Anal. Chem. 1998, 70, 1686-1691
Influence of Lipophilic Inert Electrolytes on the Selectivity of Polymer Membrane Electrodes Mathias Na 1 gele,† Yanming Mi,‡ Eric Bakker,‡ and Erno 1 Pretsch*,†
Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Universita¨ tstrasse 16, CH-8092 Zu¨ rich, Switzerland, and Department of Chemistry, Auburn University, Auburn, Alabama 36849
Lipophilic inert electrolytes, i.e., salts without ionexchange properties, may influence the selectivity of ionophore-based liquid membrane electrodes by affecting the activity coefficients in the organic phase. It is expected by a theoretical model that the addition of a lipophilic salt renders the ion-selective electrode more selective for divalent over monovalent ions. These predictions are confirmed with Ca2+-responsive membranes containing the ionophores ETH 2120, ETH 1001, and ETH 129. The effect is especially pronounced with nonpolar membrane phases containing a low concentration of charged species, where up to 2 orders of magnitude selectivity improvement is observed. Neutral carrier-based ion-selective electrode (ISE) membranes usually consist of four components: ionophore, ion-exchanger (sites), plasticizer, and polymer matrix.1 Although the presence of the ionic sites is mandatory,2,3 membranes may function without deliberately incorporated ion exchanger because of impurities in the polymer matrix (“fixed sites”)4 or in other components.5 Their concentration relative to the ionophore has an important selectivity-modifying effect due to the influence of the involved equilibria.6 Occasionally, salts of a lipophilic cation with a lipophilic anion, i.e., without ion-exchanger properties, were used as further membrane additives with the aim of reducing the membrane resistance.7,8 The various lipophilic salts had no or only a slight influence on the selectivity of the investigated potassium-7 and calcium-selective8 electrodes, respectively. Recently, it was observed that tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) has a surprisingly large effect on the Ca2+ selectivity of silicone rubber-based ISEs.9 It was argued that the lipophilic cation might be replaced by Ca2+ complexes, although this †
Swiss Federal Institute of Technology. Auburn University. (1) Umezawa, Y. Handbook of Ion-Selective Electrodes: Selectivity Coefficients; CRC Press: Boca Raton, FL, 1990. (2) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: New York, 1981. (3) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083. (4) Lindner, E.; Gra´f, E.; Nigreisz, Z.; To´th, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1988, 60, 295. (5) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, Y. Electrochim. Acta 1995, 40, 3021. (6) Eugster, R.; Gehrig, P. M.; Morf, W. E.; Spichiger, U. E.; Simon, W. Anal. Chem. 1991, 63, 2285. (7) Nieman, T. A.; Horvai, G. Anal. Chim. Acta 1985, 170, 359. (8) Ammann, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E. Anal. Chim. Acta 1985, 171, 119. ‡
1686 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
contradicts earlier observations where hints for such an exchange were only found for tetrabutylammonium but not for the much more lipophilic tetradodecylammonium salt.8 In this paper, it is shown for the first time that lipophilic salts without ion-exchange properties such as ETH 500 have an important selectivity-modifying effect by influencing the activity coefficients in the membrane. As expected theoretically and demonstrated by experiments, the selectivities of divalent over monovalent ions are massively improved if membranes of low polarity and low site concentrations are used. THEORY The phase boundary potential model10 allows for a rather simple correlation between the potentiometric selectivity coefficient and thermodynamic parameters:11
KI,Jpot )
kJzI/zJ γI[IzI+] kI (γ [JzJ+])zi/zj
(1)
J
kI and kJ are functions of the standard free energies of the ions in the membrane and aqueous phase, (k ) exp({µ°(aq) - µ°(membr)}/RT)). The ion activities in the membrane phase, i.e., the activity coefficient γ multiplied by the concentration [IzI+] and [JzJ+], refer to two different measurements with sample solutions, allowing the full replacement of the two ions by each other. It is assumed that the response toward both ions is theoretical (Nernstian), a condition that is only fulfilled if the selectivity is not excessively high or if it is determined with necessary precautions such as applying special measurement protocols12 or an ion buffer.13 In previous descriptions it was assumed that activity coefficients in the membrane phase are constant and included in the other thermodynamic terms. In this work, the addition of a lipophilic salt without ion-exchanger properties is allowed. The anion of this salt is identical to the incorporated anionic site, and the cation is a lipophilic quaternary ammonium ion. It is assumed that the influence of this salt on the membrane properties is fully (9) Oh, B. K.; Kim, C. Y.; Lee, H. J.; Rho, K. L.; Cha, G. S.; Nam, H. Anal. Chem. 1996, 68, 503. (10) Bakker, E.; Meruva, R. K.; Pretsch, E.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 3021. (11) Bakker, E.; Na¨gele, M.; Schaller, U.; Pretsch, E. Electroanalysis 1995, 7, 817. (12) Bakker, E. Anal. Chem. 1997, 69, 1061. (13) Sokalski, T.; Maj-Zurawska, M.; Hulanicki, A. Mikrochim. Acta 1991, I, 285. S0003-2700(97)00903-7 CCC: $15.00
© 1998 American Chemical Society Published on Web 03/24/1998
described by the change of the activity coefficients, and other effects on the membrane properties, such as change of its polarity, are neglected. As the concentration of lipophilic salt is increased, the ionic strength is increased as well. According to the DebyeHu¨ckel theory, the activity coefficients for divalent ions will decrease more rapidly than for monovalent ions. According to eq 1, this should give smaller selectivity coefficients for divalent over monovalent ions and, therefore, a potentiometric sensor with improved divalent ion selectivity. The possible extent of this improvement is estimated by theory. The polymeric membrane contains an ionophore L which forms a complex ILnIzI+ with the extracted ion (nI being the stoichiometry). This effect can be described with the stability constant βILn:
βILn ) I
γILn[ILnIzI+]
(2)
γI[IzI+](γL[L])nI
where species in brackets again denote concentrations. Since L is a neutral species, it is assumed that γL is constant for all cases. For a given concentration of ionophore and complex in the membrane, the ratio of the two ionic activity coefficients is constant. It is now assumed that the formed complex is sufficiently stable so that the concentration of uncomplexed ions in the membrane can be neglected in the charge balances and that every ion forms a complex with one given stoichiometry n only. Since the membrane contains the ionophore, and lipophilic anions R- and cations R+ (with anions being in excess over the cations), the simplified charge balance (RT- - RT+ ) zI[ILnIzI+]) and mass balance (LT ) [L] + nI[ILnIzI+]) have to respect both of these trapped lipophilic species in the membrane phase. The symbols RT-, RT+, and LT are the total membrane concentrations of lipophilic anionic and cationic sites and ionophore, respectively. Analogous relationships are formulated for extracted interfering ions JzJ+, which form a complex JLnJzJ+ in the membrane with the stability constant βJLn . These complex formation constants and J charge and mass balances are now inserted into eq 1 to give a simplified relationship between selectivity coefficient and activity coefficients of the formed complexes in the membrane:
Kpot I,J
)
(kJβJLn )zI/zJ J
kIβILn
×
I
(zJ{LT - (nJ/zJ)RT- - RT+}nJ)zI/zJ zI{LT -
(nI/zI)RT-
-
RT+}nI(RT-
-
RT+)zI/zJ-1
γILn
I
zI/zJ
(γJLn )
model only. The three terms of eq 3 reflect different contributions to the overall selectivity coefficient: that of the membrane solvent and the kind of ionophore, the concentration of exchangeable ions and ionophore, and the activity coefficients, respectively. The first two factors have been extensively used in the past for optimizing selectivities but the influence of the activity coefficients has not yet been considered. The concentration of exchangeable ions only has an influence if zI * zJ and/or the complex stoichiometries are different.6 Since the activity coefficients of two complexes with the same charge are probably also quite similar, an activity coefficient effect is only expected for ions of different charge. Naturally, various contributions to the selectivity factor are in part interrelated. For example, a change of the membrane solvent influences not only the distribution of the ions (term 1) but also the complex formation constants (also in term 1) and the activity coefficients (term 3). A change of net excess of anionic sites will affect the net concentration of active membrane components as well as the activity term. It appears that the most direct way of influencing the activity term alone is to introduce a salt of a lipophilic cation with a lipophilic anion. In the following, the dependence of the activity coefficient term on the ionic strength of the membrane phase is estimated. According to the Debye-Hu¨ckel theory the activity coefficient of an ion of the charge zI is given by
1 F2 logγILn ) -2.03zI2 I 8π0rNART rI + bI
where R, T, F, and NA are the gas constant, the absolute temperature, the Faraday constant, and the Avogadro number, respectively, r and 0 are the permeability of the membrane and the vacuum, respectively, bI is related to the radius of the ions, and rI is the Debye length:
rI )
0RT 2F
2
r FII
zi2A1 log γILn ) -2.03 xFII I A2r3/2
J
(14) Meier, P. C.; Morf, W. E.; La¨ubli, M.; Simon, W. Anal. Chim. Acta 1984, 156, 1. (15) Verpoorte, E. M. J.; Chan, A. D. C.; Harrison, D. J. Electroanalysis 1993, 5, 845. (16) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1.
x x
(5)
with F and II as the density and the ionic strength of the medium. The index I specifies that the parameters relate to the experimental situation where the exchangeable cations in the membranes are primary ions. For phases of low ionic strength and sufficient polarity, rI . bI, and eq 4 simplifies to
(3)
Analogous equations in the literature were lacking the activity coefficient terms and did not consider trapped cationic species in the membrane.10,14 It must be noted that ion pair equilibria are not explicitly considered in this treatment15,16 and thus eq 3 is an approximate
(4)
(6)
with
A1 )
F2 8π0NART
(7)
and
A2 )
x
0RT 2F2
Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
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The last term of eq 3 is now given by
γILn
zI2A1
zIzJA1 log ) -2.03 FI + xFIJ 3/2 x I zI/zJ (γJLn ) A2r A2r3/2 I
(9)
J
The selectivity coefficient of an ion-selective membrane for a divalent ion over a monovalent one (zI ) 2 and zJ ) 1) can be described by further simplifying eq 9:
A1F1/2 log Kpot ) C + 2.03 2(xIJ - 2xII) I,J A2r3/2
(10)
with C as the logarithm of the first two terms of eq 3. The two ionic strengths in eq 10 can be replaced with concentrations of anionic and cationic sites by inserting the simplified mass and charge balances introduced above:
log Kpot I,J ) C + 2.03
A1F1/2
2 A2r3/2
{xR
T
} (11)
- 2x1.5RT- - 0.5RT+
This equation is expected to be valid for low ionic strengths and for media of sufficiently high polarity. Otherwise, the contribution of bI is no more negligible in eq 4 and the influence of the ionic strength will be smaller than predicted. As expected intuitively, the selectivity coefficient decreases as the concentration of lipophilic salt increases. A notable feature of eq 11 is that the polarity (permeability r) of the membrane is expected to affect the influence of the membrane ionic strength on the selectivity. Consequently, divalent ion-selective membranes are expected to show a much improved selectivity over monovalent ions upon addition of lipophilic salts if the membrane matrix is of low polarity (small r). EXPERIMENTAL SECTION Reagents. Aqueous solutions were prepared with doubly quartz distilled water and salts and acids of p.a. grade from Fluka AG (Buchs, Switzerland). N,N,N′,N′-Tetracyclohexyl-1,2-phenylenedioxydiacetamide (ETH 2120), [(-)-N,N′-bis[11-(ethoxycarbonyl)undecyl]-N,N′-4,5-tetramethyl-3,6-dioxaoctanediamide (ETH 1001), N,N,N′,N′-tetracyclohexyl-3-oxapentanediamide (ETH 129), potassium tetrakis(4-chlorophenyl)borate (KTpClPB), tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500; R+ R-), bis(2-ethylhexyl) sebacate (DOS), o-nitrophenyl octyl ether (NPOE), and poly(vinyl chloride) (PVC) of high molecular weight were Fluka Selectophore. Tetrahydrofuran (THF) puriss p.a. was from Fluka. Membrane Preparation. The membranes containing ETH 2120 (Figure 2) were prepared by dissolving 0.56 wt % ETH 2120 and 0.05 wt % KTpClPB, with plasticizer and PVC (2:1 by weight; total mass, 1.2 g), in 12 mL of THF. Overnight evaporation of the THF from the solutions gave a master membrane. Weighted pieces from the master membrane and a weighted amount of ETH 500 were mixed again and dissolved in THF. The overnight evaporation of the solvent yielded membranes of different, precisely known ETH 500 concentrations. They were conditioned 1688 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
in vials with 0.1 M CaCl2 (∼10 mL) for ∼2 d and left to dry in laboratory air for 2-3 h. The series of membranes containing ETH 1001 (Figures 3 and 4) in DOS and PVC (2:1 by weight), were made in complete analogy to the ETH 2120 membranes with the concentrations specified in the figures. The membranes containing the calcium ionophore ETH 129 (Figure 5) contained 0.11 wt % KTpClPB, 0.21 wt % ETH 129, and specified amounts of ETH 500 together with DOS and PVC (2:1 by weight) with a total mass of 140 mg. These membranes were individually weighed out and cast as previously described.17 Electrode System and emf Measurements. Three disks of 7-mm diameter were cut from each of the above membranes, mounted in electrode bodies (Philips IS-561, N. V. Philips Gloeilampenfabrieken, Eindhoven, The Netherlands), and conditioned in 0.01 M CaCl2 for a few hours (typically overnight). The following cell assembly was used: Hg | Hg2Cl2 | KCl (satd) | 3 M KCl | sample solution | liquid membrane | internal filling solution | AgCl | Ag. The internal filling solution consisted of 0.01 M chloride salt of the primary ion (NaCl for ETH 2120 membranes and CaCl2 for ETH 1001 and ETH 129 membranes), with the exception of the second series of ETH 1001 membranes (Figure 4), where 0.01 M NaCl was used. The external half-cell was a free-flowing free-diffusion liquid junction calomel reference electrode.18 Emf measurements were carried out at room temperature (22 ( 1 °C). Measurements were carried out in unbuffered metal ion chloride solutions and, for ETH 129 membranes, in 10-4 M EDTA solutions containing varying concentrations of NaOH and KOH to confirm Nernstian electrode slopes. Selectivity coefficients were determined according to the separate solution method with 0.1 M metal chloride solutions.19 RESULTS AND DISCUSSION The selectivity coefficient of a polymer membrane electrode is defined by the activity of the primary and interfering ion in a membrane, each after equilibrating it with the respective solutions. Selectivity optimization so far focused on lowering the concentration of the primary ion in the membrane phase by introducing selective lipophilic complexing agents. In this work, it is demonstrated that selectivities may be improved by lowering the activity coefficient of the primary ion relative to that of the interfering ion. As known from aqueous ion chemistry, the increase in ionic strength reduces the activity coefficients of divalent ions stronger than that of monovalent ions. In analogy, the method shown here is expected to improve selectivity for ions with higher relative to those with lower valency. The improvement of Ca2+ selectivity relative to monovalent ions in various systems was chosen in this work to demonstrate the effects. Preliminary experiments were made with membranes without ionophore, i.e., based solely on an ion exchanger, but unfortunately divalent ions are so strongly discriminated in such systems that no Nernstian response toward them can be observed. Equation 3 was, therefore, derived for ionophore-based systems. The activity coefficient of the uncomplexed ion does not occur explicitly in this equation because it was replaced by that of the complex. It is however proportional to it for a given system. (17) Bakker, E. J. Electrochem. Soc. 1996, 143, L83. (18) Dohner, R. E.; Wegmann, D.; Morf, W. E.; Simon, W. Anal. Chem. 1986, 58, 2585. (19) Buck, R. P.; Lindner, E. Pure Appl. Chem. 1994, 66, 2527.
Figure 1. Chemical constitutions of the active membrane compounds (ionophores and ionic additives) of interest.
The influence of ionic strength on the selectivity coefficient is expected to be higher for a nonpolar membrane than for a polar one (the dielectric constant of the medium is in the denominator of eq 11). This notion was tested with different series of membranes based on the ionophore ETH 2120 (cf. Figure 1). Each series contained a different plasticizer and varying concentrations of the added lipophilic salt ETH 500. Figure 2 shows for three cases that the Ca2+ vs Na+ selectivity is indeed improved for all systems by increasing the concentration of ETH 500. As expected, the effect is most pronounced for membranes based on DOS (r ) 3.9) and the weakest for those with NPOE (r ) 23.9) as plasticizer. Tecoflex membranes exhibit an intermediate behavior. Although the polarity of Tecoflex membranes, which were prepared without plasticizer,20 is not known, this intermediate behavior is expected for chemical reasons. The predicted linear relationship is observed with NPOE (slope of the linear regression, 2.92) and with unplasticized polyurethane membranes (slope of the linear regression, 10.25). With DOS membranes, on the other hand, the beneficial influence of increasing the ionic strength upon increasing ETH 500 concentrations is declining, indicating that the above model is not valid over the entire concentration range. At high concentration of ETH 500, a similar effect is observed with Tecoflex membranes. Unpublished measurements on various DOS membranes only differing in the concentration of the lipophilic salt did not show any difference in their dielectric constants, so that changes of the membrane polarity as origin of the observed effects can be excluded.21 Either an increasing tendency for ion pair formation, which was not accounted for in the model, or the limited validity of the Debye-Hu¨ckel ap(20) Lindner, E.; Cosofret, V. V.; Ufer, S.; Buck, R. P.; Kao, W. J.; Neuman, M. R.; Anderson, J. M. J. Biomed. Mater. Res. 1994, 28, 591. (21) Na¨gele, M.; Lan, B. T. T.; To´th, K.; Bakker, E.; Pretsch, E., in preparation.
Figure 2. Improvement of the Ca2+/Na+ selectivity of ion-selective electrodes by adding various concentrations of the lipophilic salt ETH 500 (R+ R-) to membranes with different plasticizers. The effect is more pronounced for membranes of low polarity. Slopes of the regression lines: NPOE, 2.92; Tecoflex, 10.25.
proximation for apolar membranes may cause this deviation. Nevertheless, the model correctly predicts the observed qualitative trends for both systems. Because of the practical relevance of improving divalent ion selectivities of nonpolar membrane phases (see below), DOS was chosen as plasticizer for further experiments. The selectivity coefficients of various DOS-PVC membranes based on the calcium ionophore ETH 1001 are shown in Figure 3. The first membrane corresponds to a composition proposed earlier for blood Ca2+ measurements.22 The reduction of the concentration of ionophore and KTpClPB by a factor of ∼30 induces a dramatic loss of ∼2 orders of magnitude in the selectivities with respect to monovalent ions (membrane 2). Full selectivity is recovered if 16.3 mmol kg-1 ETH 500 is added to the membrane of otherwise unchanged composition (membrane 3). According to the model presented here, this improvement is attributed to the change in activity coefficients of the ionic species within the membrane. Similar to membranes containing ETH 2120 (Figure 2), a further increase of the concentration of ETH 500 does not have any significant effect (membrane 4 in Figure 3). As expected from theory, the selectivity over ions of the same (22) Anker, P.; Ammann, D.; Meier, P. C.; Simon, W. Clin. Chem. 1984, 30, 454.
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Figure 3. Experimental selectivity coefficients of DOS-PVC-based calcium-selective electrodes containing ETH 1001 and KTpClPB in a fixed molar ratio of 1.24. The selectivity improves as the concentration of the lipophilic salt R+ R- increases (numbers shown are concentrations in mmol kg-1).
charge as calcium (Mg2+ and Sr2+) is not significantly altered by any of these variations. These results may have important practical implications since the total concentration of ionophore can now be drastically reduced without sacrificing analytical performance. Besides saving costs, it could allow the use of alternate membrane materials where an insufficient solubility of ionophore would otherwise limit the utility of the respective ionselective membrane. In a further series of membranes, the concentration of ETH 1001 was kept constant at the conventional level of 24 mmol kg-1 but a 10 times lower concentration of KTpClPB was chosen (Figure 4). The selectivity coefficients obtained with the first membrane (without ETH 500) are comparable to those with membrane 2 in the previous figure, indicating that the dilution not of the ligand but of the anionic sites is decisively influencing the selectivity. The addition of ETH 500 again dramatically improves selectivities with respect to monovalent ions. The selectivity again does not significantly change with respect to divalent ions. As above, the effect is declining with increasing concentration of ETH 500. Since the selectivities are quite high, the results might be biased by the detection limit of the electrode. This is corroborated by the fact that better selectivities are found in Figure 4 than in the previous one, since in this case the internal filling solution did not contain any primary ion and thus lower detection limits are expected.23,24 To determine unbiased selectivity coefficients with membranes containing ionophores of even higher calcium selectivity, a Ca2+ ion buffer was used in further experiments. The complex of ETH 129 with Ca2+ has a 1:3 (ion/ligand) stoichiometry so that the optimum ratio of ligand to sites is 0.46.6 In the experiments shown in Figure 5, this ratio was kept constant and ETH 500 was again successively added to the membrane. The response to monovalent ions was determined by buffering (23) Mathison, S.; Bakker, E. Anal. Chem. 1998, 70, 303. (24) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 46, 11347.
1690 Analytical Chemistry, Vol. 70, No. 9, May 1, 1998
Figure 4. Experimental selectivity coefficients of DOS-PVC-based calcium-selective electrodes containing a large excess of ETH 1001 over KTpClPB, and with varying concentrations of ETH 500 (R+ R-; in mmol kg-1).
Figure 5. Selectivity coefficient of DOS-PVC-based calcium electrodes with a low concentration of highly selective calcium ionophore ETH 129 and optimum concentration of KTpClPB. Only the membranes containing ETH 500 (R+ R-) are selective enough to show negligible sodium interference for undiluted whole blood measurements.
the sample with EDTA to lower the detection limit and by titrating the sample with NaOH and KOH solutions. This enabled us to confirm Nernstian response slopes for discriminated monovalent ions. Again, a dramatic improvement is initially observed by adding ETH 500 to the membrane, but only a slight effect is seen above 2.6 mmol kg-1 ETH 500. The obtained selectivity coefficients are very comparable to the ones reported recently with a new conditioning method,12 even though the total concentrations of ionophore and anionic site are here far below traditional levels. The recently observed large effect of ETH 500 on the Ca2+ selectivity of silicone rubber-based ISEs9 can be explained in the light of the present results. The membranes had been prepared with a plasticizer of low polarity (dioctyl adipate, closely related to DOS applied in this paper). Because of limited solubility in plasticized silicone rubber matrix, the concentration of KTpClPB was only 2 mmol kg-1 (0.1 wt %) and thus similar to the ones in this work showing low selectivities without ETH 500 (1.9-2.4
mmol kg-1, cf. Figures 3-5). It is therefore likely that the influence of ETH 500 on the activity coefficients also had a decisive influence in that case. This shows that the method proposed here to improve divalent/monovalent ion selectivities of nonpolar membranes is not limited to PVC as matrix. Conventionally, polar membrane phases are chosen for divalent ion-selective electrodes. In some applications, however, such as in clinical chemistry for reasons of biocompatibility, nonpolar membrane phases are desirable.25 The present method of improving selectivities by lipophilic salts without ion-exchanger properties is therefore of practical interest. For example, the Ca2+/Na+ selectivity of the ETH 1001-based DOS-PVC membrane (membrane 1 in Figure 3) was claimed to be sufficient for measurements in blood.22 However, the necessary selectivity coefficient was estimated according to the Nicolskii-Eisenman equation, which is not valid in the range close to the selectivity limit. An exact estimation with an extended theory10 shows that the required selectivity coefficient log is -5.6 for a maximum influence of 1% by sodium ions. This is clearly not achieved with electrodes containing low concentrations of ionophore without ETH 500. Selectivity is, however, sufficient with the ETH 129-based DOSPVC-ETH 500 membranes shown here (Figure 5).
a large effect on the selectivities. Likely reasons why this possibility of improving selectivities was not considered previously are that such salts have only been used in a few studies and their influence is only large in nonpolar membranes containing low concentration of ionic sites. In such circumstances, the increase of the ionic strength favors the divalent over monovalent ion selectivities. Polar membranes with high site concentrations are normally used for divalent ion-selective electrodes, i.e., membranes in which the effect of the ionic strength is small. However, nonpolar membrane phases for divalent ions may be of interest for biocompatibility reasons. A further implication of the present results is that the concentration of the (often expensive) ionophores together with that of ion-exchanging sites may be significantly reduced without loss of selectivity. ACKNOWLEDGMENT The authors thank the Swiss National Science Foundation, Hitachi Ltd. (Tokyo), Orion Research Inc. (Beverly, MA), and the Petroleum Research Fund (administered by the American Chemical Society) for financial support.
CONCLUSIONS It is shown that in some cases the incorporation of lipophilic salts without ion-exchanger properties into the ISE membrane has
Received for review August 18, 1997. Accepted January 29, 1998.
(25) Meyerhoff, M. E. Trends Anal. Chem. 1993, 12, 257.
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