Beryllium-Selective Membrane Electrode Based on Benzo-9-crown-3

Anal. Chem. 1998, 70, 5259-5263

Beryllium-Selective Membrane Electrode Based on Benzo-9-crown-3 Mohammad Reza Ganjali,† Abolghasem Moghimi,‡ and Mojtaba Shamsipur*,§

Departments of Chemistry, Tehran University, Tehran, Iran, Imam Hossein University, Tehran, Iran, and Razi University, Kermanshah, Iran

A PVC membrane electrode for beryllium(II) ions based on benzo-9-crown-3 as membrane carrier was prepared. The sensor exhibits a Nernstian response for Be2+ ions over a wide concentration range (4.0 × 10-3-2.5 × 10-6 M) with a limit of detection of 1.0 × 10-6 M (9.0 × 10-3 ppm). It has a response time of ∼50 s and can be used for at least 4 months without any divergence in potential. The proposed electrode revealed very good selectivities for Be2+ over a wide variety of other cations including alkali, alkaline earth, transition, and heavy metal ions and could be used in a pH range of 2.0-6.0. It was successfully applied to the determination of beryllium in a mineral sample. The introduction of new ion-selective electrodes has caused a fundamental development in potentiometry over two past decades. There has been an increasing interest in the preparation of molecular carriers possessing electrical neutrality, lipophilic character, and capability to selectively and reversibly bind metal ions to induce a selective permeation of one type of metal ion through the membranes of electrodes.1 Among various metal ions, alkali and alkaline earth cations such as Li+, Na+, K+, and Ca2+ have been routinely measured by ion-selective electrodes in clinical applications.2-4 In recent years, several new ion-selective electrodes for alkali and alkaline earth cations such as lithium,5-7 sodium,8-11 potassium,12-15 rubidium,16 magnesium,17 calcium,18-20 †

Tehran University. Imam Hossein University. § Razi University. (1) Ammann, D.; Morf, W. E.; Anker, P.; Meier, P. C.; Pretsch, E.; Simon, W. Ion-Sel. Electrode Rev. 1983, 5, 3. (2) Mayerhoff, M. E.; Opdyche, M. N. Adv. Clin. Chem. 1986, 25, 1. (3) Moody, G. J.; Saad, B. B.; Thomas, J. D. R. Sel. Electrode Rev. 1988, 10, 71. (4) Janata, J. Anal. Chem. 1992, 64, 196R. (5) Attiyat, A. S.; Christian, G. D.; Xie, R. Y.; Wen, X.; Bartsch, R. A. Anal. Chem. 1988, 60, 2561. (6) Suzuki, K.; Yamada, H.; Sato, K.; Watanabe, K.; Hisamoto, H.; Tobe, Y.; Kobiro, K. Anal. Chem. 1993, 65, 3404. (7) Faulkner, S.; Kataky, R.; Parker, D.; Teasdale, A. J. Chem. Soc., Perkin Trans. 1995, 1761. (8) Moody, G. J.; Saad, B. B.; Thomas, J. D. R. Analyst 1988, 114, 15. (9) Lukyanenko, N. G.; Titova, N. Yu.; Karpinchik, O. S.; Melnik, O. T. Anal. Chim. Acta 1992, 259, 145. (10) Ohki, A.; Lu, J. P.; Huang, X.; Bartsch, R. A. Anal. Chem. 1994, 66, 4332. (11) Suzuki, K.; Sato, K.; Hisamoto, H.; Siswanta, D.; Hayashi, K.; Kasahara, N.; Watanabe, K.; Yamamoto, N.; Sasakura, H. Anal. Chem. 1996, 68, 208. (12) Tuladhar, S. M.; Williams, G.; D’Silva, C. Anal. Chem. 1991, 63, 2282. (13) Li, A.; Zhang, Z.; Wu, Y.; An, H.; Izatt, R. M.; Bradshaw, J. S. J. Incl. Phenom. 1993, 15, 317. ‡

10.1021/ac980340r CCC: $15.00 Published on Web 11/14/1998

© 1998 American Chemical Society

and strontium21 have been reported in the literature. Active components (ionophores) for the related ion-selective electrode membranes are usually macrocyclic polyethers. Recently, we have used crown ethers and some of their aza and thia derivatives as suitable neutral carriers in membrane transport22-30 and in PVC electrode membrane studies of different metal ions.15,31,32 Due to the lack of efficient commercial beryllium(II) ion-selective electrodes and even quite sparse literature reports on such electrodes,33 we were interested in the preparation of a PVC-based sensor for Be2+ ions. Ligands for use as ionophores in such a Be2+ ion-selective electrode should ideally fulfill certain conditions. They should be selective for Be2+ over other metal ions, they should have rapid exchange kinetics, and they should be sufficiently lipophilic to prevent leaching of the ligand into the solutions surrounding the membrane electrode. Moreover, the ligand should be nonbasic to maximize the pH range over which the ionophore can be used. In this work, we found benzo-9-crown-3 (B9C3, I) as an excellent neutral carrier in construction of a beryllium(II)-PVC membrane electrode. The small size of the B9C3 cavity, which is very convenient for the ionic size of Be2+,34 increases both the stability (14) Hauser, P. C.; Chiang, D. W. L.; Wright, G. A. Anal. Chim. Acta 1995, 302, 241. (15) Ganjali, M. R.; Moghimi, A.; Buchanan, G. W.; Shamsipur, M. J. Incl. Phenom. 1998, 30, 29. (16) Lukyanenko, N. G.; Titova, N. Yu.; Nesterenko, N. L.; Kirichenko, T. I.; Schebakov, S. V. Anal. Chim. Acta 1992, 263, 169. (17) Roulliy, M. V.; Badertscher, M.; Pretsch, E.; Suter, G.; Simon, W. Anal. Chem. 1988, 60, 2013. (18) Schefer, U., Amman, D.; Pretsch, E.; Oesch, U.; Simon, W. Anal. Chem. 1986, 58, 2013. (19) Cazaux, L.; Tisenes, P.; Picard, C.; D’Silva, C.; Williams, G. Analyst 1994, 119, 2315. (20) Nakamura, T.; Hayashi, C.; Izutsu, K. Anal. Chim. Acta 1994, 292, 305. (21) Akma, N.; Zimmer, H.; Mark, H. B. Anal. Lett. 1991, 24, 1431. (22) Dadfarnia, S.; Shamsipur, M. Bull. Chem. Soc. Jpn. 1992, 65, 2779. (23) Dadfarnia, S.; Shamsipur, M. J. Membr. Sci. 1992, 75, 61. (24) Parham, H.; Shamsipur, M. J. Membr. Sci. 1994, 86, 29. (25) Parham, H.; Shamsipur, M. J. Membr. Sci. 1994, 95, 21. (26) Akhond, M.; Shamsipur, M. Sep. Sci. Technol. 1995, 30, 3061. (27) Akhond, M.; Shamsipur, M. J. Chin. Chem. Soc. 1996, 43, 225. (28) Akhond, M.; Shamsipur, M. J. Membr. Sci. 1996, 117, 221. (29) Akhond, M.; Shamsipur, M. Sep. Sci. Technol. 1997, 32, 1223. (30) Shamsipur, M.; Akhond, M. Bull. Chem. Soc. Jpn. 1997, 70, 339. (31) Tavakkoli, N.; Shamsipur, M. Anal. Lett. 1996, 26, 2269. (32) Fakhari, A. R.; Ganjali, M. R.; Shamsipur, M. Anal. Chem. 1997, 69, 3693. (33) Moody, G. M.; Thomas, J. D. R. Lab. Pract. 1981, 30, 111. (34) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1991, 91, 1721.

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and the selectivity of its beryllium complex over those of other metal ions. In addition, the existence of a benzo ring on the crown’s ring results in its diminished solubility in aqueous solutions.

EXPERIMENTAL SECTION Reagents. To synthesize B9C3, the high-dilution cyclization reaction condition that was previously reported for deuterated homologous, B9C3-d4,35 was closely followed, as

The cyclization reaction was carried out in water using catechol and 1,5-dichlorooxapentane as starting materials and lithium hydroxide as base. The crude product was acidified and then extracted into dichloromethane. The organic phase was dried over sodium sulfate, and the solvent was removed. The resulting reddish slurry was purified by column chromatography to obtain B9C3 in 33% yield (mp ) 68-70 °C). As it was expected,36 the solution 13C NMR spectrum of B9C3 contained five sharp signals at 151.4, 123.9, 122.8, 74.0, and 72.3 ppm from TMS, while three peaks were detected in 1H NMR spectrum at 6.9-7.1 (m, 4H aromatic), 4.3 (m, 4H) and 3.8 (m, 4H) ppm. Reagent grade dibutyl phthalate (DBP), dioctyl phthalate (DOP), nitrobenzene (NB), acetophenone (AP), tetrahydrofuran (THF), oleic acid, and high relative molecular weight PVC (all from Merck) were used as received. Beryllium chloride and the nitrate salts of all other 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, 6 mg of ionophore B9C3, 116 mg of plasticizer AP, and 12 mg of additive oleic acid until the PVC was wet. Then the mixture was dissolved in 5 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-mm o.d.) was dipped into the mixture for ∼10 s so that a nontransparent membrane of ∼0.3 mm thickness was formed. The tube was then pulled out from the mixture and kept at room temperature for ∼1 h. The tube was then filled with internal filling solution (1.0 × 10-3 M BeCl2). The electrode was finally conditioned for 24 h by soaking in a 1.0 × 10-3 M solution (35) Buchanan, G. W.; Driega, A. B.; Moghimi, A.; Bensimon, C.; Bourque, K. Can. J. Chem. 1993, 71, 951. (36) Buchanan, G. W.; Driega, A. B.; Moghimi, A., Bensimon, C. Can. J. Chem. 1993, 71, 1983.

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Figure 1. Λ (S1- cm2 mo1-1) vs [B9C3]/[Mn+] in acetonitrile solution.

of beryllium chloride. 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 that result in reproducible, noiseless, and stable potentials. Emf Measurements. All emf measurements were carried out with the following assembly:

Ag-AgCl |1.0 10-3 M BeCl2 | PVC membrane | test solution | AgAgCl, 1.0 M KCl

A Corning ion analyzer 250 pH/mV meter was used for the potential measurements at 25.0 × 0.1 °C. Activities were calculated according to the Debye-Hu¨ckel procedure.37

RESULTS AND DISCUSSION To the best of our knowledge, there is no literature report on the stability of B9C3 complexes with metal ions. Thus, in preliminary experiments, the complexation of B9C3 with a wide variety of cations including alkali, alkaline earth, transition, and heavy metal ions was investigated in acetonitrile solution at 25.00 × 0.03 °C conductometrically,38-40 in order to obtain a clue about the stability and selectivity of the resulting complexes. With the exception of Li+, Be2+, and Mg2+ ions (see Figure 1), in all other cases studied, the incremental addition of B9C3 to 1.0 × 10-4 M solutions of the cations resulted in negligible change in molar (37) Kamata, S.; Bhale, A.; Fukunaga, Y.; Murata, A. Anal. Chem. 1988, 60, 2464. (38) Amini, M. K.; Shamsipur, M. Inorg. Chim. Acta 1991, 183, 65. (39) Ghasemi, J.; Shamsipur, M. J. Solution Chem. 1996, 25, 485. (40) Shamsipur, M.; Ganjali, M. R. J. Incl. Phenom. 1997, 28, 315.

conductance of the solution even at B9C3/cation mole ratios greater than 3, most probably indicating the lack of tendency for B9C3 to bind the metal ions. However, as is seen from Figure 1, in the case of Be2+ ion, addition of the ligand to the beryllium ion solution results in a rather sharp increase in the molar conductance of solution which begins to level off at mole ratios greater than unity. The slope of the corresponding mole ratio plot changes sharply at the point where the ligand-to-cation mole ratio is 1, indicating the formation of a fairly stable 1:1 complex (i.e., log K > 6).36 On the other hand, in two other cases in Figure 1, a gradual change in molar conductance of the Li+ and Mg2+ ions upon addition of the ligand is observed, which does not exhibit any considerable change in slope at the mole ratio ∼1, and the mole ratio plots do not tend to level off, even at mole ratios of >3. This behavior is indicative of the formation of weaker 1:1 complexes (i.e., log K < 3).39,40 It is interesting to note that the observed increase in the molar conductance of the bivalent Ba2+ and Mg2+ ions upon addition of the crown ether revealed the fact that the resulting complexed cations are less mobile than the solvated metal ions. This is mainly due to the high charge density of the small free metal ions which results in their increased solvation number and, consequently, in their decreased mobility in comparison with the corresponding complexed ions with B9C3. The results thus obtained clearly indicate that B9C3 should be a highly selective ionophore for Be2+ ion in the membrane phase of a PVC-based ion-selective electrode for the cation. Thus, in preliminary experiments, B9C3 was used as a neutral carrier to prepare PVC membrane ion-selective electrodes for a variety of metal ions, including alkali, alkaline earth, and transition metal ions. The potential responses for the more sensitive metal ions are shown in Figure 2. As seen, among different cations tried, Be2+ with the most sensitive response seems to be suitably determined with the membrane electrode based on B9C3. As was clearly pointed out in the preceding paragraphs, this is most probably due to the highly selective behavior of the ionophore for Be2+ over other metal ions as well as the rapid exchange kinetics of the resulting complex. It is well-known that the sensitivity and selectivity of the ionselective electrodes depend not only on the nature and amount of ionophore used but also significantly on the properties of the plasticizer employed as well as the PVC/plasticizer ratio used.15,31,32,41-43 Since in preparation of many PVC membraneselective electrodes a plasticizer/PVC ratio of ∼2 has resulted in very suitable performance characteristics,44 this ratio was kept constant in optimization of the ingredients of the Be2+ ion-selective electrode proposed (Table 1). The influence of the quantity of B9C3 in the membrane with DBP/PVC of ∼2 was investigated, and the results are shown in Table 1 (nos. 1-4). The sensitivity of electrode’s response increases with increasing B9C3 content until a value of 3% is reached. A further addition of ionophore will, however, result in (41) Koryta, J. Anal. Chim. Acta 1990, 233, 1. (42) Wolfbeis, O. S. Anal. Chim. Acta 1991, 250, 181. (43) Rosatzin, T.; Bakker, E.; Suzuki, K.; Simon, W. Anal. Chim. Acta 1993, 280, 197. (44) Norov, S. K.; Gulamov, M. T.; Zhukov, A. F.; Norova, G. M.; Mamedova, Yu. G. Zh. Anal. Khim. 1988, 43, 777.

Figure 2. Potential response of various ion-selective membranes based on B9C3. Table 1. Optimization of Membrane Ingredients composition (%) oleic slope no. PVC plasticizer B9C3 acid (mV/decade) 1 2 3 4 5 6 7 8 9 10 11 12 13

33 33 33 33 33 33 33 33 33 33 33 33 33

DBP, 66 DBP, 65 DBP, 64 DBP, 63 DOP, 64 NB, 64 AP, 64 DBP, 62 DBP, 60 DBP, 58 DBP, 56 NB, 58 AP, 58

1 2 3 4 3 3 3 3 3 3 3 3 3

2 4 6 8 6 6

10 12 17 15 15 18 18 20 23 25 24 28 29

linear range (M) 1 × 10-5-4 × 10-3 2 × 10-5-3 × 10-3 2 × 10-5-3 × 10-3 2 × 10-5-3 × 10-3 4 × 10-5-4 × 10-3 2 × 10-5-5 × 10-3 2 × 10-5-5 × 10-3 9 × 10-6-6 × 10-3 6 × 10-6-4 × 10-3 6 × 10-6-4 × 10-3 5 × 10-6-5 × 10-3 3 × 10-6-4 × 10-3 2.5 × 10-6-4 × 10-3

some decreased response of the electrode, most probably due to some inhomogenieties and possible saturation of the membrane.45 Since the nature of the plasticizer influences the dielectric constant of the membrane phase and the mobility of the ionophore molecules and their complexes,24 it is expected to play a fundamental role in determining the selective electrode characteristics. As is obvious from Table 1, among four different plasticizers examined (nos. 3 and 5-7 as well as nos. 10, 12, and 13), NB and, especially, AP results in the best sensitivity and widest linear range. The data given in Table 1 reveal that the nature and amount of additive influences the performance characteristics of the (45) Yang, X.; Kumar, N.; Chi, H.; Hibbert, D. B.; Alexander, P. W. Electroanalysis 1997, 9, 549.

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membrane sensor significantly. Addition of 6% oleic acid will increase the sensitivity of the electrode response considerably, so that the selective electrodes demonstrate a Nernstian behavior (no. 13). It is well-known that the presence of lipophilic anionic sites in cation-selective membrane electrodes not only diminishes the ohmic resistance46 and increases the response behavior and selectivity47 but also, in cases where the extraction capability of the ionophore is poor, enhances the sensitivity of the membrane electrode.1 The concentration of the internal solution BeCl2 in the electrode was changed from 1.0 × 10-3 to 1.0 × 10-5 M and the potential response of the Be2+ ion-selective electrode was obtained. It was found that variation of the concentration of the internal solution does not cause any significant difference in the 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 in the presence of 1.0 × 10-3 M BeCl2 was 24 h, after which it would generate stable potentials in contact with beryllium solutions. The critical response characteristics of the Be2+ ion-selective electrode were assessed according to IUPAC recommendations.48 The electrode shows a linear response to the activity of Be2+ ion in the range 2.5 × 10-6 to 4.0 × 10-3 M (see Figure 2). The slopes of the calibration plots were 29.0 × 0.5 mV per decade of activity change at 25 °C. The limit of detection was 1.0 × 10-6 M, as determined from the intersection of the two extrapolated segments of the calibration graph. We measured the average time required for the Be2+ ionselective electrode to reach a potential within (1 mV of the final equilibrium value after successive immersion in a series of beryllium ion solutions, each having a 10-fold difference in concentration. The static response time of the membrane electrode thus obtained was 50 s for concentrations e5.0 × 10-3 M, and potentials stayed constant for ∼10 min, after which only a very slow divergence within the resolution of the pH meter was recorded. The performance characteristics of the membrane remained unchanged when the potentials recorded either from low to high concentrations or vice versa. The membrane electrode prepared could be used for at least 4 months without any measurable divergence. To investigate the selectivity of the proposed membraneselective electrode toward Be2+ with respect to various interfering ions, the potentiometric selectivity coefficients (kBePot) were evaluated by the mixed solution method49,50 from potential measurements on solutions containing a fixed amount of Be2+ ion (1.0 × 10-4 M) and varying amounts of the interfering ions (Mn+) according to

Table 2. Selectivity Coefficients (kBePot) of Various Interfering Ions (Mn+) Mn+

kBePot

Mn+

KBePot

Li+ Na+ K+ Rb+ Cs+ Ag+ Tl+ NH4+ Mg2+ Ca2+

5.1 × 10-3 3.8 × 10-3 2.7 × 10-4 2.3 × 10-4 2.2 × 10-4 1.8 × 10-4 2.0 × 10-4 2.0 × 10-4 2.0 × 10-3 1.6 × 10-4

Sr2+ Ba2+ Co2+ Ni2+ Cu2+ Zn2+ Pb2+ Cd2+ Hg2+ UO22+ Fe3+

1.4 × 10-4 1.2 × 10-4 1.3 × 10-4 1.2 × 10-4 1.3 × 10-4 1.3 × 10-4 1.2 × 10-5 1.2 × 10-5 1.1 × 10-5 1.3 × 10-5 1.1 × 10-4

Figure 3. Effect of pH of test solution on the potential response of the Be2+ ion-selective electrode.

(1)

where E1 and E2 are the electrode potentials for the solution of Be2+ ions alone and for the solution containing interfering ions and beryllium ions, respectively. According to eq 1, the kBePot values for diverse ions can be evaluated from the slope of the graph of aBe{exp(E2 - E1)F/RT]} - aBe vs aM2/n. The resulting kBePot values are summarized in Table 2. For the polyvalent cations used, the selectivity coefficients are in the order of 10-4 or smaller, indicating that these metal ions have negligible disturbance of the functioning of the Be2+ ion-selective membrane. On the other hand, among univalent cations used, the selectivity coefficients for Li+ and Na+ ions seem to be somehow larger (in the order of 10-3). It is noteworthy that such deceptively larger kBePot values arise from the term a2/n in eq 1; the smaller the charge of interfering ion, n, the larger the selectivity coefficient, kBePot. Thus, despite their larger selectivity coefficients, the lithium and sodium ions would not disturb the functioning of the Be2+ sensor significantly. The influence of pH of the test solution on the potential response of the Be sensor was tested at 1.0 × 10-4 M Be2+ concentration over the pH range 2-10 and the results are shown in Figure 3. As seen, the potential remained constant from pH 2.0 to 6.0, beyond which a drastic drift was observed. The observed drift at higher pH values could be due to the formation of some hydroxy complexes of Be2+ ion in solution. Thus, the

(46) Ammann, D.; Pretsch, E.; Simon, W.; Lindler, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119. (47) Huser, M.; Gehrig, P. M.; Morf, W. E.; Simon, E.; Lindler, E.; Jeney, J.; Toth K.; Pungor, E. Anal. Chem. 1991, 63, 1380.

(48) IUPAC Analytical Chemistry Devision, Commission on Analytical Nommenclature. Recommendations for Nomenclature for Ion Selective Electrodes. Pure Appl. Chem. 1976, 48, 127. (49) Sheen, S. R.; Shih, J. S. Analyst 1992, 117, 1691. (50) Srinivasan, K.; Rechnitz, G. A. Anal. Chem. 1969, 41, 1203.

kBePotaM2/n ) aBe{exp[(E2 - E1)F/RT]} - aBe

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Table 3. Determination of Beryllium in a Mineral Sample (Beryl) Be content in final solution (ppm) ISE AAS

5.6 5.7

5.4 5.6

5.5 5.6

above range may be taken as the working pH range of the proposed selective electrode. The proposed Be2+ ion-selective electrode was found to work well under the laboratory conditions. It was successfully applied (51) Stocchi, E. Industrial Chemistry; Ellis Horwood Limited Publisher: New York, 1990; Vol. 1.

to the determination of beryllium in the mineral samples such as beryl. Some 0.15 g of a mineral sample was treated with sodium carbonate and sodium fluorosilicate and its beryllium content was dissolved in water (in a 1000-mL volumetric flask), as was described elsewhere.51 The beryllium content in the final solution was determined using the proposed membrane sensor and atomic absorption spectrometry and the results of triplicate measurements are compared in Table 3. The results obtained by the ionselective electrode are in satisfactory agreement with those obtained by AAS. Received for review March 26, 1998. Accepted August 18, 1998. AC980340R

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