Spectroscopic in Situ Imaging of Acid Coextraction Processes in

Figure 1 Optode responses of ∼2-μm thin NPOE−PVC films containing a lipophilic H+-selective chromoionophore to different acids. Open symbols are ...
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Anal. Chem. 1998, 70, 1176-1181

Spectroscopic in Situ Imaging of Acid Coextraction Processes in Solvent Polymeric Ion-Selective Electrode and Optode Membranes Erno 1 Lindner,† Titus Zwickl,‡ Eric Bakker,§ Bui Thi Thu Lan,† Klara To´th,† and Erno 1 Pretsch*,‡

Institute of General and Analytical Chemistry, Technical University of Budapest, 1111 Budapest, Szent Gelle´ rt te´ r 4, Hungary, 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

Time-dependent processes induced by acidic solutions in solvent polymeric membranes with a H+-selective chromoionophore are studied in a spectropotentiometric setup. They are important for understanding the response time of anion-selective optodes and the response of H+selective electrodes at low pH when anion interference is potential determining. The extent of anion-proton coextraction is characterized with extraction experiments on thin optical films (optodes) containing the same components and described by theory. Imaging experiments indicate rapid diffusion processes and unusual nonlinear steady-state concentration profiles that are explained by parallel extraction of undissociated acid into the membrane. Long-term potential drifts of the respective electrode are detected and related to the diffusion processes. Time-dependent processes in solvent polymer membranes are important with regard to the response mechanism of ion-selective electrodes (ISEs) and are also directly related to the response time of bulk optodes. Early transport studies with radio tracers on stacks of membrane slices (each 40-50 µm thick) separated again after the experiment corroborated the permselectivity of the membranes and revealed a concentration polarization of the free and complexed carrier.1,2 More recently, analogous photometric and chronoamperometric studies made use of H+-selective chromoionophores.3 Improved resolution of ∼5 µm, achieved with spatial imaging photometry, allowed the determination of apparent diffusion coefficients of water and a nitrite-selective porphyrin derivative.4,5 Recently, a new spectropotentiometric method was introduced to image the cross section of chromoionophore-based H+-selective membranes and to simultaneously record the potential-time transients induced by cation interference at high pH.6 †

Technical University of Budapest. Swiss Federal Institute of Technology. § Auburn University. (1) Wuhrmann, P.; Thoma, A. P.; Simon, W. Chimia 1973, 27, 637. (2) Thoma, A. P.; Viviani-Nauer, A.; Arvanitis, S.; Morf, W. E.; Simon, W. Anal. Chem. 1977, 49, 1567. (3) Nahir, T. M.; Buck, R. P. Helv. Chim. Acta 1993, 76, 407. (4) Li, Z.; Li, X.; Petrovich, S.; Harrison, D. J. Anal. Methods Instrum. 1993, 1, 30. (5) Li, X.; Petrovic, S.; Harrison, D. J. Sens. Actuators B 1990, 1 , 275. ‡

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Solvent polymer membranes containing a H+-selective chromoionophore and an additional cation-selective ionophore have been widely applied in cation-selective optodes.7,8 Chromoionophores without additional ionophores have been used both in pH electrodes9 and in anion-selective optodes.10 Since a Nernstian response of ISEs requires constant activities in the membrane phase and bulk optodes measure membrane concentration changes, the measuring ranges of pH electrodes and anion optodes are complementary.11 The optical and potentiometric responses can, therefore, be interrelated.12 For a given membrane, the upper detection limit of the electrode and the lower detection limit of the corresponding optode are defined by the onset of significant coextraction of acid. Since this depends on the lipophilicity of the anion, the selectivity of such optodes corresponds to the Hofmeister series.10 At high pH, on the other hand, the lower detection limit of the ISE is defined by an ion-exchange process, where H+ from the membrane is replaced by an interfering cation of the sample. The onset of this process depends on the lipophilicity of the cations.13 Such systems could be used as cationselective optodes, with selectivities defined by the lipophilicity of the cations. An important difference between the three sensing schemes, i.e., anion optodes, H+ ISEs, and cation optodes, is that anion optodes do not require the presence of lipophilic anions in the membrane. In ISEs, at least some sites must be present (the optimum is 50 mol % relative to the ionophore12), and the signal of cation optodes directly depends on the concentration of the sites. Previously, a combined optical and potentiometric technique was introduced to analyze the processes at high pH, i.e., in the cation interference of H+-selective ISEs.6 In this work, the same (6) Schneider, B.; Zwickl, T.; Federer, B.; Pretsch, E.; Lindner, E. Anal. Chem. 1996, 68, 4342. (7) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083. (8) Bu ¨ hlmann, P.; Bakker, E.; Pretsch, E. Chem. Rev., submitted. (9) Cosofret, V. V.; Nahir, T. M.; Lindner, E.; Buck, R. P. J. Electroanal. Chem. 1992, 327, 137. (10) Tan, S. S. S.; Hauser, P. C.; Chaniotakis, N. A.; Suter, G.; Simon, W. Chimia 1989, 43, 257. (11) Bakker, E.; Na¨gele, M.; Schaller, U.; Pretsch, E. Electroanalysis 1995, 7, 817. (12) Bakker, E.; Xu, A.; Pretsch, E. Anal. Chim. Acta 1994, 295, 253. (13) Bakker, E.; Willer, M.; Lerchi, M.; Seiler, K.; Pretsch, E. Anal. Chem. 1994, 66, 516. S0003-2700(97)00952-9 CCC: $15.00

© 1998 American Chemical Society Published on Web 02/14/1998

spectropotentiometric method is used to investigate processes induced in H+-selective membranes at low pH. EXPERIMENTAL SECTION Reagents. Poly(vinyl chloride) (PVC), 2-nitrophenyl octyl ether (NPOE), and the H+-selective chromoionophore N,N-diethyl5-(octadecanoylimino)-5H-benzo[a]phenoxazin-9-amine (ETH 5294) were from Fluka, Selectophore grade. Tetrahydrofuran (Fluka) used for membrane casting and all the other reagents for solution preparations were analytical grade (Merck and Fluka) and were used as received. The synthesis of the H+-chromoionophore N,Ndiethyl-5-[(2-octyleicosanoyl)imino]-5H-benzo[a]phenoxazin-9amine (ETH 2458) was previously described.14 Membranes. The membranes were prepared according to the method of Craggs et al.;15 however, to be in the linear range of the imaging camera where Beer’s law is fulfilled, the concentration of incorporated ionophore in the plasticizer-PVC membrane (2:1 w/w) was somewhat lower (0.15% w/w, 2.6 mmol kg-1) than in traditional compositions. Optode films contained 7.7 mmol kg-1 chromoionophore to allow for sufficiently large absorbance values. To ensure analogous experimental conditions between optode and ISE measurements, no lipophilic salt additive was incorporated into the membranes. The low amount of anionic sites present as impurities in PVC (70-200 µmol kg-1)16,17 is not optimal12 but is sufficient for a functioning ISE. With the exception of the optode films (see below), the final membranes were about 200-300 µm thick. Spectroscopic Imaging. The construction and performance of the spectropotentiometric imaging technique were shown and discussed in detail in a previous paper.6 The basic idea of the method is to use a ring-shaped membrane (6 mm i.d., 7 mm o.d., thickness ∼300 µm) as a spacer between two optical windows, forming a flow-through cell. As the cell is placed onto the specimen stage of an optical microscope, a segment of the membrane ring can be studied at selected wavelengths with the help of a series of interference filters, while the composition of the sample compartments may be changed with a flow-through system. Potentiometric Measurements in Macroelectrodes and Symmetrical Transport Cells. The membranes were mounted in Philips IS 561 liquid membrane electrode bodies (Mo¨ller Glasbla¨serei, Zu¨rich, Switzerland) and tested for their pH responses by titrating Britton-Robinson buffer solutions of constant chloride or nitrate concentrations (0.2 M) with 1 M NaOH. The pH response of the membranes in the acidic range was measured by titrating 100 mL of 0.1 M chloride or nitrate solution (0.005 M citrate, pH 7.5) with 0.1 M HCl or HNO3 containing 0.1 M NaCl or NaNO3 as background. A double-junction reference electrode with a free-flowing junction was applied.18 The sample pH was simultaneously monitored with a Philips glass electrode (model GA 110). (14) Bakker, E.; Lerchi, M.; Rosatzin, T.; Rusterholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211. (15) Craggs, A.; Moody, G. J.; Thomas, J. D. R. J. Chem. Educ. 1974, 51, 541. (16) Lindner, E.; Gra`f, E.; Nigreisz, Z.; To´th, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1988, 60, 295. (17) Na¨gele, M.; Pretsch, E. Mikrochim. Acta 1995, 121, 269. (18) Dohner, R. E.; Wegmann, D.; W. E., M.; Simon, W. Anal. Chem. 1986, 58, 2585.

Optode Measurements. A membrane cocktail of the same composition used for membrane casting was spun onto the surface of two silanized quartz windows at a spinning rate of 800 min-1. The ∼3-5-µm thin membrane-coated windows were fixed into a flow-through cell described earlier.19 Solutions of 0.1 M KCl, KNO3, and NaClO4 of different pH values (2 × 10-3 M citrate or 5 × 10-3 M Tris-HCl buffer) were pumped through the cell at a flow rate of 1.43 mL/min using a Watson Merlow pump (model 202F). The absorbance was recorded until complete equilibration, and a wavelength scan was carried out at each concentration level using a Unicam UV 4 spectrophotometer equipped with Vision 3.0 software. Special care was taken to avoid air contact with the membrane during the measurement to prohibit optical fluctuations. RESULTS AND DISCUSSION A newly developed spectroscopic imaging technique allows one to observe in situ time-dependent concentration changes within solvent polymeric membranes. The first paper introducing this technique presented results with ion-exchange systems.6 The present work deals with the kinetic observation and quantification of acid coextraction processes into ion-selective electrode and optode membranes doped with H+-selective chromoionophores. These experiments give valuable insights into the importance of diffusion processes for the functioning of sensors based on solvent polymeric membrane chemistry. For ion-selective electrodes, it is well established that a Nernstian response function is expected only if the concentration of the sensing ion is not significantly altered in the organic phase boundary contacting the sample solution. Consequently, this concentration has to be deliberately changed in order to induce time-dependent concentration gradients across the membrane. The bulk of optode membranes, on the other hand, have to completely equilibrate after every sample concentration change in order to yield a useful optical response. In the present paper, coextraction systems are of primary interest. They affect the upper detection limit of cation-selective electrodes and can be used to develop anion-responsive optical sensors. If the solvent polymeric film contains an electrically neutral H+selective chromoionophore C embedded in a organic membrane phase without incorporated ion-exchanger sites, the chromoionophore has the tendency to extract acids HA from the sample according to the following equilibrium: Kcoex

H+(aq) + A-(aq) + C(org) y\z A-(org) + CH+(org) (1) where (aq) and (org) denote species in the aqueous and organic phase, and Kcoex is the coextraction equilibrium constant for this process. The extent of acid coextraction is increased with lower pH, higher activity (aA-) and lipophilicity of A-, and higher concentration and basicity of C. The selectivity of the sensor is dictated by the relative lipophilicity of the sample anions and typically follows the so-called Hofmeister sequence. While a rigorous description of the response behavior was lacking in the past, this scheme has been reported before to develop anion(19) Seiler, K. Ionenselektive Optodenmembranen; Fluka Chemie AG: Buchs, 1991.

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Figure 1. Optode responses of ∼2-µm thin NPOE-PVC films containing a lipophilic H+-selective chromoionophore to different acids. Open symbols are for ETH 5294 as chromoionophore (alkyl, octadecyl); filled symbols are for ETH 2458 (alkyl, 2-octyleicosanoyl). Both compounds have the same basicity but differ in their lipophilicities.14 Solid lines are drawn according to eq 3. The selectivity follows the Hofmeister sequence, as the extracted anions are not complexed in the organic phase.

responsive sensors. The response function may be derived by introducing 1 - R as the concentration of protonated chromoionophore [CH+] relative to its total concentration, CT.20,21 This parameter is directly related to the absorbance of the optical film:

A - AD [CH+] )1-R) CT AP - A D

(2)

where A is the absorbance at a given equilibrium, and AD and AP are the absorbances with fully nonprotonated and protonated chromoionophore at the same wavelength. This relationship, combined with the fact that the concentration of extracted anions must be equal to the concentration of protonated chromoionophore for electroneutrality reasons, is combined with the coextraction equilibrium constant to give

aA-aH+ )

(1 - R)2CT RKcoex

(3)

Equation 3 may be used to quantitatively describe the optical acid coextraction response of solvent polymeric membranes containing neutral chromoionophores. Figure 1 shows a series of experiments with thin NPOE-PVC films containing either ETH 5294 (open symbols) or ETH 2458 (filled symbols) as neutral H+chromoionophores. The visible spectrum of the unprotonated and protonated chromoionophore ETH 2458 is shown in Figure 2. Both chromoionophores have virtually the same basicity,14 which is confirmed by the HNO3, experiments which correspond nearly perfectly. The solid lines are calculated according to eq 3, with CT ) 0.0077 mol kg-1 and log Kcoex ) 1.12 (for HCl), 3.10 (for HNO3), and 6.02 (for HClO4). These experiments indicate that such coextraction processes can be described quite accurately. According to theory,20,21 these coextraction constants translate into (20) Seiler, K.; Simon, W. Anal. Chim. Acta 1992, 266, 73. (21) Bakker, E.; Simon, W. Anal. Chem. 1992, 64, 1805.

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Figure 2. Spectra of the unprotonated and protonated chromoionophore ETH 2458 in a thin NPOE-PVC film as used for optode measurements (Figure 1). Virtually the same spectrum is observed for ETH 5294. opt opt ) 1.98 and log KCl,ClO ) the selectivity coefficients log KCl,NO 3 4 4.90, which are extremely close to the respective values obtained with ion-selective electrodes.22 Early reports on ETH 5294-based optodes indicated leaching problems of the chromoionophore with HCl as sample, and plasticized hydroxylated PVC was found to increase lifetimes in this respect.10 As compared to the present results, those optodes had a reduced preference of the lipophilic ions over chloride. This is in complete analogy to the influence of higher alcohols in the membrane on the selectivity of ionexchanger-based ISEs.23 In the present work, leaching limitations with HCl samples were circumvented by incorporating a more lipophilic isomeride, ETH 2458, into the optical film. With much thicker membranes as used for the imaging experiments, leaching was not found to be problematic. The optode experiments were used to predict the necessary experimental conditions for achieving a complete reconditioning of one membrane surface upon sample change. As the respective ion-selective electrode is ideally pH responsive, electrolyte coextraction must be negligible relative to the concentration of inherently present anionic sites. In Figure 1, this condition corresponds to the situation where 1 - R ≈ 0 (aClaH < 10-6). On the other hand, maximum electrolyte coextraction will protonate all available ionophore in the membrane phase boundary, and the electrode will be ideally anion responsive. In Figure 1, this condition is fulfilled where 1 - R ≈ 1 (aClaH > 10-2). Diffusion processes induced by an acidic sample were studied with the new imaging technique in three different experimental situations: (1) The simulation of an optode membrane or a solidcontact ion-selective electrode membrane, where no electrolyte is present on one side of the membrane. In this case, dry air was continuously blown through one of the sample compartments, while the sample in the other compartment was aqueous. (2) A symmetrical situation where the sample change occurs simultaneously in both aqueous compartments. This experiment was chosen since such time-dependent concentration profiles can be conveniently described by theory and the influence of errors in the not exactly known membrane thickness on the calculated

(22) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Talanta 1994, 41, 1001. (23) Ozawa, S.; Miyagi, H.; Shibata, Y.; Oki, N.; Kunitake, T.; Keller, W. E. Anal. Chem. 1996, 68, 4149.

Figure 3. Experimental time-dependent concentration profiles of the unprotonated H+ chromoionophore ETH 5294 (at 505 nm) in a ∼650-µm-thick ring membrane. The left side of the membrane was in continuous contact with air to simulate the response of an optode film. The solution on the right side of the membranes is abruptly changed from 0.1 M NaCl at pH 6.0 to 0.1 M NaCl + 0.1 M HCl so that all chromoionophore is protonated at this membrane surface, and the concentration profiles are monitored over time. Dotted lines are drawn according to eq 4.

diffusion coefficients is less critical. (3) The simulation of an ionselective electrode membrane undergoing anion interference, where one compartment contains a solution of pH 6 that induces negligible electrolyte coextraction, and the sample in the second compartment is abruptly changed to a solution that induces protonation of the chromoionophore. In some of these latter cases, the potentiometric response was simultaneously monitored. The time-dependent absorbance profiles of the protonated and unprotonated forms of ETH 5294 were used to determine apparent acid diffusion coefficients. The diffusion equation fitting the experimental conditions 1 and 2, under the assumption of a fast exchange equilibrium at the membrane/sample interface, is given by Crank:24

c - co c1 - c o

)1-

4

∞

∑

(-1)n

π n)0 2n + 1

×

exp{-D*(2n + 1)2π2t/4l2} cos

(2n + 1)πx 2l

(4)

where D* is the apparent diffusion coefficient, co and c1 are the concentrations of the species in the membrane at t < 0 and after reaching the new equilibrium (t ) ∞), respectively, l is the thickness of the membrane in experiment 1 (0 < x < l; Figure 3) and the distance measured from the middle of the membrane in experiment 2(-l < x < l; Figure 4). Images recorded at 700 nm, where the incorporated H+ chromoionophore does not absorb, showed that light-scattering effects due to heterogeneous water25 are negligible. In addition, the experimental conditions were chosen so that Beer’s law is fulfilled; hence, the concentrations in eq 4 could be calculated from absorbances (see Experimental Section and ref 6). Time-dependent concentration profiles of the unprotonated H+ chromoionophore ETH 5294 (at 505 nm) in a membrane contact(24) Crank, J. The Mathematics of Diffusion; Oxford University Press: New York, 1994. (25) Li, Z.; Li, X.; Petrovic, S.; Harrison, D. J. Anal. Chem. 1996, 68, 1717.

Figure 4. Experimental time-dependent concentration profiles of the unprotonated H+ chromoionophore ETH 5294 (at 505 nm) in a ∼650-µm-thick ring membrane. The solutions on both sides of the membrane are now symmetrically changed from 0.1 M NaCl at pH 6.0 to 0.1 M NaCl + 0.1 M HCl in order to protonate all chromoionophore at the membrane surfaces. Dotted lines are drawn according to eq 4.

ing air at the left side are shown in Figure 3. At t ) 0, the aqueous solution on the right side is changed from 0.1 M NaCl at pH 6 to 0.1 M HCl + 0.1 M NaCl. As can be seen in Figure 1, the chromoionophore is fully protonated at the membrane surface (log aHaCl ≈-2). By fitting eq 4, an apparent diffusion coefficient of D* ) (3.8 ( 0.3) × 10-8 cm2 s-1 (SD, n ) 6) is determined (theoretical curves shown as dotted lines in Figure 3). However, the calculated values strongly depend on the membrane thickness l (cf. eq 4), which is difficult to determine since the absorbances at the membrane edges are biased by diffraction and unevenness, and since the membrane thickness is not necessarily constant during the experiment. Values in the range of (2-8) × 10-8 cm2 s-1 did not lead to large deviations from the observed curves. Therefore, the experimental situation 2 was chosen, where the determination of this parameter is less critical (Figure 4). Here, a membrane is conditioned with two identical solutions (0.1 M NaCl at pH 6) at both sides. At t ) 0, the two solutions are simultaneously changed to 0.1 M NaCl at pH 1 so that the chromoionophore is again fully protonated at the membrane surfaces. Equation 4 again describes this process, but the membrane thickness is now 2l.24 By fitting eq 4 to the data, an apparent diffusion coefficient of (9.1 ( 1.3) × 10-8 cm2 s-1 (SD, n ) 5) is obtained (theoretical curves shown as dotted lines in Figure 4). These values are about 1 order of magnitude larger than those determined for the pH-sensitive chromoionophores ETH 5294 (1.1 × 10-8 cm2 s-1)26 and ETH 24396 (unprotonated, (0.72 ( 0.31) × 10-8 cm2 s-1; protonated, (1.11 ( 0.25) × 10-8 cm2 s-1) in bis(2-ethylhexyl) sebacate (DOS)-PVC) and for a series of other ionophores in PVC membranes using various plasticizers (5 × 10-9-5 × 10-8 cm2 s-1).27-29 The large values obtained indicate that the apparent diffusion coefficient is determined by both the diffusion of the protonated ionophore and that of the extracted acid. Similarly, large diffusion coefficients of (26) Nahir, T. M.; Buck, R. P. J. Phys. Chem. 1993, 97, 12363. (27) Iglehart, M. L.; Buck, R. P.; Pungor, E. Anal. Chem. 1988, 60, 290. (28) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1. (29) Li, X.; Harrison, D. J. Anal. Chem. 1991, 63, 2168.

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Figure 5. Experimental time-dependent concentration profiles of the protonated H+ chromoionophore ETH 5294 (at 678 nm) in a ∼650µm-thick ring membrane. The left side of the membrane was in continuous contact with 0.1 M NaNO3 at pH 6.0 to simulate the response of an ion-selective electrode. The solution on the right side of the membranes is abruptly changed to 0.1 M NaNO3 + 0.001 M HNO3 to protonate all chromoionophore at the membrane surface, and the concentration profiles are monitored over time.

about 1.5 × 10-7 cm2 s-1 were observed by Nahir and Buck30 for tridodecylamine in DOS-PVC membranes at pH 5 from transient current responses upon an externally applied potential step. Despite the differences between the experimental conditions (activity vs potential perturbation), the unusually large diffusion coefficients may, in both cases, have the same chemical origin. In contrast to the experiments shown above, classical H+responsive ion-selective electrode membranes are contacted at the inner side with pH-buffered electrolyte where the extent of anionproton coextraction is small. Consequently, the massive extraction of anion-proton pairs into the sample side of the membrane will eventually lead to a steady-state concentration profile where the electrolyte is continuously transported across the membrane into the inner solution side. Figure 5 shows imaging results as the sample is abruptly changed from a 0.1 M KNO3 solution of pH 6 to pH 3. This will completely protonate the H+ chromoionophore at the sample side (see Figure 1), while the chromoionophore at the inner side of the membrane remains nonprotonated, owing to the higher pH of the reference solution. Surprisingly, the attained concentration profiles are not linear, even after 20.3 h. Instead, an increasing portion of completely protonated chromoionophore is present. After 20.3 h, it extends up to the middle of the membrane, and then a linear profile toward its left side is observed. This process cannot be described by a simple diffusion process without making further assumptions that might not yet be warranted. With 0.1 M HCl + 0.1 M KCl as sample solution, analogous effects were observed (data not shown). A likely explanation for these results is the extraction of undissociated free acid from the sample, maybe in complete analogy to the extraction and diffusion behavior of water.25 As free acid is extracted, it eventually forms by itself a linear concentration profile across the ion-selective membrane, which in effect equals a pH gradient. Depending on the basicity and concentration of H+selective chromoionophore, therefore, the acid is capable of completely protonating the chromoionophore. Only as the (30) Nahir, T. M.; Buck, R. P. Talanta 1994, 41, 335.

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Figure 6. Experimental time-dependent concentration profiles of the protonated H+ chromoionophore ETH 5294 (at 678 nm) in a ∼650µm-thick ring membrane. The left side of the membrane is contacted with 0.1 M KCl at pH 6.0, while the right side of the membranes is abruptly changed to 0.1 M KCl + 0.001 M HCl to only partially protonate the chromoionophore.

concentration of acid drops below a critical value toward the left side of the membrane will the chromoionophore remain partially deprotonated. The extent of this effect depends on the concentration of extracted acid and available chromoionophore and the relative pKa values of both compounds. It is important to realize that the concentration of acid can still be quite small and induce nonlinear profiles, as long as the chromoionophore is basic enough. These results suggest that the large apparent diffusion coefficients obtained in the previous experiments (Figures 3 and 4) are influenced by diffusion of the free acid. Since in those cases equilibrium is attained, however, no unusual concentration profiles could be detected. The extraction of free, undissociated acid into the membrane should depend on the sample pH. Figure 6 shows an analogous experiment where sample conditions are changed to pH 3 (aHaCl ≈ 10-4) with chloride as the sample anion so that only about one-third of chromoionophore is protonated (see Figure 1). In this case, the expected linear concentration profile is observed. These data could now be fitted to theory24 (see ref 6 for the equation used), and an apparent diffusion coefficient of D* ) (1.0 ( 0.5) × 10-8 cm2 s-1 was observed for the protonated chromoionophore. This value is lower by about an order of magnitude than the ones obtained with strongly acidic solutions, leading to full protonation of the chromoionophore, and are close to the ones obtained earlier for chromoionophores (see above). This supports the above interpretation of the observed nonlinear concentration profiles (Figure 5), where it was assumed that the diffusion of the undissociated acid significantly contributes to the apparent diffusion coefficient. Parallel to the optical measurements shown in Figure 5, timedependent potential changes were recorded with conventional membranes and with the spectropotentiometric setup (cf. Figure 7). Upon changing the 0.1 M KNO3 solution at pH 6 to pH 3, a rapid potential step of ∼+75 mV is observed. This is much lower than that expected according to the Nernst equation (178 mV) because of the acid coextraction that reduces the concentration of the free chromoionophore and thus increases that of the protons. Such anion interference effects are well-known and understood by current theory.12 However, after a short apparent

membrane would lead to a positive drift (increasing [H+] induces a potential change of opposite sign than at the sample side), but, depending on the charge and mobility of the charged species, a slowly emerging diffusion potential could induce the same effect.

Figure 7. Long-term potential drift of regular ∼200-µm-thick H+responsive ion-selective electrode membranes under identical conditions, as shown in Figure 4.

steady state, a slow drift toward positive potential values is observed until a steady state is attained, amounting to about 8 mV over 2 h for conventional (l ≈ 200 µm) and ∼10 h for ringshaped membranes (l ≈ 650 µm). A similar type of response is observed when the inner solution is again substituted by the initial one (pH 6): a rapid decrease of ∼75 mV is followed by a slow drift toward negative potentials, the starting values being approached within the same time as for anion interference. The time periods needed to reach steady-state concentration (Figure 5) and constant potential values (Figure 7) are about the same for the membrane ring (l ≈ 650 µm). The difference between the two potential transients reflects an approximately quadratic dependence on l and again shows that the drift is related to diffusion processes. Analogous drifts were detected in the case of cation interference,6 but the time needed to reach steady state was larger by about a factor 8, in accordance with the ratio of the apparent diffusion coefficients in the presence and in the absence of acid coextraction. Although the observed drift is clearly related to membrane-internal diffusion processes, its exact reason cannot be given at this stage. A slow increase of the concentration of the protonated chromoionophore at the reference side of the

CONCLUSIONS Based on the intimate relationship of potentiometric and optical transduction, parallel and simultaneous experimentation with H+ chromoionophore-based optical and potentiometric anion sensors allows a detailed insight into time-dependent processes. A new quantitative description of the response of H+ chromoionophorebased anion optodes is given. With the help of optode measurements, experimental conditions are selected to investigate diffusion processes under partial and full protonation of the chromoionophore at the membrane surface. It is detected that a long-term potential drift is induced upon anion interference which can be related to membrane diffusion processes. Depending on the pH, the lipophilicity, and concentration of anions, apparent diffusion coefficients vary by a factor of 5-10. Fast processes are observed both optically and potentiometrically when excess acid is present to fully protonate the chromoionophore so that acid diffusion through the membrane seems to contribute significantly. Since diffusion coefficients are otherwise in the order of 10-8 cm2 s-1, coextraction-based anion optodes are not expected to respond much faster than other bulk optodes. ACKNOWLEDGMENT The authors thank the Swiss National Science Foundation, the U.S. Hungarian Science and Technology Joint Fund (Project JF No. 568), Hitachi Ltd. (Tokyo), Orion Research Inc. (Beverly, MA), and the Petroleum Research Fund (administered by the American Chemical Society) for financial support.

Received for review August 28, 1997. Accepted January 5, 1998. AC970952W

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