ANALYST, MAY 1991, VOL. 116 469 Comparative Barium Ion Sensing Qualities of Planar and Tetrahedral Tripodal Receptor Molecules Y. P. Feng, G. Goodlet, N. K. Harris, M. M. Islam", G. J. Moody and J. D. R. Thomas School of Chemistry and Applied Chemistry, University of Wales College of Cardiff, P.O. Box 912, Cardiff CF7 3TB, UK Nine acyclic polyethers, representing examples of planar and tripodal 'scorpion-like' molecules, each with oligoether 'tails' and a pair of anionic 'pincers', were evaluated as possible barium ion-selective electrodes (ISEs) when incorporated into a poly(viny1 chloride) matrix membrane with 2-nitrophenyl phenyl ether as the solvent mediator. The general performance was inferior to a traditional ISE based on the tetraphenylborate salt of the barium complex with a-( nonylpheny1)-a-hydroxy-catena-poly(oxyethy1ene) (Antarox CO-880 with 30 oxyethylene units).However, a general barium ion response seems to be favoured by a tetrahedral tripodal structure (sensor C, electrode 3)' with its design promoting good ion-dipole interactions, as seen in another study on the association constants of these acyclic polyethers with barium ions. A more extensive study of the effect of methoxylated benzyl groups on similar type groups in the pincer positions for the tetrahedral tripodal structures is indicated. Keywords: Receptor molecule; polyether; ion-selective electrode; barium; planar and tetrahedral tripodal structures Examples of strong stoichiometric complexes between alkali and alkaline earth metal cations and neutral carriers are well established.1-6 In ion-selective electrode (ISE) terms, an early example is based on the naturally occurring ionophore, valinomycin, which forms the basis of the highly selective potassium ISE.7,X However, such a rigid cyclic arrangement is not necessary for complexation, and ionophores with a more open structure have selective ion sensing capabilities.9 Systems based on complexes between polyalkoxylate systems and alkaline earth metal cations, especially barium, have been exploited for their potentiometric response in ISE mem- branes.10-15 The tetraphenylborate salt of a barium complex with a-(nonylphenyl)-o-hydroxy-catena-poly(oxyethylene) (with 30 oxyethylene units) [Ba2+ (Antarox CO-880)l [BPh4-]2, when incorporated with 2-nitrophenyl phenyl ether as the plasticizing solvent mediator in a poly(viny1 chloride)(PVC) support matrix, yields electrodes with a near Nernstian response, giving a slope of 28 mV decade-', for concentrations of barium ions between 1 x 10-1 and 1 x 10-4 mol dm-3.13 As a result of an investigation on the complexation of dibenzo-30-crown- 10,1G19 a regio-selective synthesis of acyclic polyether (based on the oxyethylene units) intermediates was devised.16 In due course, this led to the synthesis of a series of 'scorpion-like' ligands, each with oligoether 'tails' and a pair of anionic 'pincers'.*0 These were shown to be capable of metal encapsulation with the association of some members of the alkali and alkaline earth metals, with the affinity for barium being relatively strong compared with the other alkali and alkaline earth metals.'" Some possible technological benefits were indicated, such as the use of these materials to overcome clogging, e.g., by barium sulphate scale formation during oil production from oil wells.20 Additionally, there was the prospect of the application of these scorpion-like ligands as potentiometric ion sensors.Thus, some of these ligands (sensors A-I in Fig. 1 and Table 1) were studied for their suitability as possible selective sensors in PVC matrix mem- branes incorporating 2-nitrophenyl phenyl ether as the solvent mediator, and their performance was compared with that of a traditional barium sensor using [Ba2+ (Antarox CO-880)][BPh4-I2 (sensor J, Table 1). * Present address: Chemistry Department, Bangladesh University of Engineering and Technology, Dhaka.Bangladesh. Experimental Reagents Sensors A-I (Fig. 1, Table 1) were donated and synthesized by Stoddart and co-workers at the University of Sheffield, Sheffield, UK,20 and Antarox CO-880 was donated by GAF Chemicals, Manchester, UK. The [Ba2+ (Antarox c-l \o 0-x J = [ C S H ~ ~ C ~ H ~ ( ~ C H ~ C H ~ ) ~ O O H I B ~ ~ . ~ [ B P ~ ~ - Fig. 1 Structural details of sensors. A and B are planar tripodal structures, whereas C-I are tetrahedral tripodal structures. For key see Table 1470 ANALYST, MAY 1991, VOL. 116 CO-SSO)][SP~-], was prepared as described previously,l3 2-nitrophenyl phenyl ether was supplied by Eastman Kodak, Rochester, NJ, USA and PVC Breon Resin I1 EP by BP Chemicals, Barry, UK. Otherwise, analytical-reagent grade reagents were used, including the chlorides of barium, calcium, magnesium, lithium, sodium, potassium, caesium and rubidium (BDH, Poole, Dorset, UK).PVC Membrane Fabrication and E.m.f. Measurements The PVC matrix membrane ISEs were fabricated from membranes containing a sensor (2.5 mg), 2-nitrophenyl phenyl ether (360 mg) and PVC (170 mg), and assembled according to established procedures.21.22 The internal filling solution was barium chloride (0.1 rnol dm-3), and all the electrodes were conditioned in barium chloride (0.1 rnol dm-3) prior to use. The e.m.f measurements were made with a Radiometer PHM64 pH-millivoltmeter (Radiometer NS, Copenhagen, Denmark) in conjunction with a saturated calomel reference electrode (EIL Model 1370-710).A Corning pH meter and a glass electrode (EIL Model 740748) were used for pH measurements. Electrode calibrations were carried out by spiking with successive aliquots of known concentrations of the sample into doubly de-ionized water thermostated at 25 k 0.1 "C. When not in use, the electrode membranes were stored in barium chloride (0.1 rnol dm-3). Selectivity Coefficient Determination Potentiometric selectivity coefficients, (k$tt,B) were deter- mined using the separate solution method: where El and E2 are the electrode responses to the barium and interferent ion, respectively, each at a barium concentration of 1 x 10-2 rnol dm-3, S is the calibration slope and zBa and zB are the charges of the barium and interferent ions, respect- ively. For divalent interferent ions, eqn.(1) simplifies to For determination of the pH interference-free ranges, e.m.f. measurements were made on solutions of barium chloride in a 0.01 rnol dm-3 tris(hydroxymethy1)amino- methane (Trizma) buffer (obtained from Sigma, Poole, Dorset, UK), whose pH values were adjusted with 0.1 rnol dm-3 hydrochloric acid. Table 1 Structural characteristics of the planar and tetrahedral molecules used and electrode numbers (PVC matrix membrane type based on 2-nitrophenyl phenyl ether plus sensor) No. of oxyethylene Nature of X ISE Sensor units in Fig. 1 No. A 3 p-OMe-benzyl 1 B 3 Benzyl 2 C 3 Benzyl 3 D 2 H 4 E 3 H 5 F 4 H 6 G 5 H 7 H 3 CH2COOH 8 I 4 CH2COOH 9 J 30 - 10* * This electrode is based on a sensor of the tetraphenylborate salt of the barium complex with Antarox CO-880.Results and Discussion A number of points are relevant for discussion with regard to both the performance characteristics (calibration slope and kbtt,B values) of these materials as potentiometric barium ion sensors, and their selectivity characteristics, namely: (i) a general comparison of the barium ion sensing qualities of the sensors formed from compounds A-I with the traditional systems of J; (ii) a comparison of the planar tripodal (B) and the corresponding tetrahedral tripodal structure (C); (iii) the dependency of the number of oxygen atoms in the oxyethylene chain of the diphenol structures (D-G), and of the carboxyl- ates (H and I); (iv) the effect of substituting the phenolic groups of E and F by carboxylic groups (H and I), and for E by benzyl (C); and (v) a comparison of the methoxylated benzyl derivative (A) with the benzyl derivative (B) for the planar tripodal structure.Barium Ion Sensing Qualities The calibrations for barium of the various ISEs were evaluated and compared with the [Ba2+(Antarox CO-880)][BPh4-I2 mode1,13 ISE 10 (Fig. 2). All of the electrodes responded to barium ions, but to different degrees (Fig. 2); each of the new planar and tetrahedral molecule types (Fig. 1 and Table 1) were inferior to the previously established ISE 10 (Fig. 2). 8o 60 -1 UJ 40 0 v) v) 20 -20 'c: -40 -60 - 80 -100 -= E *O 60 c" 8 40 v) v) 20 > -20 E 'c: -40 LU -60 \ Ei - 80 -100 I w 60 0 40 3 2 20 E > o E 2 -20 € uj -40 -601 I ' I ' 1 ' 1 7 6 5 4 3 2 1 -Log([Ba2+l/mol dm-3) Fig. 2 Barium ion responses of various PVC matrix membrane electrodes.(a) 1, ISE 10; 2, ISE 3; 3, ISE 1; and4, ISE2: (b) 1, ISE 10; 2, ISE 7; 3, ISE 4; 4, ISE 6; and 5, ISE 5: and (c) 1, ISE 10; 2, ISE 8; and 3, ISE 9. For key see Table 1ANALYST, MAY 1991, VOL. 116 47 1 In general terms, of the electrodes based on the planar and tetrahedral scorpion-like polyethers under investigation, it is ISE 3 (sensor C) that most closely resembles the calibration characteristics of TSE 10. This is because it has a higher (near-Nernstian) e.m.f. response than any of others. The next, in order of response, are ISEs 8 (sensor H), 9 (sensor I) and 7 (sensor G). Ion-selective electrode 5 showed the best slope, and has a reasonable linear range (2.6 x 10-4-4.3 x 10-2 mol dm-3), but its overall response characteristics are greatly inferior to that of ISE 10.Sensor B (ISE 2) is inferior to C (ISE 3) and points to the tetrahedral tripodal structure being a favourable system for forming a pseudo-cavity around the metal ion in order to maximize the number of stable ion-dipole interactions as previously intimated from data for the association constants of these acyclic polyethers with barium ions.20 Association constants ( K , ) in tetrachloromethane of sensors B and C with alkali and alkaline earth metal cations as measured by the picrate extraction technique are as follows (data from refer- ence 20). For sensor B: Li+ , 160; Na+ , 1300; K+, 1600; Rb+, 610; Cs+, 220; Mg'+, 140; Ca2+, 500; Sr2+, 900; and Ba2+, 1500. For sensor C: Li+, 670; Na+, 1100; K+, 3800; Rb-+, 1100; Cs+, 800; Mg*+, 180; Ca2+, 700; Sr2+, 1700; and Ba2+ , 24 000.Regarding the effect of the dependency of the number of oxygen atoms in the oxyethylene chain, there is little difference between ISEs 4, 5 and 6 (sensors D, E and F), but ISE 7, although of short calibration (Fig. 2), indicates the extra effect of ion-dipole interaction promoted by the oxyethylene units. This is not so when ISEs 8 and 9 (sensors H and I) are compared (Fig. 2), but here the carboxylate group may have a modifying steric influence impressed on the extended oxyethylene chain (from 3 to 4 units). Appropriate groups in the 'pincer' positions certainly seem to promote barium ion-sensing (ISEs 3 and 8). After comparing the effect of the methoxylated benzyl group in the pincer positions of the planar tripodal structure (ISE 1) with just the benzyl group for ISE 2, it would seem [Fig.2(a)] that there is some virtue in a future study of the methoxylated benzyl derivative with the tetrahedral tripodal structure. Selectivity Characteristics The selectivity features of ISEs 1-10 for barium with respect to selected alkali and alkaline earth metal cations are sum- marized in Fig. 3. Again, the superiority of ISE 10 (sensor J) is demonstrated. However, it is less easy to distinguish the trends between the various other electrodes. It is of interest to note that ISE 5 (sensor E) is more selective [Fig. 3(b)] than ISE 4 (sensor D), 6 (sensor F) and 7 (sensor G). The structural reason for this is unclear. More obvious is that ISE 1 (sensor A) is more selective than ISE 2 (sensor B) [Fig.3(a)], and ISE 3 (sensor C ) is marginally more selective than ISE 2 (sensor B). For the alkali metal cations, the interferences are not as significant as indicated by the positive values for log kk?& in Fig. 3, as even when kk:t.B > 1 there can be a selectivity towards barium if B is a univalent cation.23 Nevertheless, a value of log kEt,K = 6 for JSE 3 is considerably greater than the threshold value of "2, which is necessary for the complete loss of selectivity towards barium over potassium for sensor C, and raises the question of whether the system yields a credible potassium ISE. Experiments indicated that potassium ISEs do result from the system, but they are of inferior quality to the well established ISE based on valinomycin. However, there was insufficient sensor material to permit a full definitive study, and the matter merits further investigation.With regard to barium ion selectivity, ISE 10 is still the best. 8 6 4 m + - 9 2 0) 3 0 - 2 -4 Mg Ca Li Na K 3 2 1 -m g o 0" - 1 J -2 -3 - 4 -5 Mg Ca Li Na K Rb Cs 2 1 0 g j - 1 0" -2 J -3 -4 t Mg Ca Li Na K Rb CS Element Fig. 3 Summary of selectivity coefficient data for various PVC matrix membrane electrodes. (a) 1, ISE 2; 2, ISE 3; 3 , ISE 1; and 4, ISE 10: ( b ) 1, ISE 4; 2, ISE7; 3, ISE 6; 4, ISE 5 ; and 5 , ISE 10: and (c) 1, ISE 9; 2, ISE 8; and 3 . ISE 10. For key see Table 1 Conclusion Although the general performance of ISEs, based on sensors of the planar and tetrahedral receptor molecules studied here, is inferior to that of barium ISEs based on an a-( nonylpheny1)- o3-hydroxy-catena-poly(oxyethy1ene) system, an extended study of the effect of methoxylated benzyl or similar type groups in the 'pincer' positions of the tetrahedral tripodal structures is indicated. Hunan University, China, and the British Council are thanked for supporting visiting research associateships for Y .P.F.and M.M.I., respectively. The Science and Engineering Research Council is thanked for generous financial support under their Chemical Sensors research initiative which has made possible the support of N.K.H. and of the provision of the new type receptor molecule sensors from Dr. J. F. Stoddart (University of Sheffield). Discussions with Dr. Stoddart and Dr. J . F. Costello (University of Sheffield), and with Dr. D.J . 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Paper 01049340 Received November 2nd, 1990 Accepted Januury 1 Oth, I991