Spectropotentiometry: A New Method for in Situ Imaging of

The server is currently under maintenance and some features are disabled. Share Article. ACS Network; Twitter; Facebook; Google+; CiteULike; Email...
0 downloads 0 Views 346KB Size
Download PDF
Anal. Chem. 1996, 68, 4342-4350

Spectropotentiometry: A New Method for in Situ Imaging of Concentration Profiles in Ion-Selective Membranes with Simultaneous Recording of Potential-Time Transients Bernhard Schneider, Titus Zwickl, Beat Federer, and Erno 1 Pretsch*

Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Universita¨ tstrasse 16, CH-8092 Zu¨ rich, Switzerland Erno 1 Lindner

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

A new spectropotentiometric imaging technique is introduced for studying transport processes in connection with cation interference in H+-selective liquid polymer membranes that contain an indicator dye as chromoionophore. Mounted in a specially designed cell under a microscope, the inner and outer rims of a membrane ring are contacted with electrolyte solutions of identical composition. After the system reaches equilibrium, the pH of the inner solution is raised to a value at which cation interference does occur. Pictures are taken with a nominal resolution of 2.25 µm in selected wavelength ranges using a CCD imaging camera, and potential-time transients are recorded simultaneously. Concentration profiles of the protonated and unprotonated forms of the chromoionophore are calculated from the pictures and plotted as a function of time. On the basis of these results, the ratedetermining process is evaluated, and the possible sources of the nonmonotonic potential-time transients are discussed. The diffusion coefficients of the protonated and unprotonated indicator are obtained by fitting the appropriate diffusion equations to the concentration profiles. The new technique allows a more profound comparison of membrane compositions in order to optimize them systematically. In spite of the widespread applications of solvent polymeric membrane-based ion-selective electrodes (ISEs), there are still basic discrepancies in the interpretation of their response mechanism.1-4 Concerning the origin of ion selectivity and the rate of ISE response, controversy arose about the importance of interfacial ion-exchange and membrane bulk diffusion processes. The relevance of the latter was supported by the fact that transport and potentiometric selectivities of ionophores are related to each other. Numerous intuitive experimental techniques have been (1) Buck, R. P. In Ion-Selective Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum Press: New York, 1978; Vol. I, Chapter 3. (2) Morf, W. E.; Simon, W. Helv. Chim. Acta 1986, 69, 1120-1136. (3) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: New York, 1981. (4) Pungor, E. Pure Appl. Chem. 1992, 64, 503-507.

4342 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

developed with a view to understanding the response mechanism and corroborating the potential-determining process.5-15 Simon and co-workers published a number of papers on carrier-based membranes, supporting the close correlation between potentiometric and transport ion selectivities. The latter were determined under dc current flow by applying a large external field of up to 50 V to the membrane to induce ion transport.5,6,16 Xie and Cammann9 and Lindner et al.12 used ac impedance measurements and showed that the ratios of the exchange current densities for primary and interfering ions in poly(vinyl chloride) (PVC) or silicone rubber membranes correlate well with the respective pot . With the help of radiotracers, selectivity coefficients, Ki,j Horvai et al.17 found a perfect correlation also between the ionexchange selectivities of liquid membranes and the respective pot Ki,j data. On the other hand, measurements on thin optode membranes by Bakker et al.15 revealed that deviations from the phase boundary potential model occur only with massive extraction of sample anions or interfering cations into the membrane. The differences in interpreting the data obtained with ISEs can be attributed partly to the lack of a setup that would allow concentration profiles to be followed in the membrane during experiments. Transport and chronoamperometric studies on (5) Wuhrmann, P.; Thoma, A. P.; Simon, W. Chimia 1973, 27, 637-639. (6) Thoma, A. P.; Viviani-Nauer, A.; Arvanitis, S.; Morf, W. E.; Simon, W. Anal. Chem. 1977, 49, 1567-1572. (7) Horvai, G.; Graf, E.; To´th, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1986, 58, 2735-2740. (8) Lindner, E.; To´th, K.; Pungor, E.; Berube, T. R.; Buck, R. P. Anal. Chem. 1987, 59, 2213-2216. (9) Xie, S.-L.; Cammann, K. J. Electroanal. Chem. 1988, 245, 117-122. (10) Iglehart, M. L.; Buck, R. P.; Horvai, G.; Pungor, E. Anal. Chem. 1988, 60, 1018-1022. (11) To´th, K.; Lindner, E.; Pungor, E.; Zippel, E.; Kellner, R. Fresenius Z. Anal. Chem. 1988, 331, 448-453. (12) Lindner, E.; Niegreisz, Z.; To´th, K.; Pungor, E.; Berube, T. R.; Buck, R. P. J. Electroanal. Chem. 1989, 259, 67-80. (13) Nahir, T. M.; Buck, R. P. J. Phys. Chem. 1993, 97, 12363-12372. (14) Tohda, K.; Umezawa, Y.; Yoshiyagawa, S.; Hashimoto, S.; Kawasaki, M. Anal. Chem. 1995, 67, 570-577. (15) Bakker, E.; Na¨gele, M.; Schaller, U.; Pretsch, E. Electroanalysis 1995, 7, 817-822. (16) Morf, W. E.; Wuhrmann, P.; Simon, W. Anal. Chem. 1976, 48, 1031-1039. (17) Horvai, G.; Horva´th, V.; Farkas, A.; Pungor, E. Magy. Ke´ m. Foly. 1988, 94, 367-370. S0003-2700(96)00424-6 CCC: $12.00

© 1996 American Chemical Society

stacks of slices (each 40-50 µm thick) separated again after the experiment proved the permselectivity of the membranes and the concentration polarization of the free and complexed carrier.5,6,18 Although FT-IR/ATR11,19 and optical second harmonic generation (SHG)14 measurements sustained the existence of a thin space charge region, a high-resolution true imaging technique for studying membranes was described only by Harrison and coworkers, who determined concentration profiles of water20-23 and of a porphyrin complex24 in ISE membranes as a function of time using a spatial imaging photometer. The availability of ionselective chromoionophores25 has brought about additional possibilities for speciation within the membrane bulk.15 Theoretical simulations of diffusion potentials have been carried out for various ISE membranes26-28 but have not been verified so far for lack of experimental data. In the present paper, spectropotentiometric imaging is introduced as a new method for studying transport phenomena in membranes, allowing a simultaneous electrochemical control. It has been designed to visualize the transport of species in the bulk of ISE membranes in parallel with the measurement of transient potentials and is used to follow the redistribution of membrane components of H+-selective electrodes during cation or anion29 interference. The new technique is equally suited for studying concentration profiles during chronoamperometric and chronopotentiometric experiments. Also, by incorporating new lipophilic, water-sensitive dyes into membranes, water concentration profiles20 can be determined.30 Hence, with this new technique, not only are apparent diffusion coefficients in ISE membranes accessible, but also the rate-determining process can be evaluated and potential-time transients interpreted. Finally, membrane compositions may thus be optimized, since the performance of different ionophores, plasticizers, matrices, and lipophilic ionic additives can be compared by observing the prevailing processes. EXPERIMENTAL SECTION Reagents. The membrane constituents, poly(vinyl chloride) (PVC), bis(2-ethylhexyl) sebacate (DOS), and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB) were Selectophore from Fluka AG (Buchs, Switzerland). The ionophores, benzeneacetic acid 4-[[9-(dimethylamino)-5H-benzo[a]phenoxazin5-ylidene]amino]-11-[(1-butylphenyl)oxy]-11-oxaundecyl ester (ETH 2439) and the reaction product of 5-amino-9-(diethylamino)-5Hbenzo[a]phenoxazine with carboxylated PVC-methylaminododecanoic acid (ETH 3531), were synthesized in our laboratory.25,31 (18) Nahir, T. M.; Buck, R. P. Helv. Chim. Acta 1993, 76, 407-415. (19) Umezawa, K.; Lin, X. M.; Nishizawa, S.; Sugawara, M.; Umezawa, Y. Anal. Chim. Acta 1993, 282, 247-257. (20) Li, X.; Petrovich, S.; Harrison, D. J. Sens. Actuators, B1 1990, 275-280. (21) Chan, A. D. C.; Li, Z.; Harrison, D. J. Anal. Chem. 1992, 64, 2512-2517. (22) Li, Z.; Li, X.; Petrovich, S.; Harrison, D. J. Anal. Methods Instrum. 1993, 1, 30-37. (23) Li, Z.; Li, X.; Petrovic, S.; Harrison, D. J. Anal. Chem. 1996, 68, 17171725. (24) Li, X.; Harrison, D. J. Anal. Chem. 1991, 63, 2168-2174. (25) Bakker, E.; Lerchi, M.; Rosatzin, T.; Rusterholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211-225. (26) Buck, R. P.; Sandifer, J. R. J. Phys. Chem. 1973, 77, 2122-2128. (27) Sandifer, J. R. Anal. Chem. 1989, 61, 2341-2347. (28) Sandifer, J. R. Ion-Transfer Kinetics; VCH: Weinheim, 1995. (29) Lindner, E.; Schneider, B.; Zwickl, T.; Federer, B.; Lan, B. T. T.; To´th, K.; Pretsch, E. Manuscript in preparation. (30) Zwickl, T.; Schneider, B.; Cimerman, Z.; Schaller, U.; Pretsch, E. Manuscript in preparation.

Figure 1. Experimental setup for spectropotentiometric imaging.

All other reagents were analytical grade (Merck, Fluka) and used without further purification. Membranes. Rings ∼500-700 µm wide were punched from master membranes, which were prepared according to a standard procedure.32 Since the rings also serve as fittings in the spectropotentiometric cell, the total amount of components was adjusted to yield thicknesses of 200-300 µm. From optical measurements on a series of membranes differing in their ionophore concentrations, the range of the latter was determined (∼0-6.5 mmol kg-1), in which Lambert-Beer’s law is obeyed. Membranes with different molar concentrations of KTFPB (relative to the ionophore) were also tested, the results given here being obtained with ∼80 mol %. Spectropotentiometric Imaging. The experimental setup is shown in Figure 1. A specially designed flow-through thin-layer potentiometric cell with two compartments, formed with the help of two concentric O-rings, is placed on the specimen stage of an optical microscope. The inner O-ring is the ISE membrane ring itself (∼6 mm i.d., ∼7 mm o.d., thickness ∼0.2 mm; cf. Figures 1, 2b), whereas the outer O-ring (16 mm i.d., buna-nitrile) is placed around this ring and has the same thickness. The lower cell window is made of finely polished plexiglass and accommodates the solution delivery channels, while the upper one is a silanized quartz plate (Figure 2a). The flow pattern with the potentiometric cell arrangement is given schematically in Figure 2b. Through continuously flowing electrolyte solutions (∼0.6 mL h-1), the inner and outer rims of the membrane ring are in electrolytic contact with a double-junction free-flowing reference electrode.34 Thus, the membrane potential across the ring is measured between the inner and outer compartments. A segment of the membrane ring was studied at room temperature under the microscope (Olympus BH-2, Olympus Optical (Schweiz) AG, Schwerzenbach, Switzerland) by illuminating it with a halogen lamp of controlled source voltage. The appropriate wavelengths of the protonated and unprotonated forms of the ionophores25 were selected with the help of interference filters (Balzers AG, Balzers, Lichtenstein). A charge-coupled device (CCD) imaging camera (Model ST-6, Santa Barbara (31) Rosatzin, T. R.; Holy, P.; Seiler, K.; Rusterholz, B.; Simon, W. Anal. Chem. 1992, 64, 2029-2035. (32) Craggs, A.; Moody, G. J.; Thomas, J. D. R. J. Chem. Educ. 1974, 51, 541544.

Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

4343

Figure 2. Flow-through thin-layer cell used in spectropotentiometric experiments. (a) Cross section of the cell. (b) Dimensions of the membrane ring, directions of electrolyte flow, and connection of reference electrodes.

Instruments Group, Santa Barbara, CA) was used to record the microscope pictures. Its temperature was set to -30 °C with a built-in two-stage thermoelectric cooler to reduce the dark current. The pictures taken with the CCD camera consisted of 375 × 245 pixels, corresponding to a resolution of 2.25 µm (at 10-fold magnification of the microscope), determined with an objective micrometer (No. 35037, Olympus Optical) having 0.01 mm calibration marks. Each pixel registered the light intensity between 0 and 65 535 arbitrary units. Experimental conditions were adjusted so as to make use of the linear range of the system. The intensity of the brightest pixel was set to ∼80% of the 4344

Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

maximum by (1) optimizing the ionophore concentration in the membrane (see above), (2) selecting the appropriate wavelength (which does not necessarily correspond to the absorption maximum), and (3) choosing the best exposure time. Within a preselected ring segment, the median of the intensity values for 100-200 pixels along lines perpendicular to the radius was evaluated in order to minimize the noise caused by membrane inhomogeneities. The negative logarithms of the intensities thus obtained are a direct measure of the relative concentrations of the protonated or free chromoionophore, depending on the wavelength chosen, as was verified through a series of calibration

measurements. Absorbance profiles across the membrane were calculated from the pictures using a LabView program (National Instruments, Austin, TX) developed in our laboratories by using the intensities measured after initial conditioning as reference. This procedure also reduces membrane inhomogeneity effects. The reference curves (shifted to absorbance 0) are shown in the figures as superimposed dotted lines marked with I0. Potentiometric Measurements with Macroelectrodes and in a Symmetrical Cell. The pH response of membrane disks (7 mm diameter, ∼280 µm thickness) mounted in Philips IS 560 electrode bodies (Mo¨ller Glasbla¨serei, Zu¨rich, Switzerland) was determined in the presence of different interfering cations. The lower detection limit is at pH 8.6, and the Nernstian response range extends to pH < 2. It should be noted that this membrane composition was chosen for high optical sensitvity, but it is not optimal with regard to the measuring range.33 A 0.005 M citrate or acetate buffer (100 mL) of pH 4.0 and 0.1 M cationic background was titrated with 0.1 M NaOH solutions, being 0.001 M each in citric and boric acids or 0.005 M in NaOAc, respectively, employing a Metrohm 665 Dosimat controlled by a home-made LabView program for constant pH change. The solution pH was monitored with a glass electrode (Philips, GA 110) and a doublejunction free-flowing reference electrode.34 In addition to the spectropotentiometric experiments, transient potentials were recorded under similar conditions but using membranes of ∼280 µm thickness in a symmetric cell.35 This setup allowed even small long-term potential drifts to be measured reliably. The temperature was 21 ( 2 °C. Calculations. Diffusion coefficients were calculated by using Mathematica (Wolfram Research Inc., Champaign, IL) on a Silicon Graphics Iris Crimson workstation with eqs 1 and 2. The procedure “FindMinimum” was applied to perform nonlinear fits of membrane parameters to the absorbance curves measured. RESULTS AND DISCUSSIONS Free Diffusion of the Chromoionophore. In this work, the pH-sensitive chromoionophore ETH 2439 is used to monitor the concentration profiles of its protonated and unprotonated forms in ion-selective DOS/PVC membranes. To measure the free diffusion of both forms uninfluenced by other species, one membrane with chromoionophore and another one without (blank) are cut in halves. At the starting time, t ) 0, two moieties, one of each membrane, are united by moistening the cut edges with a very little THF and pressing them together (Figure 3a). As observed in earlier experiments on membrane stacks,5,6,18 redistribution of the ionophore sets in immediately. This diffusioncontrolled process can be followed with high resolution (Figure 3b) by means of the new setup described here. The diffusion of the chromoionophore into a semiinfinite matrix is represented by the following relationship:36

A(x,t) ) 1/2A0 erfc

x 2x(Dt)

(1)

where A0 is the initial absorbance (at t ) 0) of the compound in (33) Bakker, E.; Xu, A.; Pretsch, E. Anal. Chim. Acta 1994, 295, 253-262. (34) Dohner, R. E.; Wegmann, D.; Morf, W. E.; Simon, W. Anal. Chem. 1986, 58, 2585-2589. (35) Rumpf, G. G.; Du ¨ rselen, L. F. J.; Bu ¨ hler, H. W.; Simon, W. In Contemporary Electroanalytical Chemistry; Ivaska, A., Lewenstam, A., Sara, R., Eds.; Plenum Press: New York, 1990.

Figure 3. (a) Assemblage of two DOS/PVC membrane halves, one with and the other without indicator dye, for monitoring the free diffusion of chromoionophores. (b and c) Time-dependent absorbance profiles for ETH 2439 (evaluated from pictures taken at 450 nm; exposure, 3 s) and ETH 3531 (from pictures taken at 505 nm; exposure, 1.6 s), respectively.

the loaded membrane moiety and D its diffusion coefficient; x is the distance, t the time, and erfc denotes the error function. By fitting eq 1 to the absorbance profiles obtained from the images taken at 450 nm, the diffusion coefficient of the free, unprotonated ionophore can be determined. The resulting value of (2.8 ( 0.4) × 10-8 cm2 s-1 (SD, n ) 3) compares well with data determined earlier by other methods for valinomycin (1.5 × 10-8 and 3.0 × 10-8 cm2 s-1) in bis(1-butylpentyl) adipate- and 4-nitrophenyl octyl ether (o-NPOE)-based PVC membranes, respectively,37 and for the Ca2+-selective ionophore (-)-(R,R)-N,N′-bis[11-(ethoxycarbonyl)undecyl]-N,N′,4,5-tetramethyl-3,6-dioxaoctanediamide (ETH 1001) in o-NPOE/PVC (5 × 10-8 cm2 s-1),38 as well as for the pH-sensitive chromoionophore N-[9-(diethylamino)-5H-benzo[a]phenoxazin-5-ylidene]octadecanamide (ETH 5294) in DOS/PVC (1.1 × 10-8 cm2 s-1)13 and the NO2--selective bromo(pyridine)(5,10,15,20-tetraphenylporphyrinato)cobaltate in bis(2-ethylhexyl) adipate (5 × 10-9 cm2 s-1).24 (36) Crank, J. The Mathematics of Diffusion; Oxford University Press: New York, 1993. (37) Iglehart, M. L.; Buck, R. P.; Pungor, E. Anal. Chem. 1988, 60, 290-295. (38) Armstrong, R. D.; Horvai, G. Electrochim. Acta 1990, 35, 1-7.

Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

4345

In an analogous experiment using ETH 3531, the chromoionophore part of which is immobilized on PVC (see Experimental Section), the initial concentration distribution remained practically the same in the two halves of the united membrane for nearly 5 days (Figure 3c). This covalently anchored chromoionophore gave successful ion-selective optodes of extended lifetime,31 as well as ISEs with satisfactory pH response.39 However, the response times of membranes containing fixed or low-mobility ionophores generally are significantly longer than those of the corresponding mobile ones.12,39 Diffusion Processes as a Consequence of Cation Interference. In a pH-sensitive liquid membrane, the concentration of the protonated ionophore is determined by that of its lipophilic negative sites if it is equilibrated with an aqueous solution in which the ISE shows Nernstian response.15,40 Deviations from this behavior owing to anion or cation interference are expected at very low or high sample pH values, respectively.33 Strongly acidic solutions, especially when they contain lipophilic anions, lead to the coextraction of acid into the membrane so that the concentration of the protonated chromoionophore increases at the cost of the unprotonated form.29,33,41,42 Spectropotentiometric investigations on this process will be the topic of a forthcoming paper. At high pH, protons in membrane are exhanged with interfering cations of the sample so that the concentration of the protonated ionophore decreases. Eventually, the chromoionophore in the surface layer of the membrane is fully deprotonated so that the ISE response is controlled by the interfering cation and becomes pH-independent.33 The thereby induced diffusion processes in the membrane bulk are followed by the novel spectropotentiometric method. The concentrations of the protonated and unprotonated ionophore in a section of the membrane ring are monitored as a function of time, simultaneously also measuring the membrane potential. To begin with, both rims of the membrane ring are conditioned in a flow-through manner (Figure 2b) with an acetate buffer (0.005 M) of pH 3.9, which contains 0.1 M Na+ as interfering ion. As soon as the cell voltage is constant, pictures are taken at three different wavelengths: the absorbances at 700 and 450 nm are a measure of the concentration of the protonated and unprotonated chromoionophore, respectively,25 and those obtained at 766 nm reveal the influence of light scattering and might provide information on the presence of heterogeneous water.22 At t ) 0, the conditioning buffer on the inner side of the membrane ring is exchanged instantly by an equimolar mixture of NaCl and NaOH of pH 12.7, again 0.1 M in Na+. With this solution, severe cation interference is expected since the lower detection limit of the electrode is at pH ∼8.6. To follow the ion-exchange process, pictures are then taken at the above three wavelengths at selected moments. After steady state is reached, reconditioning of the membrane ring is started by replacing the inner solution with the initial acetate buffer of pH 3.9, upon which a series of pictures are recorded as mentioned before. In Figures 4 and 5, the absorbances obtained during the process of cation interference and reconditioning at the three wavelengths mentioned are plotted as a function of the distance, (39) Lindner, E.; Cosofret, V. V.; Kusy, R. P.; Buck, R. P.; Rosatzin, Th.; Schaller, U.; Simon, W.; Jeney, J.; To´th, K.; Pungor, E. Talanta 1993, 40, 957-967. (40) Buck, R. P.; To´th, K.; Gra´f, E.; Horvai, G.; Pungor, E. J. Electroanal. Chem. 1987, 223, 51-66. (41) Buck, R. P.; Cosofret, V. V.; Lindner, E. Anal. Chim. Acta 1993, 282, 273281. (42) Boles, J. H.; Buck, R. P. Anal. Chem. 1973, 45, 2057-2062.

4346

Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

x (eq 2, below). The absorbances near the edges of the ring are biased by diffraction, unevenness, and/or light scattering by water droplets.20-22 The interface between membrane and solution, therefore, cannot be investigated with this method. The absorbances of the protonated and unprotonated forms in the membrane ring equilibrated at pH 3.9 (I0 in Figure 4a,b) are almost constant over the entire ring segment observed. Upon increasing the pH of the inner solution (i.e., right side of Figure 4a-c) to 12.7, interfering cations (Na+) are extracted into the membrane so that the concentration of the protonated ionophore continuously decreases from the inner rim to the bulk of the membrane, whereas that of the unprotonated form increases (cf. Figure 4a,b). The resulting absorbance curves for both forms are practically mirror images of each other, with the only exception that their ranges are different owing to the difference in the molar extinction coefficients of protonated and unprotonated chromoionophore. At steady state, reached after ∼75 h, the concentration profiles are close to linear. However, since their shape is a function of the exchange rate of cations at the solution/membrane interface and of the diffusion rate within the latter, they might be nonlinear.29,43 The fact that no significant concentration change is detectable at the outer (left-hand) rim indicates that the diffusion process within the membrane is rate-determining. This is in accordance with earlier findings obtained by voltammetric investigations of the water/nitrobenzene interface, which showed that the complexation-decomplexation reaction is fast for all ionophores studied, no kinetic limitations of the phase transfer having been observed.44 The ion-exchange process as a consequence of cation interference is evident from the concentration profiles of the membrane ring and is highly reversible as shown during reconditioning (Figure 5a,b). Upon changing the pH of the inner solution to 3.9 again, the linear concentration profiles, passing through nonlinear interim stages, return to the original absorbance curves (cf. Figures 4a,b and 5a,b). The absorbances recorded at 766 nm during the process of cation interference (Figure 4c) and reconditioning (Figure 5c) are not influenced by the free or protonated form of the chromoionophore. Light scattering through water droplets is the most likely interpretation.21 Interestingly, the extent of the heterogeneous water region, which increases during cation interference, seems to decrease again during reconditioning. Similar reversibility of water droplet formation was reported very recently.23 These changes might be explained qualitatively in that the concentration of (uncomplexed) Na+, as indicated by that of the protonated chromoionophore, is at its highest near the membrane surface. A quantitative interpretation, however, is impossible since light scattering is a complex function of the diameter and number of the water droplets21 and since the position of the membrane rims might change because of swelling and shrinking during the experiment. It has been observed that the water content of PVC membranes depends on the composition of the conditioning solution.23 According to our recent results (unpublished), its influence is especially pronounced if uncomplexed (hydrophilic) ions are involved, such as in the present case. To determine the diffusion coefficients of the protonated and unprotonated forms of ETH 2439, the absorbance profiles recorded during the process of cation interference were used. The (43) Buck, R. P.; Nahir, T. M.; Cosofret, V. V.; Lindner, E.; Erdosy, M. Anal. Proc. Inc. Anal. Commun. 1994, 31, 301-312. (44) Vanysek, P. Electrochemistry on Liquid/Liquid Interfaces; Springer: Berlin, 1985.

Figure 4. Time-dependent absorbance changes for H+-selective ETH 2439/DOS/PVC membrane ring induced by cation (Na+) interference at pH 12.7 from the electrolyte solution contacting the inner rim (right side), relative to the equilibrium value obtained for the symmetrically conditioned ring at pH 3.9 (broken line). Theoretical curves (eq 2) correspond to individual fits. Recordings at (a) 700, (b) 450, and (c) 766 nm.

Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

4347

Figure 5. Time-dependent absorbance changes for H+-selective ETH 2439/DOS/PVC membrane ring during reconditioning of the inner rim with the initial solution of pH 3.9 (right side), relative to the equilibrium value at pH 3.9 for the symmetrically conditioned membrane (broken line). Theoretical curves calculated with appropriate values of D. Recordings at (a) 700, (b) 450, and (c) 766 nm. 4348 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

Figure 6. Potential-time transients recorded at 21 ( 2 °C for a H+-selective ETH 2439/DOS/PVC membrane disk of ∼280 µm thickness (in a symmetric cell) and for an analogous membrane ring of ∼675 µm width (in the spectropotentiometric setup) during cation interference at pH 12.7 and reconditioning at pH 3.9 (lower arrows).

diffusion equation fitting the experimental conditions, i.e., a fast exchange equilibrium at the membrane/sample interface and relative absorbance, is given according to Crank:36

( ) (

)

Dn2π2t x 2 ∞ Al cos(nπ) nπx sin Ax,t ) Al + exp l πn)1 n l l2

∑

(2)

where Ax,t is the absorbance of the observed species in the membrane for distance x and time t of diffusion, Al is the absorbance of the observed species at the disturbed membrane interface (x ) l) at t > 0, D is the diffusion coefficient (cm2 s-1) of the observed species in the membrane, l is the width (cm) of the membrane ring, t is the time of diffusion (s), and n is a running index. This equation describes the diffusion of electrically neutral species. Its use in the present context assumes that the influence of membrane internal potential gradients is negligible. The absorbance profiles recorded between ∼9 min and 24 h were selected for curve fitting. Measurements at t < 9 min were not used because, at the beginning of an experiment, uncertainties in t are relatively large and absorbances only vary near the membrane edges where they are biased. On the other hand, measurements at t > 24 h (close to the steady state) were also ignored since they are less informative. Besides Al and D, the positions of the edges of the membrane ring (defined by l and the center of the ring segment), difficult to determine exactly, were also used as fitting parameters in the computations. Thereby, about 675 ( 55 µm was obtained for the relevant diffusion path length (l). The different fittings yielded diffusion coefficients of (0.72 ( 0.31) × 10-8 cm2 s-1 (SD, n ) 8) and (1.11 ( 0.25) × 10-8 cm2 s-1 (SD, n ) 8) for the protonated and unprotonated forms of the chromoionophore in the cation interference experiments, respectively (calculated curves in Figure 4a,b). Due to the large noise, no fits were made on the basis of the reconditioning experiments (Figure 5a,b). The fact that similar

diffusion coefficients of the chromoionophore were obtained for the free diffusion (Figure 3) and in the case of cation interference (Figures 4,5) indicates that, as suggested by Nahir and Buck,18 the contribution of the diffusion (or “hopping”) of mobile H+ is negligible. Analogous experiments (not shown) were carried out for Na+, K+, and Cs+ as interfering cation on membrane rings with a somewhat higher KTFPB content (94 mol % relative to ETH 2439). The average diffusion coefficients calculated for the unprotonated and protonated forms during the cation interference process are the same within experimental errors for all cases investigated. A closer inspection of the figures shows that the theoretical curves fit nicely to the observed ones for the unprotonated chromoionophore (Figure 4b,5b). The fits are also (very) good for CH+ at the beginning of the experiment (Figure 4a, t e 1 h, 25 min), but some systematic deviations between theoretical and observed curves occur above t ) 1 h, 25 min (in Figure 4a) and for the beginning of the reconditioning experiment (Figure 5b). This might indicate that the model assumptions are not entirely fulfilled in these cases. Interpretation of Transient Potentials. For two membranes of identical composition but different length (l) of diffusion path, Figure 6 shows the potential transients recorded after a sudden pH change of the inner solution from 3.9 to 12.7 at otherwise constant concentration of interfering Na+ (0.1 M). Both response functions exhibit an overshoot: with l ≈ 280 and 675 µm, after an initial fast potential decrease by 255 and 260 mV, respectively, and a short apparent steady state, a slow drift toward positive values is observed, amounting to 25.3 and 24.2 mV over 13 and 75 h for the respective diffusion paths, until a steady state is attained. A similar type of response is observed when the inner solution is again substituted by the initial one (pH 3.9): a fast increase of ∼250 mV is followed by a slow drift toward negative potentials, the starting values being approached within the same time as for cation interference. The time periods needed to reach Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

4349

the membranes with l ≈ 280 and 675 µm has a quadratic dependence on the membrane thickness. Also, the simultaneously recorded absorbances show that the potential varies until the concentrations reach a steady-state value in the whole of the membrane. The potential drift may be induced by various diffusion-related mechanisms. Both a slowly developing diffusion potential26,27 and/or a slow change in membrane composition at the interfaces45 may contribute to it. The relevance of the two mechanisms is the topic of current investigations.

Figure 7. Schematic representation of the concentration profiles of the unprotonated (C) and protonated (CH+) chromoionophore (a) in a membrane symmetrically conditioned at pH 3.9; (b and c) immediately after increasing the pH on the right side of the membrane from 3.9 to 12.7 and at steady state, respectively.

constant potential values and steady-state concentrations within the membrane (see Figure 4a,b) coincide. The long-term drift observed here for the first time may be caused by changes in boundary phase and/or diffusion potentials. When increasing the pH of the solution on the right side of the membrane schematically represented in Figure 7, equilibrium H+ activities in both phases at the interface are reached almost instantly. As a consequence of the gradual exchange of H+ for Na+, the concentration of the free chromoionophore, C, increases, while that of the protonated chromionophore, CH+, decreases. The decrease in H+ activity in the surface layer of the membrane causes the phase boundary potential to increase, which is the very reason for the non-Nernstian potential change (about -260 instead of -514 mV at 21°; Figure 6) according to the ion-exchange model.15 Since this exchange process is fast, it cannot contribute to the long-term potential drift, which, therefore, must have its origin in diffusion processes. This interpretation is supported by the fact that the time needed to reach steady-state potential for (45) Fu, B.; Bakker, E.; Yun, J. H.; Yang, V. C.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 2250-2259. (46) To´th, K.; Lan, B. T. T.; Jeney, J.; Horva`th, M.; Bitter, I.; Gru ¨ n, A.; AÅ gai, B.; To¨ke, L. Talanta 1994, 41, 1041.

4350 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996

CONCLUSIONS The spectropotentiometric technique introduced in this work opens up new possibilities of investigating ISE response mechanisms. Long-term potential drifts of H+-selective liquid membranes induced by cation interference are observed for the first time and are shown to be related to diffusion processes within the membrane. Diffusion coefficients of the protonated and unprotonated chromoionophore ETH 2439 were evaluated on the basis of simultaneous absorbance measurements. Now that they are known it may be possible to calculate diffusion coefficients of interfering cations by fitting an appropriate model28 to the respective transient potential curves. Information on cation mobilities in pH-sensitive membranes could also be obtained by following the deprotonation process of a covalently bonded chromoionophore. The spectropotentiometric investigations are, of course, limited to compounds absorbing in the UV/visible. However, ionophores are available for cations other cations than H+,46 and current work includes the covalent attachment of chromophores to lipophilic sites and further ion-selective ligands. The diffusion of interfering cations into the membrane was brought about by concentration changes. As shown recently by Nahir and Buck,13 it can also be induced by external potentials, which would provide another approach to spectropotentiometric investigations. Moreover, a new combined optopotentiometric sensing scheme based on ion-selective chromoionophores is feasible. Besides the selectivity of the ion complexation, differing diffusion coefficients could serve as a further selection principle. ACKNOWLEDGMENT E.L. thanks the Swiss National Science Foundation for supporting his stay in Zu¨rich. We thank Dr. D. Wegmann for careful reading of the manuscript. Received for review April 29, 1996. Accepted September 4, 1996.X AC9604245 X

Abstract published in Advance ACS Abstracts, October 15, 1996.