Anal. Chem. 1998, 70, 295-302
Ion-Selective Electrodes Based on Two Competitive Ionophores for Determining Effective Stability Constants of Ion-Carrier Complexes in Solvent Polymeric Membranes Eric Bakker*,† and Erno 1 Pretsch‡
Department of Chemistry, Auburn University, Auburn, Alabama 36849, and Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), CH-8092 Zu¨ rich, Switzerland
A new potentiometric method to determine effective complex formation constants in organic membrane phases is reported in detail. It demands measurements with two different membranes, one containing a highly selective reference ionophore (in this case for H+) and anionic sites and another containing the same components and additionally the lipophilic ionophore to be characterized. The response characteristics of both electrodes can be compared and related to the effective complex formation constant of the ion carrier in the membrane. Potentiometric experiments with membranes containing a series of highly selective H+-ionophores confirm that alkali metal ions are not complexed by these ionophores. The ionophores valinomycin, BME-44, ETH 2120, tert-butylcalix[4]arene tetraethyl ester, and ETH 1810 are characterized using this potentiometric technique with potassium, sodium, and lithium ions in the sample. The complex formation constants are generally large and correspond very well to data obtained with a previously established optical method. Carrier-based ion-selective electrodes are established analytical tools for the direct selective detection of a large variety of analytes in complex samples. It is primarily the stability constant of the ion-ionophore complex within the membrane phase that dictates the practical selectivity of the sensor.1 Since these ion carriers are designed to be highly lipophilic or even immobilized onto the polymeric backbone of the membrane material, they are virtually insoluble in aqueous solutions. While a number of studies have been reported to characterize the complex formation constants of selected ion carriers in methanol or ethanol, such constants have been found to be relatively small,2 and direct quantitative correlations to the selectivity behavior of ion-selective electrode membranes could be performed only in special cases.3,4 It would †
Auburn University. Swiss Federal Institute of Technology. (1) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev., in press. (2) Eyal, E.; Rechnitz, G. A. Anal. Chem. 1971, 43, 1090. (3) Morf, W. E.; Bliggensdorfer, R.; Simon, W. Anal. Sci. 1989, 5, 453. (4) Morf, W. E.; Simon, W. In Ion-Selective Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum Press: New York, 1978. ‡
S0003-2700(97)00878-0 CCC: $15.00 Published on Web 01/15/1998
© 1998 American Chemical Society
clearly be desirable to characterize these ionophores in the same organic solvent or matrix they are designed for. Recently, we proposed a spectrophotometric method on thin plasticized poly(vinyl chloride) films to determine apparent complex formation constants of ion carriers directly within the polymeric phase.5 The constants obtained were larger by orders of magnitude than those traditionally obtained in polar solvents. Extended models were proposed to characterize ionophores that form complexes of mixed stoichiometries. Although the method seems theoretically wellfounded, it has not yet been widely adopted by other research groups. The possibility of assessing such stability constants potentiometrically would allow a straightforward and experimentally simpler characterization of new ionophores. Solvent polymeric membrane electrodes primarily respond to ion activities on both sides of the aqueous-organic phase boundary. The incorporation of a highly selective ion carrier into the membrane phase should induce a substantial potential change at the sample-membrane phase boundary, since the ion activity within the organic phase is dramatically altered. This effect could, therefore, in principle, be used to determine the formation constant of the ion-ionophore complex. Normally, however, this same change occurs simultaneously at the membrane-inner filling solution interface, thus eliminating any net effect on the potentiometric response. One possible way to overcome this problem might be to work with solid contact electrodes, given that the potential change at the solid phase boundary remains independent of the incorporated ionophore. The potentiometric response of two different membranes, one with and one without added ionophore, could then be compared to gain information about the extent of ion complexation within the membrane phase. Although active research to stabilize such solid contact interfaces has been successful,6 the relevant processes that occur at membrane-solid contact interfaces are still not understood well enough to justify such an approach at this time. Moreover, other sampleindependent potential contributions always exist that vary from electrode to electrode and would have to be eliminated as well. (5) Bakker, E.; Willer, M.; Lerchi, M.; Seiler, K.; Pretsch, E. Anal. Chem. 1994, 66, 516. (6) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, K.; Nishizawa, S.; Umezawa, Y. Electroanalysis 1995, 7, 811.
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998 295
In a recent communication, we introduced a new potentiometric method to determine effective formation constants of ionionophore complexes in solvent polymeric membranes.7 The influence of the second phase boundary potential is eliminated by doping the membrane with a second ionophore that is responsive to a reference ion, in this case H+. It is possible to yield absolute, rather than relative, formation constants by using reference ionophores that do not complex any ions other than H+. This paper establishes the new method in detail for the characterization of alkali metal-selective ionophores. THEORY Two equivalent approaches are given here to relate the potentiometric responses of two-ionophore membranes to the apparent stability constant of the ion-ionophore complex in the membrane. The treatment here is valid for cation-selective ionophores that are electrically neutral. Other systems have not been studied yet. The first approach generalizes the simplified treatment given recently in a short communication,7 where the potential shift in the range of metal ion response is related to the activity difference of the metal ion within two membrane phases. This activity difference translates into an effective complex formation constant, as long as the stoichiometry of the formed complex and the membrane composition are known. The second treatment determines selectivity coefficients from these measurements in analogy to the fixed interference method as recommended by IUPAC.8,9 These selectivity coefficients are related to overall ion-exchange constants which are routinely used to characterize optical films, and which enable a comparison of both methods. These data are directly translated into effective complex formation constants as well. Both approaches give equivalent results. Ion-selective membranes that contain a neutral hydrogen ion carrier (C) with or without the presence of an additional metal ion carrier (L) will, in a certain pH range, show Nernstian pH response behavior. As shown earlier,10 such a response can be described by assuming that the hydrogen ion concentration within the membrane phase is largely sample-independent. Other sample cations (even the metal ion Mz+) will here not interfere, and the membrane potential can be expressed by the sample-membrane phase boundary potential as follows:11
EH ) E° +
RT [C]aH ln F [CH+]
(7) Bakker, E.; Pretsch, E. J. Electrochem. Soc. 1997, 144, L125. (8) Buck, R. P.; Lindner, E. Pure Appl. Chem. 1995, 66, 2527. (9) 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, 48, 127. (10) Bakker, E.; Na¨gele, M.; Schaller, U.; Pretsch, E. Electroanalysis 1995, 7, 817. (11) Bakker, E.; Xu, A.; Pretsch, E. Anal. Chim. Acta 1994, 295, 253.
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
EH ) E° +
RT (CT - RT )aH ln F R -
(2)
T
where CT and RT- are the total membrane concentrations of H+ carrier and lipophilic anionic site. It is here assumed that the concentration of free hydrogen ions in the organic phase is negligibly small compared to that of protonated carrier, which is a reasonable assumption for excess of carrier over anionic sites. In this important ideal pH response region, two different membrane electrodes, one with and one without additional ion carrier L, will behave identically (see Figure 1). This allows us to standardize the potential response of both electrodes, i.e., to correct for any differences in the experimental E° values. Cation Response of Membranes Containing H+ Carrier and Anionic Sites Only. At high pH, the following equilibrium will be shifted toward the right side, and H+ will be exchanged by the sample cations Mz+ (with charge z+): Kexch
Mz+(aq) + zCH+(org) y\z Mz+(org) + zH+(aq) + zC(org) (3) It is here assumed that Mz+ does not interact with C in the organic phase. Eventually, H+ is displaced from the phase boundary region by Mz+. In this important activity range, the electrode response is independent of pH. This can be described mathematically by inserting the overall ion-exchange constant for eq 3,
Kexch )
( )
[Mz+] aM [CH+] [C]aH
z
(4)
(1)
where EH is the measured potential in the Nernstian pH response range, all constant potential contributions are included in E°, [C] and [CH+] are the concentrations of the nonprotonated and protonated forms of the H+ carrier in the organic phase boundary region contacting the sample, and aH is the sample hydrogen ion activity, while R, T, and F are the gas constant, the absolute
296
temperature, and the Faraday constant. With all the membranes considered here, the phase boundary potential at the inner side of the membrane, i.e., at the membrane-inner electrolyte interface, is fully dictated by the incorporated H+-ionophore and the pH of the inner electrolyte and, therefore, equal for all electrodes. This contribution is, therefore, included in E° and not shown in the equations that follow. After appropriate charge and mass balances are inserted, eq 1 simplifies to
into eq 1:
EM ) E° +
(
)
aM RT Kexch ln zF [Mz+]
(5)
The Nernstian electrode response to the sample activity of Mz+ alone is obtained by inserting the simplified charge balance (RT) z[Mz+]) into eq 5:
EM ) E° +
(
)
zaM RT ln -Kexch zF RT
(6)
Equation 6 was obtained by assuming that the extracted sample cation remains uncomplexed in the organic phase.
Figure 1. Schematic representation of the extraction processes that govern the electrode response of the two different membranes that are necessary to determine effective complex formation constants of neutral lipophilic ion carriers L. Rectangular boxes that also contain the neutral hydrogen carrier C and lipophilic anionic sites R- denote the organic membrane phase. Two separate Nernstian response ranges are analyzed for both electrodes, one toward H+ only and the other to the extracted metal cation M+. The selectivity coefficients of hydrogen over M+ ions are compared for both membranes to determine the complex formation constant for the ML+ complex (see text).
Cation Response of Membranes Containing H+ Carrier, Mz+ Carrier, and Anionic Sites. Evidently, eq 5 is a rather general description of the Mz+ response behavior of potentiometric membranes containing the hydrogen ion-selective carrier C. As the cation response characteristics of an identical membrane as above, but with additional Mz+ carrier L, are determined, the extracted cations will now be complexed. For the activity range at high pH where the electrode response is completely pH independent, the following charge balance is expected for the formation of up to 1:3 complexes:
RT- ) z([Mz+] + [MLz+] + [ML2z+] + [ML3z+])
(7)
In cases where a significant excess of free ionophore is present in the membrane, it can be safely assumed that the concentration of uncomplexed ions is small relative to the one of the complex. To simplify the treatment further, only one complex stoichiometry n is allowed, so that eq 7 modifies to
RT- ) z[MLnz+]
(8)
The concentration of complex can be related to the free ion concentration in the membrane by considering the complex formation constant for MLnz+:
βMLn )
[MLnz+]
(
(
)
aMz(LT - nRT-/z)n RT βMLnKexch ln EM ) E° + zF R T
(11)
Apparently, membranes containing an additional carrier (L) that binds to sample cations Mz+ will show higher potentials in the cation interference region as compared to those of otherwise identical membranes containing H+ ionophore only. This means that Mz+ interference occurs earlier, i.e., at lower sample pH values. Determination of Effective Complex Formation Constants. The potential shift in this Mz+ response region, induced by adding the ligand L to the reference membrane, is a direct measure for the concentration decrease of Mz+ as a result of complexation with the carrier L. The E° values of both electrodes in the ideal pH response range are theoretically identical, as long as the inner electrolyte composition induces a interference-free pH response at the second interface. In practice, small changes in the E° values can be accounted for by setting their values equal (see eq 1). The potential difference in the cation interference region can then be expressed mathematically by subtracting eq 6 from eq 11 (assuming identical samples in both measurements):
∆EM )
RT ln((LT - nRT-/z)nβMLn) zF
(12)
(9)
[Mz+][L]n
The uncomplexed ion concentration [Mz+] in eq 5 can now be replaced by inserting eq 9, to obtain
EM ) E° +
mass and charge balances:
)
aM[L]n RT βMLnKexch ln zF [MLnz+]
(10)
The concentrations of free and complexed carrier can be expressed by total membrane concentrations after considering
This equation can be used for the determination of effective complex formation constants from EMF data, as long as the total membrane concentrations are known and an appropriate number for n as the stoichiometry of the formed complexes is assumed. The electrode response toward Mz+ in the cation interference region is assumed to be Nernstian. An analogous approach is to consider the difference of the lower detection limits of the two electrodes. According to IUPAC, the detection limit is defined as the cross section of the two extrapolated Nernstian response ranges of the calibration curve.9 The sample activities aH and aM at this detection limit can also be Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
297
used to determine the selectivity coefficient of H+ over Mz+ in accordance to the fixed interference method:9
Kpot HM )
aH(DL) (aM(DL))1/z
(13)
Equation 13 is applicable if both ions are fully potentialdetermining in the respective portions of the calibration curve. This is usually achieved with samples that contain a high concentration of interfering electrolyte and a sufficiently concentrated pH buffer. The detection limit can be described with the equations established above by solving for the point where EH ) EM.11 For the electrode membrane containing no Mz+-selective ionophore L, the following relationship is obtained by setting eqs 2 and 6 equal:
Kpot HM (without L) )
aH(DL) (aM(DL))
) 1/z
(RT-)1-1/z (zKexch)1/z CT - RT(14)
In complete analogy, the lower detection limit for a membrane containing both carriers C and L can be described by combining eqs 2 and 11:
Kpot HM (with L) )
aH(DL) (aM(DL))1/z
)
(RT-)1-1/z (z(LT - nRT-/z)nβMLnKexch)1/z (15) CT - RT
Apparently, the ratio of the two selectivity coefficients can be directly related to the complex formation constant of the carrier L:
Kpot HM (with L) Kpot HM (without L)
) ((LT - nRT-/z)nβMLn)1/z
(16)
This treatment should be equivalent to the one shown in eq 12, since Nernstian electrode slopes to H+ and Mz+ are assumed with both approaches. The main advantage of the latter treatment may be that information about the membrane selectivity is obtained as well, and that the two different membrane responses can be analyzed separately, i.e., without standardization of E° values as demanded for the former approach. EXPERIMENTAL SECTION Reagents. The salts, acids, and the membrane components valinomycin, BME-44, tert-butylcalix[4]arene tetraethyl ester, ETH 1810, ETH 2439, ETH 5350, dioctadecylmethylamine (DODMA), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na(TFPB)), dioctyl sebacate (DOS), high molecular weight poly(vinyl chloride) (PVC), and tetrahydrofuran (THF) were purchased in the highest quality available from Fluka Chemika-Biochemika 298
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
(Ronkonkoma, NY). The synthesis of the pH chromoionophores ETH 2458 and ETH 5418 has been described previously.12,13 Aqueous solutions were prepared by dissolving the appropriate salts in doubly quartz-distilled water. Membrane Preparation and EMF Measurement. Ionselective electrode membranes were cast by dissolving the carriers and the tetraphenylborate derivative salt (Na(TFPB)), together with PVC and plasticizer (1:2 by weight), to give a total cocktail mass of 180 mg, in 1.5 mL of THF and pouring it into a glass ring (2.2 cm i.d.) affixed onto a microscopic glass slide. The specific membrane compositions were as follows (numbers in parentheses are concentrations in millimoles per kilogram)): valinomycin (10.4), ETH 2458 (9.7), and Na(TFPB) (5.1); BME-44 (10.7), ETH 2458 (10.4), and Na(TFPB) (5.6); tert-butylcalix[4]arene tetraethyl ester (10.1), ETH 2458 (10.4), and Na(TFPB) (5.0); ETH 2120 (21.3), ETH 2458 (9.6), and Na(TFPB) (4.9); ETH 1810 (20.9), ETH 2458 (9.5), and Na(TFPB) (5.3). The compositions of the membranes with H+-carrier and anionic site only were as follows: ETH 2439 (9.8) and Na(TFPB) (5.3); ETH 2458 (9.8) and Na(TFPB) (5.4); ETH 5418 (10.5) and Na(TFPB) (5.2); ETH 5350 (9.6) and Na(TFPB) (4.9); DODMA (9.6) and Na(TFPB) (5.2). The concentrations given here are relative to total membrane mass. The solvent THF was allowed to evaporate overnight. For each electrode, a 6 mm diameter disk was cut with a cork borer from the parent membrane and incorporated into a Phillips electrode body (IS-561, Glasbla¨serei Mo¨ller, Zu¨rich, Switzerland). A 0.1 M KCl solution, buffered to pH 4.0 in 1 mM citric acid and 1 mM boric acid, served as the internal filling solution for the assembled electrodes. The electrodes were conditioned in a solution identical to the inner filling solution overnight before measurement. All membrane electrode potential measurements were performed at laboratory ambient temperature (21.5 ( 0.5 °C) and, to minimize the noise, in unstirred solutions versus a double-junction free-flowing calomel reference electrode with 1 M LiOAc as the bridge electrolyte.14 The sample solutions, containing a 0.1 M chloride salt of either potassium, sodium, or lithium, in 1 mM citric acid and 1 mM boric acid, were dropwise titrated with a 1 M hydroxide solution of the same metal ion from low to high pH. The sample pH was simultaneously monitored with a pH glass electrode. Calculations. Activity coefficients were calculated according to Meier15 by assuming pure 0.1 M alkali metal chloride solutions, and selectivity coefficients were determined according to the fixed interference method.9 For the measurement with membranes containing the extremely basic H+-ionophore ETH 5350 (see Figure 3), selectivity coefficients were determined by fitting the data to the Nicolskii equation. ∆EM values were determined after adjusting for variations in the Nernstian pH response portions (E° values) of the individual electrodes. This resulted in shifting all potential values of the entire response curve of one of the two electrodes by typically less than 10 mV. Standard deviations were determined from measurements of three membranes that were made from the same parent membrane. (12) Lerchi, M.; Bakker, E.; Rusterholz, B.; Simon, W. Anal. Chem. 1992, 64, 1534. (13) Bakker, E.; Lerchi, M.; Rosatzin, T.; Rusterholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211. (14) Dohner, R. E.; Morf, W. E.; Simon, W.; Wegmann, D. Anal. Chem. 1986, 58, 2585. (15) Meier, P. C. Anal. Chim. Acta 1982, 136, 363.
RESULTS AND DISCUSSION Ideally, the lower detection limit of a polymer membrane ionselective electrode is dictated by the limited selectivity toward other sample ions that start to ion-exchange with the ion initially contained in the membrane.16 In that case, the lower detection limit is a characteristic of the relative preference of the membrane for one ion over another. For classical electrodes containing one ionophore and appropriate ionic sites, this selectivity is given by the relative tendencies of the ions to partition into the organic phase, and by the extent of complexation by the incorporated ionophore. The goal of this paper is to reveal these separate processes experimentally. It is proposed to dope the membrane with an additional ionophore which exhibits a very high binding selectivity for a second, so-called reference ion. In this situation, the two ionophores will compete with each other since only a limited number of ions may be extracted for electroneutrality reasons. At the lower detection limit, the competing ion is now complexed by the second ionophore. Hence, separate sections of the calibration curve exist that are each dictated by the binding properties of one of the two ionophores only. It is expected that the same reference ionophore can be used in membranes containing different ion carriers of interest. Depending on the extent of complex formation, the range of cation interference will occur at different ion activities. Hence, these separate measurements can be used to reveal relative complex formation constants of these ionophores. Fortunately, the choice of reference ionophore is quite free as long as the compound is not strongly complexing the same ion as the ionophore of interest. An ideal reference ionophore, of course, would not complex any ions other than the reference ion. The range of cation interference of a membrane containing only reference ionophore and anionic sites would then reflect the extraction process of ions that are solvated by the membrane components or ion-paired by the incorporated ionic site only. Neglecting the influence of ion pairing, such experiments could then be used to determine absolute complex formation constants. Since this assumption cannot be exactly valid,17,18 the complex formation constants obtained here should be termed effective or apparent constants, not thermodynamic ones. Nonetheless, such data directly reflect the activity decrease of an ion in the membrane upon incorporation of an ionophore into the membrane. Such experiments reveal the overall ion binding process within the sensing film. Various neutral H+-ionophores were evaluated as candidates for reference ionophores (for structures, see Figure 2). The compounds shown in Figure 2 were initially used as chromoionophores in optical sensors13 and first shown to be functional as H+-ionophores in ion-selective electrode membranes by the group of Buck.19 Previous extraction experiments on thin polymeric films containing these compounds have indicated that these compounds exhibit negligible binding affinity toward potassium ions.13 This would satisfy the requirement stated above to yield absolute binding constants of neutral ionophores in polymeric membranes. To confirm this potentiometrically, the cation (16) Bakker, E. Anal. Chem. 1997, 69, 1061. (17) Verpoorte, E.; Chan, A.; Harrison, D. J. Electroanalysis 1993, 5, 845. (18) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1. (19) Cosofret, V. V.; Nahir, T. M.; Lindner, E.; Buck, R. P. J. Electroanal. Chem. 1992, 327, 137.
Figure 2. Structures of the neutral H+-ionophores that were studied as potential reference ionophores.
interference range of ion-selective electrode membranes containing a series of neutral H+-carriers together with a lipophilic tetraphenylborate derivative were evaluated with Li+, Na+, and K+ as interfering sample cations. The different pH response functions in buffered KCl solutions are shown in Figure 3, while the selectivity coefficients and overall exchange constants are given in Table 1. Figure 4 shows that, on a logarithmic scale, the selectivity coefficients of H+ over a given alkali metal ion are linearly dependent on the basicity of the ionophore, with a slope of unity. This conforms perfectly to theoretical predictions according to eq 14, since Kexch is proportional to the acidity constant of the H+-ionophore.13 It is also striking that, for each electrode, the ratios of the selectivity coefficients are identical for these interfering ions (shown as distances in Figure 4). In fact, it is the same relative selectivity one obtains with ion-selective electrodes containing the tetraphenylborate derivative only20 (see inset in Figure 4). These experiments strongly indicate that extracted alkali metal ions are not bound by the H+-ionophore but are rather stabilized by the membrane solvent and/or the incorporated anionic site. This suggests that any of the investigated H+-ionophores may be used as a reference ionophore to determine absolute stability constants of ionophore complexes in ion-selective electrode membranes. In subsequent studies, ETH (20) Schaller, U. Ph.D. Thesis, ETH Zu ¨ rich, 1994.
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
299
Figure 3. pH response functions of PVC-DOS membranes containing different H+-ionophores in buffered 0.1 M KCl solutions. Table 1. Selectivity Coefficients and Overall Ion-Exchange Constants for H+-Ionophores in DOS-PVC Membranes Containing Na(TFPB)a ion J+
log Kpot HJ
log Kexch
K+ Na+ Li+
ETH 5418 -6.19 ( 0.03 -6.69 ( 0.03 -6.78 ( 0.03
-8.46 ( 0.03 -8.96 ( 0.03 -9.05 ( 0.03
K+ Na+ Li+
ETH 2439 -7.35 ( 0.01 -7.86 ( 0.01 -8.04 ( 0.01
-9.70 ( 0.01 -10.22 ( 0.01 -10.40 ( 0.01
K+ Na+ Li+
ETH 2458 -9.26 ( 0.01 -9.74 ( 0.02 -9.85 ( 0.01
-11.62 ( 0.01 -12.09 ( 0.02 -12.21 ( 0.01
K+ Na+ Li+
DODMA -9.28 ( 0.04 -9.73 ( 0.03 -9.81 ( 0.02
-11.64 ( 0.04 -12.09 ( 0.03 -12.17 ( 0.02
K+ Na+ Li+
ETH 5350 -10.84 ( 0.02 -11.19 ( 0.01 -11.45 ( 0.01
-13.17 ( 0.02 -13.51 ( 0.01 -13.77 ( 0.01
Figure 4. Correlation of selectivity coefficients log Kpot HM (with M ) Li+, Na+, and K+) of PVC-DOS membranes containing various neutral H+-ionophores to the logarithmic acidity constants pKa of these same compounds determined previously13 in pH-buffered methanol solutions (see Figure 2 for chemical structures). Straight lines are shown with slope of -1. The linear relationship confirms that the extracted alkali metal ions are not appreciably complexed by the hydrogen carrier. (Inset) Selectivity behavior of a membrane containing Na(TFPB) only.20
a Standard deviations are shown from measurements with three electrodes of identical composition.
2458 was chosen as the reference ionophore on the basis of its high lipophilicity and conveniently accessible pH measuring range. Two-ionophore membranes were characterized with five different carriers selective for potassium, sodium, and lithium (see Figure 5) in DOS-PVC (2:1) membranes containing the H+carrier ETH 2458. The results for one of those (BME-44) were reported in a recent short communication.7 As shown in Figure 6, the incorporation of a potassium carrier in addition to ETH 2458 leads to a large positive potential shift. Apparently, cation interference occurs earlier, i.e., at lower pH values as compared to an analogous membrane but without additional carrier. Since it can be assumed that the H+-carrier does not complex potassium (see above), this shift may be related to the effective complex formation constant of the ionophore. Apparently, the potential 300 Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
Figure 5. Structures of the studied alkali metal ionophores.
shift is larger for valinomycin than for BME-44, suggesting that the latter one forms weaker complexes. Indeed, Table 2 shows that the obtained complex formation constants, assuming 1:1
Figure 6. pH response functions of PVC-DOS membranes containing the H+ carrier ETH 2458 and the lipophilic anionic additive Na(TFPB) (lower curve) and, in addition, either valinomycin (top) or BME-447 as potassium ion carrier in 0.1 M KCl (see Figure 5 for chemical structures). The horizontal shift in the detection limit as well the vertical potential shift in the cation interference region between the membrane with and without additional potassium carrier may be used to determine the effective complex formation of the carrier. Potassium ions form stronger complexes with valinomycin than with BME-44 (see Table 2). Table 2. Experimental EMF Differences between DOS-PVC (2:1) Membranes Containing the H+ Carrier ETH 2458 and Anionic Site Additive Na(TFPB) and Ones with Additional Ionophore La ion M+
∆EM (mV)
K+ Na+ Li+
411 ( 2 255 ( 2 259 ( 3
Valinomycin (n ) 1) 7.03 ( 0.03 4.36 ( 0.03 4.44 ( 0.05
9.32 ( 0.03 6.65 ( 0.03 6.74 ( 0.05
K+ Na+ Li+
324 ( 1 166 ( 2 151 ( 1
BME-44 (n ) 1) 5.54 ( 0.02 2.84 ( 0.03 2.59 ( 0.02
7.87 ( 0.02 5.17 ( 0.03 4.92 ( 0.02
Na+ K+ Li+
log([M+(L)]/[M+(C)])
log βMLn
tert-Butylcalix[4]arene Tetraethyl Ester (n ) 1) 303 ( 2 5.19 ( 0.04 7.56 ( 0.04 141 ( 2 2.42 ( 0.04 4.80 ( 0.04 63 ( 2 1.08 ( 0.04 3.46 ( 0.04
Na+ K+ Li+
259 ( 1 153 ( 2 193 ( 1
ETH 2120 (n ) 2) 4.43 ( 0.02 2.62 ( 0.03 3.30 ( 0.01
8.33 ( 0.02 6.52 ( 0.03 7.21 ( 0.01
Li+ K+ Na+
207 ( 6 74 ( 1 114 ( 2
ETH 1810 (n ) 2) 3.6 ( 0.1 1.26 ( 0.01 1.95 ( 0.04
7.6 ( 0.1 5.21 ( 0.01 5.90 ( 0.04
a The potential difference ∆E M between the two respective electrodes in the cation interference range yields effective stability constants βMLn for the ion-ionophore complex. Standard deviations are shown from measurements with three electrodes of identical composition.
complexes between potassium and ionophore, are large and quite different from each other. For the valinomycin membrane, it seems difficult to identify an activity range of Nernstian pH response, and the inflection point of the curve was chosen in the present case. Apparently, the cation interference range has some overlap to the region of anion interference, i.e., anion coextraction into the membrane. In more extreme cases, smaller sample
Figure 7. pH response functions of PVC-DOS membranes containing the H+ carrier ETH 2458 and the lipophilic anionic additive Na(TFPB) (lower curve) and, in addition, either tert-butylcalix[4]arene tetraethyl ester (top) or ETH 2120 as sodium ion carrier in 0.1 M NaCl (see Figure 5 for structures and Table 2).
potassium concentrations may be chosen to broaden the pH response range. The two potassium carriers, valinomycin and BME-44, have previously also been characterized with an analogous spectrophotometric method on thin PVC films.5 These logarithmic stability constants, 9.3 and 7.9, respectively, correlate very well to the ones presented here. This result is reassuring since the model assumes no influence of diffusion potentials or other diffusion-related effects that would complicate the treatment presented here. It is likely that other experimental cases will not correspond so well if such effects would occur. Two-ionophore membranes for sodium (ETH 2120 and the calix[4]arene derivative) and one being lithium selective (ETH 1810) were also evaluated. The results for the sodium membranes are shown in Figure 7 and in Table 2. Since the positive potential shift is larger for the calix[4]arene-based membrane, it apparently complexes sodium ions more strongly. Indeed, it is well documented in the literature that membranes containing the calixarene ionophore are much more selective than ones doped with the ETH ligand. On the basis of the data presented here, this improvement is partly caused by an increased stabilization by the carrier, since, for sodium, ∆EM is 303 mV for the calixarene membrane and only 259 mV for the one containing ETH 2120. However, the calixarene ionophore also complexes the interfering ions less strongly than the ETH ligand (see Table 2), and both of these effects explain the dramatically improved ion selectivity of the respective electrode. It should be mentioned that higher membrane selectivities have been found with ETH 2120 in membranes containing more polar plasticizers.21 For the ETH 2120 ionophore, a complex stoichiometry of n ) 2 was assumed. Since the assumed complex formation stoichiometries are different for the two sodium ionophores, the actual values for the complex formation constants cannot directly be compared to each other. It is most appropriate to compare the net decrease of the free sodium ion concentration within the organic phase due to complexation with the carrier, which is directly related to the ∆EM values discussed above. (21) Maruizumi, T.; Wegmann, D.; Suter, G.; Ammann, D.; Simon, W. Mikrochim. Acta 1986, I, 331.
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
301
Table 3. Experimental Selectivity Coefficients log Kpot HJ and log Kpot IJ for DOS-PVC (2:1) Membranes Containing + the H Carrier ETH 2458 and Anionic Site Additive Na(TFPB) with Additional Ionophore L, and Comparison to Literature Values ion M+
log Kpot HJ
log Kpot IJ
log Kpot IJ (lit.)
K+ Na+ Li+
Valinomycin (I+ ) K+) -2.23 ( 0.03 0 -5.38 ( 0.02 -3.17 ( 0.04 -5.41 ( 0.05 -3.20 ( 0.06
0 -4.024 -4.324
K+ Na+ Li+
BME-44 (I+ ) K+) -3.72 ( 0.02 0 -6.90 ( 0.02 -3.18 ( 0.03 -7.26 ( 0.02 -3.54 ( 0.03
0 -3.325 -3.825
Na+ K+ Li+
tert-Butylcalix[4]arene Tetraethyl Ester (I+ ) Na+) -4.55 ( 0.03 0 0 -6.84 ( 0.04 -2.29 ( 0.05 -2.726 -8.77 ( 0.04 -4.22 ( 0.05 -3.426
Na+ K+ Li+
ETH 2120 (I+ ) Na+) -5.31 ( 0.01 0 -6.64 ( 0.03 -1.33 ( 0.03 -6.55 ( 0.01 -1.24 ( 0.01
0 -1.521 -1.221
Li+ K+ Na+
ETH 1810 (I+ ) Li+) -6.3 ( 0.1 0 -8.00 ( 0.01 -1.7 ( 0.1 -7.79 ( 0.03 -1.5 ( 0.1
0 -1.627 -1.527
The lithium carrier ETH 1810 was characterized in a similar way by comparing the response of membranes with ETH 2458 and with or without lithium carrier in pH-buffered lithium ion solutions. A complex stoichiometry of n ) 2 was assumed here on the basis of crystallographic data.22 As shown in Table 2, the stabilization of lithium ions within the membrane phase is rather weak. Nonetheless, complexes in the membrane phase are evidently much more stable than those in polar solvents such as ethanol, where logarithmic stability constants in the order of 1.3 were found with a stoichiometry of n ) 1 only.23 One would expect that two-ionophore membranes would, with the exception of H+, show selectivity behavior similar to that of classical membranes without reference ionophore. The twoionophore membrane electrodes mentioned above were characterized in the three pH-buffered electrolyte solutions with LiCl, NaCl, or KCl. The respective selectivity coefficients are, together with literature values for regular one-ionophore membranes,21,24-27 (22) Aeschimann, R. Ph.D. Thesis, ETH Zu ¨ rich, 1990. (23) Bliggensdorfer, R.; Suter, G.; Simon, W. Helv. Chim. Acta 1989, 72, 1164. (24) Jenny, H.-B.; Riess, C.; Ammann, D.; Magyar, B.; Asper, R.; Simon, W. Mikrochim. Acta 1980, 2, 309. (25) Lindner, E.; Toth, K.; Jeney, J.; Horvath, M.; Pungor, E.; Bitter, I.; Agai, B.; Toke, L. Mikrochim. Acta 1990, 1, 157. (26) Cadogan, A.; Gao, Z. Q.; Lewenstam, A.; Ivaska, A.; Diamond, D. Anal. Chem. 1992, 64, 2496. (27) Metzger, E.; Ammann, D.; Asper, R.; Simon, W. Anal. Chem. 1986, 58, 132.
302
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
shown in Table 3. Evidently, ions that are ordinarily discriminated by the respective carrier-based electrode are discriminated here as well. At this stage, it would be premature to quantitatively compare the data and find reasons for some of the deviations that were found, since experimental selectivity coefficients can vary widely, depending on exact membrane compositions and experimental conditions. Besides these discrepancies, it is reassuring that two-ionophore electrodes show, with the exception of hydrogen ions, very similar selectivity over other ions as membranes without reference ionophore. CONCLUSIONS The new proposed method to use potentiometric membranes containing a highly selective H+ carrier in addition to the metal ion carrier to be characterized promises to be a valuable tool to gain information about the absolute stabilization of metal ions within the membrane phase. A model was introduced that analyzes the two Nernstian response ranges of the electrodes toward hydrogen and metal ions, respectively. Simple results could be obtained by only considering the separate responses to the ions and not the actual mixed ion response (i.e., the curvature of the functions shown). Experiments were performed with membranes containing five different alkali metal ion carriers. The effective complex formation constants correspond very well for the two potassium-selective carriers that have been characterized with a previously established optical method.5 This result further corroborates that the response behavior of solvent polymeric membrane ion-selective electrodes and optodes can be quantitatively described with analogous equilibria. The obtained constants are generally much larger for the membrane phase as compared to the ones determined classically in polar solvents. The selectivity of the carrier in the cation interference range is satisfactorily maintained. It is planned to perform experiments with systems selective for heavy metal ions and for divalent ions such as calcium and magnesium. This approach will also be expanded to determine mixed stoichiometries of ion-carrier complexes by studying a series of membranes containing varying concentrations of membrane components. ACKNOWLEDGMENT Financial assistance from the Petroleum Research Fund (administered by the American Chemical Society), Hitachi Ltd., and Orion Research Inc. is appreciated.
Received for review August 14, 1997. Accepted October 21, 1997.X AC970878H X
Abstract published in Advance ACS Abstracts, December 15, 1997.
