Anal. Chem. 2005, 77, 276-283
PVC Membrane Potentiometric Sensor Based on 5-Pyridino-2,8-dithia[9](2,9)-1,10-phenanthrolinephane for Selective Determination of Neodymium(III) Mojtaba Shamsipur,*,† Morteza Hosseini,‡ Kamal Alizadeh,‡ Mir Fazlollah Mousavi,‡ Alessandra Garau,§ Vito Lippolis§, and Abdollah Yari†
Departments of Chemistry, Razi University, Kermanshah, Iran, Tarbiat Modarres University, Tehran, Iran, and Dipartimento di Chimica Inorganica ed Analitica, Universita´ degli Studi di Cagliari, Cagliari, Italy
Spectroflourometric studies on the binding properties of 5-pyridino-2,8-dithia[9](2,9)-1,10-phenanthrolinephane (L) toward La3+, Sm3+, Gd3+, Yb3+, and Nd3+ in methanol solution revealed the occurrence of both 1:1 and 2:1 (ligand/metal) complexation with a stability order of Nd3+ > Yb3+ > Gd3+ > Sm3+ > La3+. Consequently, L was used as a suitable neutral ionophore for the preparation of a novel polymeric membrane-selective electrode for Nd3+ ion. The electrode exhibited a Nernstian response over a wide concentration range (1.0 × 10-6-1.0 × 10-2 M) with a low limit of detection of 7.9 × 10-7 M. The electrode possesses a fast response time of Yb3+ > Gd3+ > Sm3+ > La3+. To obtain more information about the optimized structures of L and its 1:1 and 2:1 complexes with the lanthanide ions used, (53) Pistolis, G.; Malliaris, A. J. Phys. Chem. 1996, 100, 15562. (54) Eddaboudi, M.; Coleman, A. W.; Prognon, P.; Lopez-Mabia, P. J. Chem. Soc., Perkin Trans. 2 1996, 955. (55) Nicely, V. A.; Dye, J. L. J. Chem. Educ. 1971, 48, 443.
cation
Log K1
Log K2
Log K1K2
La3+ Sm3+ Gd3+ Yb3+ Nd3+
4.40 ( 0.05 4.76 ( 0.05 5.16 ( 0.03 6.08 ( 0.04 6.95 ( 0.03
2.70 ( 0.04 3.32 ( 0.04 4.12 ( 0.02 4.20 ( 0.04 4.80 ( 0.04
7.10 8.08 9.28 10.28 11.75
the molecular structures of the uncomplexed ligand and its complexes with the lanthanide ion were built with the Hyperchem program. The structure of free ligand was optimized using the 6.31 G* basic set at the restricted Hartree-Fock (RHF) level of theory. The optimized structure of the ligand was then used to find the initial structures of its lanthanide complexes. Finally, the structures of the resulting 1:1 and 2:1 sandwich complexes were optimized using the Lan12mb basis set at the RHF level of theory. No molecular symmetry constraint was applied; rather, full optimization of all bond lengths, bond angles, and torsion angles was carried out using the Gaussian 98 program.56 The optimized structures are shown in Chart 1. As is obvious from Chart 1, in the case of free ligand, the pyridine and phenanthroline moieties are folded over the ligand cavity, in two opposite directions, to result the in lowest conformational energy. While, in the case of the 1:1 complex, the presence of lanthanide ion resulted in the formation of a more or less planar ligand molecule, over which the cation is located and is involved in bond formation with the pyridine and phenanthroline nitrogens as well as the two donating sulfur atoms of the ring. The 1:1 complex thus formed facilitates the formation of a 2:1 sandwich complex by involvement of a second ligand molecule in bond formation with the central lanthanide ion, although in some bent form to minimize the possible intermolecular repulsive forces. As is obvious from Chart 1C, in the 2:1 complex, the lanthanide ion is involved in bond formation with the pyridine and phenanthroline nitrogen atoms and only one of the two sulfur atoms of each L molecule. It is interesting to note that the ∆E values for the resulting 1:1 and 2:1 complexes were calculated as -452.43 and -605.55 kcal mol-1, respectively. Thus, based on the increased selectivity of L for neodymium(III) over other lanthanide ions, as was concluded from the data given in Table 1, as well as its high lipophilic character and enhanced molecular rigidity, due to the presence of both pyridine and phenanthroline groups, the ligand was expected to act as a suitable ionophore for Nd3+ ion in a PVC membrane electrode. In preliminary experiments, it was found that, while the use of an ionophore-free PVC membrane resulted in no measurable re(56) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Startmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, K.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A., Gaussian Inc., Pittsburgh, PA, 1998.
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Chart 1. Optimized Structures of Free L (A) and Its 1:1 (B) and 2:1 (C) Complexes
Table 2. Optimization of Membrane Ingredients
sponse with respect to Nd3+, the addition of L shows a Nernstian response for the cation in the range of 1.0 × 10-2-1.0 × 10-6 M (Figure 2). Meanwhile, the ligand L was also used as a neutral carrier to prepare PVC membrane electrodes for a variety of metal ions including the lanthanide ions other than Nd3+. The potential responses of some of the most sensitive electrodes based on L are also shown in Figure 2. As is obvious from Figure 2, among different cations tested, Nd3+ with the most sensitive response seems to be suitably determined with the electrode. This is due
composition (%) no. PVC plasticizer 1 2 3 4 5 6 7 8 9 10 11 12 13
30 30 30 30 30 30 30 30 30 30 30 30 30
60, NPOE 68, NPOE 67, NPOE 66,NPOE 65,NPOE 56, NPOE 50, NPOE 46, NPOE 56, NPOE 50, NPOE 46, NPOE 56, BA 56, DBP
additive
slope linear range ionophore (mV decade-1) (M)
10, OA
10, OA 16, OA 20, OA 1, NaTPB 2, NaTPB 3, NaTPB 10 10
2 3 4 5 4 4 4 4 4 4 4 4
5.2 ( 0.1 9.1 ( 0.1 13.4 ( 0.2 11.9 ( 0.1 18.4 ( 0.3 20.1 ( 0.2 18.6 ( 0.2 17.3 ( 0.1 18.7 ( 0.2 17.8 ( 0.1 18.3 ( 0.2 12.2 ( 0.1
10-5-10-1 10-5-10-1 10-6-10-1 10-6-10-1 10-6-10-1 10-6-10-1 10-6-10-1 10-6-10-1 10-5-10-1 10-5-10-1
to the selective behavior of the PVC membrane system against Nd3+ in comparison to the metal ions tested, including other lanthanide ions. Membrane Composition. Besides of the critical role of the nature of the ionophore in preparing PVC membrane electrodes, it is well understood that the performance characteristics for the ionophore-incorporated PVC membrane may also be very dependent on electrode composition and the nature of the solution of which the electrodes are composed.18-20,39-45,57 Thus, different aspects of the composition of membranes based on L for Nd3+ ion were optimized, and the results are summarized in Table 2. As expected, the amount of ionophore was found to affect the PVC membrane sensitivity (nos. 2-5). The calibration slope increased with increasing L content until a value of 4% was reached. However, further addition of the ionophore resulted in a diminished response slope of the electrode, most probably due to some inhomogeneity and possible saturation of the membrane.58 The potentiometric response of the membrane ion-selective electrodes based on neutral ionophores is greatly influenced by the polarity of the membrane medium, which is defined by the Figure 2. Potential responses of various cation-selective electrodes based on L. The PVC membrane composition used is that given for membrane 7, Table 2. 280
Analytical Chemistry, Vol. 77, No. 1, January 1, 2005
(57) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083. (58) Ammann, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119.
dielectric constants of the major membrane components.57-60 The influence of the nature of plasticizer on the Nd3+ response was studied on electrodes containing three types of plasticizers having different dielectric constants, namely, BA, DBP, and NPOE. As shown in Table 2, NPOE with the highest dielectric constant in the series resulted in the best sensitivity of the potential responses. It should be noted that the nature of the plasticizer affects not only the dielectric constant of membrane phase but also the mobility of ionophore molecules and the state of the ligands.57,59,61 We have previously reported that, in the presence of the same ionophore (i.e., 1,3,5-trithiane), a change in the nature of the plasticizer will result in altering selectivity of the PVC membrane system toward Ce3+ 41 and La3+ 42 ions. It is well known that the incorporation of lipophilic additives can significantly influence the performance characteristics of a membrane sensor.18-20,38-45,57-64 The presence of additives not only improves the response characteristics and selectivity57,62 but also may catalyze the exchange kinetics at the sample-membrane interface.65 In this work, we examined the influence of both OA and NaTPB, as suitable lipophilic additives, on the response characteristics of the proposed PVC membrane, and the results are also included in Table 2. We have recently reported the first use of oleic acid as a very suitable lipophilic additive in inducing permselectivity to some PVC-based ion selective electrodes.18-20,41-45,66 The data given in Table 2 indicate that, in the absence of a proper additive, the sensitivity of the PVC membrane based on L is quite low (nos. 2-5, with slopes of < 13.4 mV decade-1). However, the presence of either 16% oleic acid (no. 7) or 2% NaTPB (no. 10), as suitable lipophilic additives, will improve the sensitivity of the Nd3+ sensor considerably (with a slopes of 20.1 and 18.7 mV decade-1, respectively). It is interesting to note that, in membrane 7, the molar ratio of the ionophore to oleic acid is ∼1/4, which implies that oleic acid is not primarily a phasetransfer catalyst but also contributes to the complexation mechanism, as described by Eugster et al.67 Moreover, with a fraction of 16 wt %, oleic acid is expected to contribute significantly to the dielectric constant of the membrane in addition to the plasticizer. As is obvious from Table 2, membrane 7 with a PVC/NPOE/ OA/L percent ratio of 30:50:16:4 resulted in Nernstian behavior of the membrane electrode over a wide concentration range. Effect of Internal Solution. Based on the generally adopted ion-selective response formalism,57 the internal solution may affect the electrode response when the membrane internal diffusion potential is appreciable. Thus, the proposed sensor was examined at different concentrations of inner reference solution, as it is described in the Experimental Section. It was found that the (59) Yang, X.; Kumar, N.; Chi, H.; Hibbert, D. D.; Alexander, P. N. W. Electroanalysis 1997, 9, 549. (60) Morf, W. E. The Principles of Ion-Selective Electrodes and Membrane Transport; Elsevier: New York, 1981. (61) Masoda, Y.; Zhang, Y.; Yan, C.; Li, B. Talanta 1998, 46, 203. (62) O. Lindner, E.; Graf, E.; Niegriesz, Z.; Toth, K.; Purgor, E.; Buck, R. P. Anal. Chem. 1988, 60, 295. (63) Rosatzin, R.; Bakker, E.; Suzuki, K.; Simon, W. Anal. Chim. Acta 1993, 280, 197. (64) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66, 391. (65) Gehrig, P. M.; Morf, W. E.; Welti, M.; Pretsch, E.; Simon, W. Helv. Chim. Acta 1990, 73, 203. (66) Ganjali, M. R.; Moghimi, A.; Buchanan, G. W.; Shamsipur, M. J. Inclusion Phenom. 1998, 30, 24. (67) Eugster, R.; Spichiger, U. E.; Simon, W. Anal. Chem. 1993, 65, 689.
Figure 3. Calibration graph for the Nd3+ ion-selective electrode at pH 5.0. The PVC membrane composition used is that given for membrane 7, Table 2.
variation of the concentration of the internal solution (in the range of 1.0 × 10-2-1.0 × 10-4 M Nd(NO3)3) 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. Response Characteristics of the Membrane Electrode. The critical response characteristics of the proposed Nd3+-selective electrode were investigated according to IUPAC recommendations.47,48 The emf response of the membrane electrode at varying Nd3+ concentrations (Figure 3) depicts a rectilinear range from 1.0 × 10-2 to 1.0 × 10-6 M with a Nernstian slope of 20.0 ( 0.2 mV decade-1. The limit of detection was 7.9 × 10-7 M, as determined from the intersection of the two extrapolated segments of the calibration plots. According to the first IUPAC recommendation,47,68 the practical response time is defined as “the length of time which elapses between the instant at which an ion-selective electrode and a reference electrode are brought into contact with a sample solution (or the instant at which the concentration of the ion of interest in a solution in contact with an ion-selective electrode and a reference electrode is changed) and the first instant at which the potential of the cell becomes equal to its steady-state value within 1 mV”. While the 1994 IUPAC recommendation defines the response time as “the time which elapses between the instant at which an ionselective electrode and a reference electrode (ISE cell) are brought into contact with a sample solution (or at which the activity of the ion of interest in a solution is changed) and the first instant at which the emf/time slope (∆E/∆t) becomes equal to a limiting value on the basis of the experimental conditions and/or requirements concerning the accuracy”.48,68 Thus, the practical response time required for the Nd3+ sensor to reach a potential within (1 mV of the final equilibrium value after successive immersion of a series of neodymium ion solutions, each having a 10-fold difference in concentration, was measured. As is obvious from Figure 4, the practical response time of the membrane electrode thus obtained was
