A Chloride Ion-Selective Solvent Polymeric Membrane Electrode

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Anal. Chem. 1997, 69, 1038-1044

A Chloride Ion-Selective Solvent Polymeric Membrane Electrode Based on a Hydrogen Bond Forming Ionophore Kang Ping Xiao, Philippe Bu 1 hlmann, Seiichi Nishizawa,† Shigeru Amemiya, and Yoshio Umezawa*

Department of Chemistry, School of Science, The University of Tokyo, Hongo, Tokyo 113, Japan

A chloride-selective solvent polymeric membrane electrode based on a neutral hydrogen bond forming ionophore, poly(vinyl chloride) (PVC), the plasticizer 2-nitrophenyl octyl ether (o-NPOE), and cationic sites (50 mol %, relative to the ionophore) is described. At pH 7.0 (HEPES buffer), this electrode responds to chloride in a linear range from 10-5 to 10-2 M with a slope of (-54.0 ( 1.0) mV/decade and a detection limit of (6.5 ( 3.0) × 10-6 M. As compared to conventional anion-exchanger electrodes, the interference of SCN-, Br-, NO3-, I-, and even salicylate is considerably reduced, as shown by the selectivity coefficients determined with the matched potential method (MPM) in the chloride concentration range 10-2.34-10-2.04 M (log Kpot Cl,j (MPM): Sal , +1.8; SCN , +1.6; NO3-, +0.7; I-, +0.5; Br-, +0.4). Because the discrimination of the more hydrophilic anions SO42-, HSO3-/SO32-, OAc-, HCO3-, and H2PO4-/HPO42- is too large for a determination of accurate selectivity coefficients in this high chloride concentration range, corresponding values for the range 10-5.00-10-4.70 M Cl- have been 2determined (log Kpot Cl,j (MPM): SO4 , -1.2; HSO3 / 2SO3 , -2.0; OAc , -2.3; HCO3 , -2.6; H2PO4 / HPO42-, < -3.5), showing here, too, a good selectivity for Cl-. The chloride ion concentration in certified control horse serum was determined by the standard addition method as 102.1 mM with a coefficient of variation of 0.42% (n ) 6). This result is in excellent agreement with the certified value of 102.3 mM, as determined by coulometry, and demonstrates the applicability of this sensor for measurements in biological samples. The selectivity upon storage of the electrode in buffer solution for 4 weeks decreases but is still considerably better than that for a freshly prepared anion-exchanger electrode.

to the AgCl surface and is therefore not suitable for the analysis of biological samples,3,4 solvent polymeric membrane ion-selective electrodes based on anion-exchangers have found wide application in physiology and clinical chemistry.5,6 However, anion-exchanger electrodes have all roughly the same sequence of selectivity, which follows the order of the free energy of hydration of the analyte anions (Hofmeister series):

ClO4- > SCN- > I- > Sal- > NO3- > Br- > Cl- > HCO3- > OAc- > SO42- > HPO42Many efforts have been made to modify the selectivity of these electrodes by carefully choosing the membrane plasticizers, but the improvements in Cl- selectivity have been only moderate. Consequently, lipophilic anions like SCN- and Br- but also the more hydrophilic bicarbonate are major interferents in clinical analysis.5-7 The development of carrier-based ion-selective electrodes (ISEs) has, therefore, been of prime interest. ISEs based on trialkyltin compounds8-13 and mercury organic derivatives14,15 as well as Mn(III)16 and In(III)4 porphyrins have been reported, but so far an electrode with fully satisfactory selectivity and longterm stability is not known yet. Whereas all the ionophores that have been used so far are organometallic compounds, the present paper discusses an electrode based on an ionophore that binds Cl- by formation of hydrogen bonds. We have recently shown that neutral bis-thiourea ionophores form complexes with monoanions that may act as hydrogen bond

† Present address: Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-77, Japan. (1) Bu ¨ hrer, T.; Gehrig, P.; Simon, W. Anal. Sci. 1988, 4, 547-557.

(2) Umezawa, Y. Handbook of Ion-Selective Electrodes: Selectivity Coefficients; CRC Press: Boca Raton, 1990. (3) Hartman, K.; Leterotti, S.; Osswald, H. F.; Oehme, M.; Meier, P. C.; Ammann, D.; Simon, W. Mikrochim. Acta 1978, 2, 235-246. (4) Park, S. B.; Matuszewski, W.; Meyerhoff, M. E.; Liu, Y. H.; Kadish, K. M. Electroanalysis 1991, 3, 909-916. (5) Oesch, U.; Ammann, D.; Pham, H. V.; Wuthier, U.; Zu ¨ nd, R.; Simon, W. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1179-1186. (6) Wegmann, D.; Weiss, H.; Ammann, D.; Morf, W. E.; Pretsch, E.; Sugahara, K.; Simon, W. Mikrochim. Acta 1984, 3, 1-16. (7) Hu ¨ bl, W.; Wejbora, R.; Shafti-Keramat, I.; Haider, A.; Hajdusich, P.; Bayer, P. M. Clin. Chem. 1994, 40, 1528-1531. (8) Frant, M. S.; Ross, J. W. U.S. Patent 3,406,102, 1968. (9) Wuthier, U.; Pham, H. V.; Rusterholz, B.; Simon, W. Helv. Chim. Acta 1986, 69, 1435-1441. (10) Wuthier, U.; Pham, H. V.; Zu ¨ nd, R.; Welti, D.; Funck, R. J. J.; Bezegh, A.; Ammann, D.; Pretsch, E.; Simon, W. Anal. Chem. 1984, 56, 535-538. (11) Pham, H. V.; Pretsch, E.; Fluri, K.; Bezegh, A.; Simon, W. Helv. Chim. Acta 1990, 73, 1895-1904. (12) Fluri, K.; Koudelka, J.; Simon, W. Helv. Chim. Acta 1992, 75, 1013-1022. (13) Hauser, P. C. Anal. Chim. Acta 1993, 278, 227-232. (14) Rothmaier, M.; Simon, W. Anal. Chim. Acta 1993, 271, 135-141. (15) Rothmaier, M.; Schaller, U.; Morf, W. E.; Pretsch, E. Anal. Chim. Acta 1996, 327, 17-28. (16) Kondo, Y.; Bu ¨hrer, T.; Seiler, K.; Fro ¨mter, E.; Simon, W. Pflu ¨ gers Arch. 1989, 414, 663-668.

1038 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997

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The need for chloride ion (Cl-) determination in clinical analysis and environmental monitoring has led to a number of methods for the measurement of this analyte. Many conventional methods, such as volumetric and coulometric titrimetry that are based on coprecipitation with Ag+ or Hg(II), are time-consuming and/or require use of reagents. The use of ion-selective electrodes, on the other hand, is simple and even allows in vivo measurements.1 While direct potentiometric determination of Clwith the use of the AgCl-based solid-state electrodes2 suffers from a poor discrimination of bromide and from protein adsorption

© 1997 American Chemical Society

acceptors, such as dihydrogen phosphate, acetate, or chloride, while anions with only a very weak hydrogen bond acceptor strength, such as hydrogen sulfate, perchlorate, or nitrate, form only very weak complexes.17,18 Though these ionophores were found to bind dihydrogen phosphate more strongly than the other anions, their use did not result in ion-selective electrodes with a high selectivity for phosphate.19 Instead, an ISE with R,R′-bis(N′-phenylthioureylene)-m-xylene as ionophore was found to be sulfate selective.19 The complexation of sulfate with this ionophore led to a much improved discrimination of lipophilic ions such as thiocyanate, and the very high free energy for the transfer of phosphate from the sample solution into the membrane phase (end position in Hofmeister series!) prevented a significant phosphate interference. We now present a new ISE based on bisthiourea ionophore 1, giving a high selectivity for Cl-. EXPERIMENTAL SECTION Reagents. Dimethyl sulfoxide-d6 (DMSO-d6) was bought from E. Merck (>99.5%, Darmstadt, Germany). 2-Nitrophenyl octyl ether (o-NPOE), HEPES (2-[4-(2-hydroxyethyl)-1-piperazine]ethanesulfonic acid), ACES (2-[(carbamoylmethyl)amino]ethanesulfonic acid), and MES (2-[N-morpholino]ethanesulfonic acid monohydrate) were purchased from Dojindo Laboratories (Kumamoto, Japan). Di(2-ethylhexyl) sebacate (DOS) was from Tokyo Kasei (Tokyo, Japan), and chlorinated paraffin (Grade 63L, 62.5% chlorine content) was a generous gift from ICI (Tokyo, Japan). Poly(vinyl chloride) (PVC) was obtained from Wako Pure Chemical Industries (Osaka, Japan). Tridodecylmethylammonium chloride (TDDMACl) from Aldrich Chemical Co. (Milwaukee, WI) was purified by recrystallization from ethyl acetate, and tetrahydrofuran (THF) from Wako Pure Chemical Industries was freshly distilled over NaOH. Anions of the highest grade commercially available were acquired as sodium salts and used without further purification. The synthesis of ionophore 1 was reported previously.18 A control serum made from horse blood was supplied from HECTEF (Health Care Technology Foundation, Tokyo, Japan). Deionized and charcoal-treated water (18.2 MΩ cm specific resistance) was prepared by a Milli-Q Type I reagent grade water system (Millipore Corp., Bedford, MA) and used for all potentiometric experiments. 1H NMR Spectroscopy. 1H NMR spectra were obtained on a Bruker AM500 spectrometer (500 MHz; Bruker, Fa¨llanden, Switzerland). All chemical shift values are reported in parts per million (ppm), using the residual solvent signal of DMSO (δ 2.55) as reference. To determine the complexation constant, the concentration of Cl- (used as N(C4H9)4+ salt) in a 1.0 mM solution of host 1 in DMSO-d6 was increased from 0 to 18.0 mM. DMSOd6 was dried over molecular sieves (3 Å). To prevent an increase of the concentration of the residual water in this solvent and prevent propagation of errors in the course of the titration, all solutions were prepared separately from two stock solutions containing either host only or a mixture of host and guest. All experimental data obtained by this method were analyzed with a nonlinear regression package and an appropriate binding isotherm model20 using Mathematica 2.2 (Wolfram Research Inc., Champaign, IL). (17) Nishizawa, S.; Bu ¨hlmann, P.; Iwao, M.; Umezawa, Y. Tetrahedron Lett. 1995, 36, 6483-6486. (18) Bu ¨ hlmann, P.; Nishizawa, S.; Xiao, K. P.; Umezawa, Y. Tetrahedron, in press. (19) Nishizawa, S.; Bu ¨ hlmann, P.; Xiao, K. P.; Umezawa, Y. Manuscript in preparation. (20) Connors, K. A. Binding Constants; John Wiley & Sons: New York, 1987.

Membranes and Cell Assembly. The membranes were prepared as described previously21 and then cut into small disks of 7 mm diameter and mounted onto Philips electrode bodies (Model IS-561, Philips Electronic Instruments Co., Mahwah, NJ). The cell assembly for the potentiometric measurements was as follows:

Ag/AgCl|3 M KCl||0.1 M HEPES||buffered sample solution|membrane|0.01 M KCl, 0.1 M HEPES|Ag/AgCl HEPES buffer solutions containing 0.01 M KCl were used to control the pH of the internal filling solution for all chlorideselective electrodes.22 Only for the determination of the optimum pH buffer, 1.0 M CH3COOLi replaced the 0.1 M HEPES as the external filling solution of the reference electrode, and 0.01 M KCl/0.1 M the buffer of interest replaced 0.01 M KCl/0.1 M HEPES as the internal filling solution of the ion-selective electrode. Membranes based on ionophore 1 (1 wt %), TDDMACl (50 mol %), o-NPOE, and PVC were used to determine the best buffer. Aliquots of concentrated buffer solutions of pH 7.0 (adjusted with NaOH) were added to the sample solutions containing 1.016 × 10-3 M Na2HPO4 and 9.993 × 10-4 M KH2PO4 (pH 7.0).23 The presence of the phosphate allowed control of the pH, even at low concentrations of the zwitterionic buffers. EMF Measurements. A Philips type electrode body, a double-junction type Ag/AgCl reference electrode (Denki Kagaku Keiki (DKK) Co., Tokyo, Japan), and an ion meter (Model IOL 50, DKK) were used for all EMF measurements. The pH values of all solutions were measured with a pH electrode (Type 6157, DKK), and EMF responses were determined at a temperature of 23 ( 1 °C. The pH of the HEPES buffered sample solutions was adjusted with NaOH to 7.0 to resemble physiological conditions. Each membrane was conditioned with a 0.1 M buffered chloride solution overnight before the measurements. Anion concentrations rather than activities are used because it is difficult to estimate activity coefficients in zwitterionic buffers. Selectivity coefficients were determined using the matched potential method.24 Resistance Measurements. Membrane resistances were determined using the method of potential reduction by a known shunt25,26 with membranes conditioned overnight in 0.1 M NaCl. A y-t recorder (Hitachi 056, Tokyo, Japan) was used to record the potential as a function of time to distinguish between ohmic drop and membrane polarization. RESULTS AND DISCUSSION Cl- and other halide anions are well known to be good hydrogen bond acceptors. This can, for example, be seen from the very large spectral shifts in infrared27 and 1H NMR spectra17,28-33 of hydrogen bond donors (X-H) upon formation of hydrogen (21) Tohda, K.; Umezawa, Y.; Yoshiyagawa, S.; Hashimoto, S.; Kawasaki, M. Anal. Chem. 1995, 67, 570-577. (22) Eugster, R.; Gehrig, P. M.; Morf, W. E.; Spichiger, U. E.; Simon, W. Anal. Chem. 1991, 63, 2285-2289. (23) Perrin, D. D. Buffer for pH and Metal Ion Control; Chapman and Hall: London, 1974. (24) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1995, 67, 508-518. (25) Ammann, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119-129. (26) Oesch, U.; Simon, W. Anal. Chem. 1980, 52, 692-700. (27) Allerhand, A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1963, 85, 1233-1237. (28) Beer, P. D.; Hesek, D.; Hodacova, J.; Stokes, S. E. J. Chem. Soc., Chem. Commun. 1992, 270-272. (29) Beer, P. D.; Dickson, C. A. P.; Fletcher, N.; Goulden, A. J.; Grieve, A.; Hodacova, J.; Wear, T. J. Chem. Soc., Chem. Commun. 1993, 828-830.

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Figure 1. Structure of ionophore 1.

bonds of the type X-H‚‚‚halide anion. Also, the shorter distances for C(H)‚‚‚Cl- than for C(H)‚‚‚ClR in crystal structures, where ClR indicates a covalently bound chlorine atom, suggest attractive interactions between Cl- and hydrogen bond donors.34 The strength of the interaction between Cl- and a large number of hydrogen bond donors has been determined experimentally by mass spectrometry,35-38 and hydrogen bonding in chloride-water clusters has been quantitatively discussed by quantum chemical means.39 Not surprisingly, hydrogen bonds to halide ions have been recently used for the design of several synthetic anion receptors. Several pyrrole-containing macrocycles were shown to bind Cl- upon protonation,40 and formation of hydrogen bonds between amides and Cl- was shown to significantly increase the stability of complexes in which Cl- is electrostatically bound to cobalticinium or ruthenium(II) bipyridyl moieties.28-31 Recently, Reinhoudt et al. have reported that neutral (thio)urea-derivatized p-tert-butylcalix[4]arenes and p-tert-butylcalix[6]arenes bind halide anions exclusively through hydrogen bonding.32,33 For the present case, complexation of the bis-thiourea ionophore 1 and Cl- in DMSO-d6 was confirmed by 1H NMR spectroscopy. Figure 2 shows the chemical shifts of three hydrogens of ionophore 1 as observed in a titration with tetrabutylammonium chloride. Downfield shifts for all the nitrogen-bound hydrogens of the ionophore were observed upon complexation of Cl-. The chemical shifts of the two NH adjacent to the xanthene spacer, Ha (Figure 1), and the two NH adjacent to the butyl groups, Hb, in the complex were larger by 1.09 and 1.21 ppm, respectively, than in the free ionophore. A smaller shift by 0.39 ppm was also observed for the aromatic hydrogen adjacent to the thiourea group (Hc). Nonlinear fitting shows that all these shifts can be explained as due to formation of 1:1 complexes with a complexation constant of 102.91 M-1. This shows a high stability (30) Beer, P. D.; Drew, M. G. B.; Hazlewood, C.; Hesek, D.; Hodacova, J.; Stokes, S. E. J. Chem. Soc., Chem. Commun. 1993, 229-231. (31) Beer, P. D.; Szemes, F. J. Chem. Soc., Chem. Commun. 1995, 2245-2247. (32) Scheerder, J.; Fochi, M.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Org. Chem. 1994, 59, 7815-7820. (33) Scheerder, J.; Engbersen, J. F. J.; Casnati, A.; Ungaro, R.; Reinhoudt, D. N. J. Org. Chem. 1995, 60, 6448-6454. (34) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063-5070. (35) Yamdagni, R.; Kebarle, P. J. Am. Chem. Soc. 1971, 93, 7139-7143. (36) Cumming, J. B.; French, M. A.; Kebarle, P. J. Am. Chem. Soc. 1977, 99, 6999-7003. (37) Paul, G. J. C.; Kebarle, P. Can. J. Chem. 1990, 68, 2070-2077. (38) Larson, J. W.; McMahon, T. B. J. Am. Chem. Soc. 1984, 106, 517-521. (39) Xantheas, S. S. J. Phys. Chem. 1996, 100, 9703-9713. (40) Sessler, J. L.; Cyr, M.; Furuta, H.; Kral, V.; Mody, T.; Morishima, T.; Shionoya, M.; Weghorn, S. Pure Appl. Chem. 1993, 65, 393-398.

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Figure 2. 1H NMR chemical shifts of the hydrogen in the NH adjacent to the xanthene spacer, Ha (cf. Figure 1), the NH adjacent to the butyl groups, Hb, and the aromatic hydrogen adjacent to the thiourea group, Hc, in a titration with tetrabutylammonium chloride (ionophore 1 0.001 M, in DMSO-d6, 300 K): (b) experimental data; (s) nonlinear fit based on a 1:1 complexation model.

of the 1:1 complex between ionophore 1 and Cl-, particularly when considering that DMSO diminishes the extent of complexation by forming hydrogen bonds to the ionophore and by solvating the Cl- ion by ion-dipole interactions.18 In analogy to previously reported anion complexes of bis-thiourea ionophores,17,18 the stability of this 1:1 complex can be explained as due to formation of four hydrogen bonds and high preorganization of the ionophore by the rigid spacer xanthene. As usual in host-guest chemistry, other complex stoichiometries may, however, occur if complexation is observed at higher ionophore concentrations or if another solvent is used. Whereas for usual ISE measurements the selection of an appropriate pH buffer is rarely a major problem, choice of the buffer when using ionophores with hydrogen bond donor groups such as in ionophore 1 requires special attention. In view of the relatively high stabilites of the acetate and dihydrogen phosphate complexes of ionophore 1 (104.57 and 104.74 M-1, respectively, in DMSO),18 complexation between the oxoanionic groups of many commonly used buffers had to be expected. Furthermore, hydrochloric or sulfuric acid cannot be used to adjust the pH because the ISE based on ionophore 1 responds to the corresponding anions (vide infra). To find a buffer giving an even smaller EMF response than phosphate buffer, which itself gives rise to a small EMF response (vide infra), the highly hydrophilic pH buffers HEPES, ACES, and MES were tested at pH 7.0. As Figure 3 shows, no EMF response to HEPES buffer even at a concentration of 0.1 M was observed, while ACES led to a weak and MES to a large response. Considering that, at pH 7.0, all three buffers occur in a zwitterionic and an anionic form, the major factors that explain the extent of buffer interference seem to be the lipophilicity of these anions, their concentration in the sample solution, and the strength of the complex formed between these anions and ionophore 1. Relative hydrophobicities of the three buffer anions were estimated on the basis of a method proposed by Leo et al.,41,42 according to which the logarithm of a partition coefficient, P, for the partitioning of a solute between an organic and an aqueous phase can be calculated as the sum of the (41) Fujita, T.; Iwasa, J.; Hansch, C. J. Am. Chem. Soc. 1964, 86, 5175-5180. (42) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525-613.

Figure 3. Potentiometric responses to various buffers for an electrode membrane based on ionophore 1 (1 wt %), 50 mol % TDDMACl (relative to the ionophore), o-NPOE (66 wt %), and PVC (33 wt %): (×) HEPES, (O) ACES, and (+) MES. All sample solutions contained 2.0 mM phosphate buffer, and the pH was adjusted to 7.0 with NaOH.

logarithms of the partition coefficient for a parent compound and the π-terms characterizing the hydrophobicities of its substituents. The anions of HEPES, ACES, and MES having ethanesulfonate as a common parent, the estimated log P values of the Nmorpholino (-1.1), N′-(2-hydroxyethyl)-N-piperazino (-1.3 to -1.5), and (carbamoylmethyl)amino (-2.4 to -2.6) substituents indicate an increasing lipophilicity in the order of ACES < HEPES < MES. On the other hand, the ratio of the anionic and zwitterionic forms at pH 7.0 increases in the order of HEPES (0.28) < ACES (1.25) < MES (7.08), as can be calculated from their pKa values. In view of the order of these lipophilicities and also of the concentrations of the anionic forms, it appears not surprising that the largest EMF is observed for MES. However, lipophilicity and anion concentration seem not to explain that no potentiometric response is observed for HEPES, while ACES leads to a response of -33 mV at a concentration of 0.1 M. Complexation between these buffer anions and the ionophore may be partly responsible for the selectivities among the buffers. HEPES and ACES having both hydrogen bond donor and acceptor groups, various complex stoichiometries are possible,43 making an accurate interpretation very difficult. We conclude that the choice of the buffer when using ionophores with hydrogen bond donor groups, such as in ionophore 1, must be determined experimentally. Due to the lack of any response to HEPES for these o-NPOE-based ISEs, 0.1 M HEPES was used for all further experiments. The selectivity of an ISE is codetermined by the solvation of the complexes between ionophore and primary and interfering ions, by desolvation of the free ionophore in the case of different stoichiometries of the complexes of primary and interfering ions, and by the solvation of interfering ions that do not form a complex with the ionophore. The plasticizer of a solvent polymeric ISE membrane has, therefore, a large influence on the observed selectivity of ion-selective electrodes. Furthermore, the nature of the plasticizer also governs, to a large extent, the solubility of the ionophore in the ISE membrane.44 To determine the influence of the plasticizer on the response of electrodes based on ionophore 1, electrodes containing ionophore 1, 50 mol % TDDMACl, and (43) Amemiya, S.; Bu ¨ hlmann, P.; Tohda, K.; Umezawa, Y. Anal. Chim. Acta, in press. (44) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: New York, 1981.

Cl

Figure 4. Potentiometric responses to chloride of the electrode membranes containing 1 wt % ionophore, 50 mol % TDDMACl (relative to the ionophore), and (O) DOS, (4) chloroparaffin, or (b) o-NPOE as plasticizer (all samples buffered with 0.1 M HEPESNaOH to pH 7.0).

as plasticizer DOS, chloroparaffin, or o-NPOE were used. EMF responses of these electrodes to Cl- are shown in Figure 4. While the o-NPOE-plasticized membrane responds to chloride in a linear response from 10-5 to 10-2 M Cl- with a nearly Nernstian slope (-54.0 ( 1.0 mV/decade) and a detection limit of (6.5 ( 3.0) × 10-6 M Cl- (determined according to IUPAC recommendation),45 the membranes plasticized with DOS and chloroparaffin gave no and only an extremely poor response, respectively. The response of the DOS-plasticized membrane to unbuffered Cl- solutions showed, however, a nearly Nernstian slope (-53.0 ( 1.0 mV/ decade from 10-6 to 10-2 M Cl-), suggesting that the lack of a response to HEPES-buffered Cl- solutions is due to an interference from the buffer. On the other hand, the chloroparaffin-plasticized membrane gave a poor response to Cl- even in absence of HEPES. In an attempt to determine the reason for the completely different behaviors of the differently plasticized membranes, they were observed by light microscopy, but no phase separation, aggregation, or crystallization of membrane components could be observed. The values of the membrane resistance of 7.5 × 103, 195, and 89 kΩ for the chloroparaffin-, DOS- and o-NPOE-plasticized membranes indicate that a high resistance may be the reason for the poor response of the chloroparaffin-plasticized membrane. On the basis of these results, o-NPOE was used as the plasticizer for all further experiments. Deliberately added ionic sites are now routinely used to reduce the interference of lipophilic counterions and the electric membrane resistance, control the potentiometric selectivity, and shorten response times, and they actually have been shown to be a necessity for counterion-independent EMF responses of neutral carrier ISEs.46-52 An established theory describes how potentiometric selectivities depend on the stoichiometry of the (45) Guilbault, G. G.; Durst, R. A.; Frant, M. S.; Freiser, H.; Hansen, E. H.; Light, T. S.; Pungor, E.; Rechnitz, G.; Rice, N. M.; Rohm, T. J.; Simon, W.; Thomas, J. D. R. Pure Appl. Chem. 1976, 46, 127. (46) Buck, R. P. Anal. Chem. 1976, 48, R23-R39. (47) Gehrig, P.; Morf, W. E.; Welti, M.; Pretsch, E.; Simon, W. Helv. Chim. Acta 1990, 73, 203-212. (48) Chan, A. D. C.; Harrison, D. J. Anal. Chem. 1993, 65, 32-36. (49) Hauser, M.; Gehrig, P. M.; Morf, W. E.; Simon, W.; Lindner, E.; Jeney, J.; To´th, K.; Pungor, E. Anal. Chem. 1991, 63, 1380-1386. (50) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66, 391-398.

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Table 1. Response Characteristics and Selectivity Coefficients of Electrodes Based on Ionophore 1 and of Anion-Exchanger Electrodes mole ratio of ionophorea and ionic sites

detection limit (M) linear range (M) slope (mV/decade) pot c log KCl,SCN pot log KCl,Br pot log KCl,HCO 3

6 wt % TDDMACl

35 mol %

50 mol %

100 mol %

150 mol %

this workb

Simon et al.6

(1.3 ( 0.4) × 10-5 10-4-10-2 -50.8 ( 1.2 +1.8 +0.6 d

(6.5 ( 3.0) × 10-6 10-5-10-2 -54.0 ( 1.0 +1.6 +0.4 d

(7.7 ( 3.0) × 10-6 10-5-10-2 -55.0 ( 1.0 >+2.6 +0.8 d

(6.3 ( 1.2) × 10-5 10-4-10-2 -58.8 ( 2.1 >+2.6 +1.0 d

(2.1 ( 0.1) × 10-4 10-3-10-2 -54.0 ( 1.0 +4.0 +1.1 -1.0

9 × 10-5- 10-1 -60.0 ( 0.9 +3.8e +1.4e -1.0e

a Membrane composition: ionophore 1, 1 wt %; o-NPOE, ∼66 wt %; PVC, 33 wt %; and TDDMACl. b Membrane composition: ionic sites (TDDMACl), 6 wt %; o-NPOE, 63 wt %; PVC, 31 wt %. c Selectivity coefficients measured by the matched potential method in a Cl- concentration pot range of 10-2.34-10-2.04 M. d KCl,HCO is too small to be accurately determined at 10-2.34 M Cl-. e Selectivity coefficients as measured with the 3 separate solution method in 0.1 M TRIS-H2SO4 buffer at pH 8.36 ( 0.05.

Figure 5. Potentiometric anion responses of an o-NPOE-plasticized electrode membrane containing 1 wt % ionophore and 50 mol % TDDMACl (relative to the ionophore). The sample solutions were buffered to pH 7.0 with 0.1 M HEPES-NaOH.

complexes between ionophore and primary and interfering ions.53,54 These stoichiometries are, however, in general not known prior to potentiometric experiments. The optimum ratio of ionic sites and a new ionophore remains, therefore, ultimately an experimental task. The potentiometric responses to chloride of membranes made of 1 wt % ionophore 1 and 35, 50, 100, and 150 mol % TDDMACl to provide lipophilic cationic sites were tested. The ion-exchanger electrode without ionophore used for comparative purposes contained 6 wt % TDDMACl, as reported previously in the literature.6 Membranes with only 1 wt % TDDMACl but no ionophore gave sub-Nernstian slopes and had only a very short lifetime; they were, therefore, not further investigated. All the membranes containing ionophore 1 responded to chloride within seconds, and the drift of the EMF for each membrane was smaller than 0.1 mV/min. Response characteristics of all electrodes are listed in Table 1, showing that all membranes containing 1 wt % ionophore 1 have lower detection (51) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, Y. Electrochim. Acta 1995, 40, 3021-3027. (52) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, K.; Nishizawa, S.; Umezawa, Y. Electroanalysis 1995, 7, 811-816. (53) Bakker, E. J. Electrochem. Soc. 1996, 143, L83-L85. (54) Bakker, E.; Meruva, R. K.; Pretsch, E.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 3021-3030.

1042 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997

limits and broader ranges of the linear response than the ionexchanger membranes containing 6 wt % TDDMACl. With the exception of the membrane containing only 35 mol % ionic sites, all membranes gave rise to nearly Nernstian slopes. While the membranes containing 1 wt % ionophore 1 and 100 mol % TDDMACl were very similar to membranes with 50 mol % ionic sites, increasing the site ratio to 150 mol % worsened the detection limit, which is apparently due to a very low concentration of the free ionophore in the membrane at this site ratio.22 The chloride selectivities of all the membranes to thiocyanate, bromide, and bicarbonate, which are the main interfering ions in the analysis of Cl- in blood, were measured by the matched potential method24 in the chloride concentration range from 10-2.34 to 10-2.04 M. This range was chosen because the average value of chloride concentration in normal blood is about 10-1 M Cl-; therefore, sample dilution, as typically used for the potentiometric analysis of Cl-, easily allows dilution to give roughly 10-2 M solutions. It was found that all the membranes containing 1 wt % ionophore 1 have such a large preference for chloride over bicarbonate that accurate pot data for KCl,HCO could not be obtained at this high chloride 3 pot concentration range. On the other hand, log KCl,HCO of the ion3 exchanger TDDMACl membrane was determined to be -1.0. Membranes containing 1 wt % ionophore 1 also discriminate SCNand Br- to a larger extent than the TDDMACl membrane. Selectivity. The membrane containing 1 wt % ionophore 1, 50 mol % TDDMACl, 33 wt % PVC, and 66 wt % o-NPOE being by far superior to the other membranes in view of chloride response and selectivity, the selectivity of membranes with the same composition was determined in more detail (Figure 5, Table 2). As the responses to many strongly discriminated ions were subNernstian, selectivity coefficients were determined by the matched potential method (MPM).24 Because selectivity coefficients in such cases depend on the Cl- ion concentration range for which pot they are measured,24 KCl,j values for two representative concentration ranges were determined. One of them was chosen to be 10-2.34-10-2.04 M Cl-, representing a typical Cl- concentration upon dilution of blood samples (vide supra), and the other was chosen to be 10-5.00-10-4.70 M, allowing the selectivity over highly discriminated ions to be quantified. Results are shown in Table pot 2, which also gives maximum selectivity coefficients KCl,j required for determining chloride concentrations in blood samples, as calculated for the separate solution method (SSM) by use of the Nicolsky-Eisenman formalism and assuming representative pot physiological concentration ranges.5 Though these KCl,j values

pot Table 2. Selectivity Coefficients, log KCl, j, for Electrodes Based on Ionophore 1 and an Anion-Exchanger Electrode pot log KCl,j (MPM)

pot log KCl,j (SSM)

anion

ISE based on ionophore 1a

ISE based on ionophore 1b

TDDMACl electrodec

ClSalSCNNO3IBrSO42HSO3-/SO32OAcHCO3H2PO4-/HPO42-

0 +0.7 +1.0 +0.2 -0.2 +0.2 -1.2 -2.0 -2.3 -2.6