electrode possesses the advantages of low resistance, short conditioning time, fast response, and, especially, very good selectivities over a wide variety of other anions. The electrode can be used for at least 10 months without any considerable divergence in potentials. It was used as an indicator electrode in potentiometric titration of triiodide ions. Ion-selective electrodes based on solvent polymeric membranes with incorporation of ion carriers are shown to be very useful tools for chemical, clinical, and environmental analyses as well as in process monitoring.1-3 In addition to the cation-selective electrodes, recently the design and synthesis of sensory molecules for anion-selective electrodes have become a challenging subject.4-6 The useful anion-selective electrodes reported in recent years have been mainly based on quaternary ammonium salts,7-10 metal complexes,11-14 organometallic compounds,15-18 and metalloporphyrin derivatives.19-23 †
Tehran University. Razi University. (1) Amman, D.; Morf, W. E.; Anker, P.; Meier, P. C.; Pretsch, E.; Simon, W. Ion-Sel. Electrode Rev. 1983, 5, 3. (2) Moody, G. J.; Saad, B. B.; Thomas, J. D. R. Sel. Electrode Rev. 1988, 10, 71. (3) Mayerhoff, M. E., Opdyche, M. N. Adv. Clin. Chem. 1986, 25, 1. (4) Chang, Q.; Park, S. B.; Kliza, D.; Cha, G. S.; Yim, H.; Meyerhoff, M. E. Am. Biotechnol. Lab. 1990, 8, 10. (5) Wotring, V. J.; Johnson, D. M.; Daunert, S.; Bachas, L. G. In Immunochemical Assay and Biosensor Technology for the 1990s: Nakamura, R. M., Kasahara, Y., Rechnitz, G. A., Eds.; American Society of Microbiology: New York, 1992; pp 355-376. (6) Rothmaier, M.; Simon, W. Anal. Chim. Acta 1993, 271, 135. (7) Umezawa, Y.; Kataoka, M.; Tokami, W. Anal. Chem. 1988, 60, 2392. (8) Wotring, V. J.; Johonson, D. M.; Bachas, L. G. Anal. Chem. 1990, 62, 1506. (9) Nomura, S. Analyst 1995, 120, 503. (10) Ozawa, S.; Miyagi, H.; Shibata, Y.; Oki, N.; Kunitake, T.; Keller, W. E. Anal. Chem. 1995, 68, 4149. (11) Yuan, R.; Chai, Y. Q.; Liu, D.; Gao, D.; Li, J. Z.; Ya, R. Q. Anal. Chem. 1993, 65, 2572. (12) Gao, D.; Li, J. Z.; Yu, R. Q. Anal. Chem. 1994, 66, 2245. (13) Ying, M.; Yuan, R.; Zhang, X. M.; Song, Y. Q.; Li, Z. Q.; Shen, G. L.; Yu, R. Q. Analyst 1997, 122, 1143. ‡
1350 Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
It is well known that the use of metal-ion-containing ionophores as anion carriers in solvent polymeric membrane electrodes may result in potentiometric anion selectivity patterns significantly different from the so-called Hofmeister selectivity sequence (i.e., selectivity based solely on the lipophilicity of anions).24 Such antiHofmeister selectivity order is believed to originate from the selective axial ligation between the metal ion centers and certain anions.22 Thus, the nature of the central metal is expected to play an important role in the realization of the selectivity patterns observed. We have recently introduced several PVC-based membrane sensors for K+,25 Be2+,26 Zn2+,27 Pb2+,28-31 and Hg2+ ions32 using different noncyclic and macrocyclic ligands as ion carrier. Due to the vital importance of iodide determination in chemical, industrial, and clinical analyses, we were interested in preparation of a new solvent polymeric membrane sensor for selective monitoring of iodide (or triiodide) ion in solution. In this paper we report a highly selective PVC membrane electrode for I3- ion based on a chargetransfer complex between iodine and 2,4,6,8-tetraphenyl-2,4,6,8tetraazabicyclo[3.3.0]octane as (TPTABO‚I+)I3-. In a recent publication,wehavereportedthespectroscopicstudyof(TPTABO‚I+)I3in chloroform solution and its isolation and characterization in (14) Li, Z. Q.; Yuan, R.; Ying, M.; Song, Y. Q.; Shen, G. L.; Yu, R. Q. Anal. Lett. 1997, 30, 1455. (15) Chaniotakis, N. A.; Jurkschat, K.; Ruhlemann, A. Anal. Chim. Acta 1993, 282, 345. (16) Hisamoto, H.; Siswanta, D.; Nishihara, H.; Suzuki, K. Anal. Chim. Acta 1995, 304, 171. (17) Badr, I. H. A.; Meyerhoff, M. E.; Hassan, S. S. M. Anal. Chem. 1995, 67, 2613. (18) Rothmaier, M.; Schaller, U.; Morf, W. E.; Pretsch, E. Anal. Chim. Acta 1996, 327, 17. (19) Jyo, A.; Minakami, R.; Kanda, Y.; Egawa, H Sens. Actuators B 1993, 13/ 14, 200. (20) Blair, T. L.; Allen, J. R.; Daunert, S.; Bachas, L. G. Anal. Chem. 1993, 65, 2155. (21) Gao, D.; Gu, J.; Yu, R. Q.; Zhen, G. D. Analyst 1995, 120, 499. (22) Malinowska, E.; Meyerhoff, M. E. Anal. Chim. Acta 1995, 300, 33. (23) Sun, C.; Zhao, J.; Xu, H.; Sun, Y.; Zhang, X.; Shen, J. Talanta 1998, 46, 15. (24) Hofmeister, P. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247. (25) Ganjali, M. R.; Moghimi, A.; Buchanan, G. W.; Shamsipur, M. J. Incl. Phenom. 1998, 30, 29. (26) Ganjali, M. R.; Moghimi, A.; Shamsipur, M. Anal. Chem. 1998, 70, 5259. (27) Pouretedal, H. R.; Shamsipur, M. Fresenius J. Anal. Chem. 1998, 362, 415. (28) Tavakkoli, N.; Shamsipur, M. Anal. Lett. 1996, 29, 2269. (29) Tavakkoli, N.; Khojasteh, Z.; Sharghi, H.; Shamsipur, M. Anal. Chim. Acta 1998, 360, 203. (30) Rouhollahi, A.; Ganjali, M. R.; Shamsipur, M. Talanta 1998, 46, 1341. (31) Ganjali, M. R.; Rohollahi, A.; Mardan A. R.; Hamzeloo, M.; Moghimi, A.; Shamsipur, M. Microchem. J. 1998, 60, 122. (32) Fakhari, A. R.; Ganjali, M. R.; Shamsipur, M. Anal. Chem. 1997, 69, 3693. 10.1021/ac981077x CCC: $18.00
© 1999 American Chemical Society Published on Web 02/24/1999
crystalline form.33 It is noteworthy that, to the best of our knowledge, this is the first triiodide ion-selective electrode ever reported in the literature. EXPERIMENTAL SECTION Reagents. Reagent grade benzyl acetate (BA), oleic acid, iodine, tetrahydrofuran (THF), and high relative molecular weight PVC (all from Merck) were used as received. TPTABO (I) and Ph Ph
Ph Ph
its iodine charge-transfer complex (TPTABO‚I+)I3- were prepared as described elsewhere.33,34 The resulting (TPTABO‚I+)I3- solid complex was recrystallized from reagent grade acetone and vacuum-dried before use. The potassium and sodium salts of all anions 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 60 mg of powdered PVC, 100 mg of plasticizer BA, 10 mg of additive oleic acid, and 10 mg of ionophore (TPTABO‚I+)I3- in 10 mL of THF. 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 (3-5 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 KI + 1.0 × 10-3 M I2). The electrode was finally conditioned by soaking in 1.0 × 10-2 M triiodide solutions for 1 h. A silver/silver chloride wire was used as an internal reference electrode. EMF Measurements. All EMF measurements were carried out with the following assembly:
Ag-AgCl|3 M KCl|internal solution (1.0 × 10-3 M KI + 1.0 × 10-3 M I2)|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.35 RESULTS AND DISCUSSION It is well established that the selective interaction between a given analyte anion and a lipophilic carrier within the membrane (33) Rouhollahi, A.; Kakanejadifard, A.; Farnia, S. M. F.; Shamsipur, M. Polish J. Chem. 1997, 71, 731. (34) Farnia, M.; Kakanejadifard, A.; Karimi, S.; Todaro, L. J. Iran. J. Chem., Chem. Eng. 1993, 12, 57. (35) Kamata, S.; Bhale, A.; Fukunaga, Y.; Murata, H. Anal. Chem. 1998, 60, 2464.
Figure 1. Potential response of various anion-selective membranes based on (TPTABO‚I+)I3-.
is essential for the development of anion-selective polymeric membranes that exhibit anti-Hofmeister potentiometric selectivity patterns.11-23 In the case of organometallic compounds15-18 and metalloporphyrin derivatives,19-22 the anion selectivity is mainly governed by the specific interaction between the central metal and anions rather than the lipophilicity of the anions or simple opposite charge interactions with anionis. In this work, we were interested in investigating the possibility of the use of a 2:1 charge-transfer complex between I2 and TPTABO (I), formulated as (TPTABO‚I+)I3-, as an ion carrier in PVC-based membranes for I3- ion. The charge-transfer complex shows a high stability in chloroform solution (log Kf ) 8.07 at 25 °C) and possesses a high degree of lipophilicity.33 It is interesting to note that, to the best of our knowledge, there is no literature report on the use of charge-transfer complexes in the preparation of ion-selective electrodes. Thus, in preliminary experiments, (TPTABO‚I+)I3- was used as a neutral carrier to prepare PVC membrane ion-selective electrodes for a wide variety of anions. The potential responses of various anion-selective electrodes based on the charge-transfer complex used are shown in Figure 1. As seen, with the exception of I3- ion, all anions tested show negligible responses in the concentration range 1.0 × 10-1-1.0 × 10-7 M, due to their very weak interactions with the membrane. Noteworthy, among these anions, I- shows a higher potential response at higher concentrations. This is most probably due to the oxidation of I-, to a limited extent, and consequent formation of a low-level amount of I3- ion Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
1351
Table 1. Optimization of Membrane Ingredients
no.
PVC
1 2 3 4 5 6 7
56.2 35.3 52.9 44.5 33.3 31.6 32.4 a
composition plasticizer ionophore 37.6 58.8 35.3 44.5 55.7 52.6 54.1
oleic acid
slope (mV decade-1)a
5.9 5.9 5.5 5.5 10.5 5.4
3.0 6.3 15.1 28.6 54.8 33.2 50.9
6.2 5.9 5.5 5.5 5.3 8.1
RSD on the values lies within (0.8. Figure 2. Response-time profile of the I3- ion-selective electrode.
solution.36 However, the triiodide ion results in a near-Nernstian potential response at a wide concentration range. It is well understood that the sensitivity, linearity, and selectivity obtained for a given ionophore depend significantly on the membrane composition and nature and amount of additive used.1,2,13,17,25-32,37,38 Thus, the influence of the membrane composition and amount of oleic acid as a lipophilic additive on the potential response of the I3- sensor was investigated, and the results are summarized in Table 1. It is seen that, among different membrane compositions tested, membrane 5 with 5.5% ionophore (TPTABO‚I+)I3-, 33.3% PVC, 55.7% plasticizer BA, and 5.5% additive oleic acid offers in the best sensitivity, with a nearNernstian slope of 55 mV/decade. It has been shown that the incorporation of lipophilic additives not only can significantly enhance the potentiometric anion selectivity of membrane electrodes17,37,39 but also diminishes the ohmic resistance and increases the response behavior.1,14 We have recently found that the use of oleic acid as a very efficient lipophilic additive can have an important influence on the response properties of a variety of membrane-based ion-selective electrodes.26,30-32 The critical response characteristics of the electrode were assessed according to IUPAC recommendations.40 The EMF response of the PVC membrane at varying concentrations of triiodide ion (Figure 1) indicates a rectilinear range from 5.0 × 10-2 to 3.5 × 10-6 M (r g 0.999). The slopes of the calibration curves were 54.7 ( 0.8 mV/decade of I3- ion concentration. The limit of detection, as determined from the intersection of the two extrapolated segments of the calibration graph, was 2.0 × 10-6 M. The average time required for the membrane electrode to reach a potential response within (1 mV of the final equilibrium value after successive immersion of a series of I3- solutions, each having a 10-fold difference in concentration, was investigated. A potential-time plot for the electrode response is given in Figure 2. The static response time of the PVC membrane thus obtained was 50 s for concentrations e1.0 × 10-3 M. It should be noted that the equilibrium potentials essentially remained constant for about 10 min, after which only a very small divergence within (36) Christian, G. D. Analytical Chemistry; Wiley & Sons: New York, 1986. (37) Bakker, E.; Malinowska, E.; Schiller, R. D.; Meterhoff, M. E. Talanta 1994, 41, 881. (38) Kim, W.; Sung, D. D.; Cha, G. S.; Park, S. B. Analyst 1998, 123, 379. (39) Schaller, U.; Bakker, E.; Spichiger, D. N.; Pretsch, E. Anal. Chem. 1994, 66, 391. (40) IUPAC Analytical Chemistry Devision, Commission on Analytical Nommenclatue Recommendations for Nommenclature of Ion Selective Electrodes. Pure Appl. Chem. 1976, 48, 127.
1352 Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
Figure 3. Effect of pH of test solution on the potential response of the I3- ion-selective electrode.
the resolution of the pH meter was recorded. The standard deviation of 10 replicate measurements is (0.2 mV. The membrane sensors prepared could be used for at least 10 months without any measurable divergence. The pH dependence of the potential response of the proposed electrode in the pH range 2-11 was tested, and the results are shown in Figure 3. As can be seen, the potential response remains almost constant over the pH range 4.0-10.5. In highly alkaline media, the potential decreased sharply, most probably due to the disproportion reaction between I2 and OH- resulting in the formation of hypoiodate and iodide,36 both of which are insensitive to the membrane electrode. On the other hand, at pH values lower than 4.0, the electrode potential rises sharply. This is probably due to simultaneous response of the electrode to oppositely charged H3O+ and I3- ions.41 The contribution of H3O+ to the potential counteracts that of I3-. Perhaps the most important characteristic of an anion-selective membrane electrode is its relative response for the primary anion over other anions present in solution, which is usually expressed pot in terms of potentiometric selectivity coefficient (KA,B ). However, the methods based on the Nicolsky-Eisenman equation for the determination of potentiometric selectivity coefficients (e.g., the fixed interference method and the mixed solution method) suffer some limitations in terms of values for ions of unequal charges, non-Nernstian behavior of interfering ions, and activity dependence of values.42,43 Thus, in this work, the recommended matched potential method,42-44 which is totally independent of the Nicolsky-Eisenman equation, was used to overcome the abovestated difficulties. According to this method,44 a specified activity (41) Sun, B.; Fitch, P. G. Electroanalysis 1997, 9, 494. (42) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1995, 67, 507. (43) Bakker, E. Electroanalysis 1997, 9, 7. (44) Gadzekpo, V. P.; Christian, G. D. Anal. Chim. Acta 1984, 164, 279.
Table 2. Selectivity Coefficients (KMPM A,B ) of Various Interfering Anions anion
KMPM A,B
anion
KMPM A,B
FClBrSCNNO3CN-
