Anal. Chem. 1994,66, 3592-3599
This Research Contribution is in Commemoration of the Life and Science of
I. M. Kolthoff (1894-1993).
Carboxylated Poly(viny1 chloride) as a Substrate for Ion Sensors: Effects of Native Ion Exchange on Responses Vaslle V. Cosofret, Richard P. Buck,' and Mlklos Erdosy Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
The ion-exchange properties of carboxylated poly(viny1 chloride) (PVC-COOH) are described and interpreted by theory and experiments for H*and M&+-sensitivemembrane sensors. With the new ion-exchange data for PVC-COOH membranes relativeto neutral carrier-PVC, a number of discrepantfeatures for PVC-COOH, noted previously, can be explained. Among these are the following: the relatively high bulk conductivity of plasticized PVC-COOH compared with plasticized high molecular weight PVC; changes in conductivity of PVC-COOH membranes containing different basicity amino compounds; variation of carrier loading-determined selectivities for PVCCOOH membranes when plasticizers of high vs low dielectric constant are substituted; reversal of response signs when PVCCOOH is used as a matrix for anion exchangers. The paper enunciates another response selectivity principle based on the ratio of opposite charge fixed and mobile sites. Use of carboxylated poly(viny1 chloride) (PVC-COOH) as an ISE substrate is not n e ~ . l - Anzai ~ et a1.6 prepared a pH sensor using tridodecylamine proton neutral carrier (TDDA) with o-nitrophenyl octyl ether (o-NPOE) as plasticizer. They found a limited pH response (pH range 4-10; sensitivity, 50 mV/pH unit). However, this response was sufficient to permit pH changes from penicillin hydrolysis after adsorption immobilization of penicillinase on the membrane surface to be detected.6 We will show that the quantity of carrier was not sufficient to produce optimal pH response. But even with a correct excess of carrier relative to sites, the linear pH response range of PVC-COOH membranes is limited and depends on plasticizer polarity; higher dielectric constant o-NPOE shows greater linearity range and better slope than low dielectric constant dioctyl sebacate (DOS).3 However, with DOS plasticizer in PVCCOOH, pH response extends farther into the acid solution range compared with the same carrier system in high molecular (1) Lindner, E.; Graf, E.; Niegreisz, Z.; Toth, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1988, 60, 295-301. (2) Ma, S. C.; Chaniotakis, N. A.; Meyerhoff, M. E. Anal. Chem. 1988, 60, 2293-2299. (3) Lindner, E.; Cosofret, V. V.; Kusy, R. P.; Buck, R. P.; Rosatzin, T.; Schaller, U.;Simon, W.; Jeney, J.; Toth, K.; Pungor, E. Talanta 1993, 40, 957-967. (4) Satchwill, T.; Harrison, D. J. J. Electroanal. Chem. 1988, 202, 75-81. (5) Pace, S. Eur. Patent Appl. 138, 150, 1985. (6) Anzai, J.; Shimada, M.; Osa, T., Chen, C. W. Bull. Chem. SOC.Jpn. 1987, 60, 41334137. (7) Buck, R. P.; Toth, K.; Graf, E.; Horvai, G.; Pungor, E. J . Electroanal. Chem. 1987, 223, 51-66.
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Analytical Chemistry, Vol. 66,No. 21, November 1, 1994
weight PVC. This extension may be related to the intrinsic response of the COOH groupsa3 In our most recent work* we demonstrated that the dissociation process of PVC-COOH depends on the polarity of the plasticizer as well as on the pH of the bathing solution. There is a very distinct and characteristic increase of conductivity upon soaking in pH 7 carrier-free membranes with o-NPOE, compared with DOS plasticizer. The latter system actually become more resistive on soaking, except when a reactive basic proton carrier is present. Then the conductivity remains nearly constant and is determined partly by the carrier loading. Although water uptake is slow in PVC-COOH,3 there is marked difference between resistance changes caused by water uptake compared with ion-forming neutralization interactions between basic amine carriers and the COOH groups.8 Carboxylated poly(viny1 chloride), density 1.390 g/mL, is an article of commerce and was synthetically prepared to take advantage of improved bonding to surfaces through the 1.8 wt % ' COOH.4 The determination of ion-exchange capacity used LiOH solutions to force ion exchange and to replace protons by Li+ ions in nonplasticized "dry" powder. The Li+ was later stripped and analyzed by atomic absorption and led to a capacity of 276 mmol of COOH/kg, although the calculated value is about 400 mmol/kg (560 mM).' Plasticized membranes made from PVC-COOH with DOS and other common esters with dielectric constants 3-5 were analyzed and found to contain only about 0.3 mmol of COOH/ kg. Even when plasticized membranes contained complexforming neutral carriers, viz. valinomycin, analysis of sites by potassium content gave only 1.48 mmol/kg. These discrepant mass balances on sites were further emphasized using the Donnan exclusion failure method to estimate site densities of plasticized PVC-COOH relative to high molecular weight PVC bathed in KSCN ~olutions.~ Both membranes contained valinomycin and DOS. Since the concentration of maximum response is proportional to site concentration, a value of 5-1 5 mmol/kg is inferred for PVCCOOH using observed 0.1-0.3 mmol/kg for PVC. Carboxylated PVC is known to contain some other trapped anionic site impurities, and their concentration levels are equivalent to regular high molecular weight PVC as evaluated from FTIR spectra.* (8)Cosofret,V.V.;Kao, W. J.;Lindner,E.;Erdosy,M.;Anderson, J. W.;Neuman, M. R.; Buck, R. P. Analyst, in press.
0003-27OOf94fO366-3592$04.50/0 0 1994 American Chemical Society
The discrepancy between apparent COOH content and ion-exchange capacity was considered to be a problem of accessibility of protons and different degrees of weakness of the COOH in less plasticized vs more plasticized regions of the polymer membrane. Meyerhoff et ala2showed that major removal of H+ required strong base treatment and showed by FT-IR spectroscopy that no carboxylate ions were present in the normal acidic form. The two research group^^?^ commented on the minor or negligible effects of COOH in PVC on the properties of potassium and calcium ion-selective electrodes based on PVC-COOH, which were compounded optimally with added tetraphenylborate (TPB) mobile sites and an ideal ratioof excesscarrier toTPB Selectivities were more affected by the addition of plasticizers with higher dielectric constants, viz. o-NPOE, e t o = 23.9 than by substituting PVC-COOH for high molecular weight PVC. Since the theory of Donnan exclusion depends on the ratio of site density to salt extraction c~efficient,~ it was not possible to conclude that increased dielectric constant of plasticizer was preferentially aiding COOH dissociation (increasing site density) rather than improving the extraction of a salt or an acid. Both would lead to increased conductivities. With the new ion-exchange data for PVC-COOH membranes relative to neutral carrier-PVC, and aminated PVC membrane data, a number of other discrepant features for PVC-COOH, noted previously,lv2can be explained. Among these are the following: (1) relatively high bulk conductivity of PVC-COOH compared with ordinary PVC in T H F solution'o and in compounded, plasticized membra ne^;^,' (2) changes in conductivity of PVC-COOH membranes containing different basicity amino compounds;* (3) variations of hydration time, response range, and counterion selectivity of neutral carrier membranes for cations by virtue of the intrinsic ionexchange selectivity of the RCOO- itselC3 (4) variations of carrier loading-determined selectivities for PVC-COOH membranes when plasticizers of high vs low dielectric constant are substituted; ( 5 ) reversal of response signs when PVCCOOH is used as a substrate for anion exchangers; (6) unexpected variations of co-ion interferences when tetraalkylammonium salts are added to anion-exchanger systems in PVC-COOH. The emphasis of this work is demonstration of native responses of PVC-COOH membranes to pH, pNa, pK, and pCa. Consequently, no trapped mobile sites are added to these membranes. All of the native effects are catalogued first. Finally, some additional responses that suggest themselves, e.g., (1) responses of a stronger acid additive, dinonylnathphthalenesulfonic acid (DNNS), a mobile trapped cation exchanger, and (2) alteration of known responses of trapped, cationic mobile sites for anion exchange (TDA+Br-), are illustrated with striking new results.
EXPERIMENTAL SECTION Reagents. For all experiments, deionized water (Barnstead NAN-0-Pure 11) and chemicals puriss or p.a. grade were (9) Buck, R. P.; Cosofret, V. V.; Lindner, E. Anal. Chim. Acta 1993, 282, 273281. (IO) Cosofret, V. V.; Lindncr, E.; Buck, R. P.; Kusy, R. P.; Whitley, J . Q.J. Electroanal. Chem. 1993, 345, 169-1 8 I. ( 1 1 ) Cosofret, V. V.; Lindner, E.; Buck, R. P.; Kusy, R. P.; Whitley, J. Q. Elecfroanalysis 1993, 5 , 725-730.
Table 1. Compodtlon of Polymerlc Membranes Used for Potentlometrlc Evaluation
membrane'
polymer
A B C D E F G H I J K L M N 0 P
PVC-COOH PVC-COOH HMW-PVC HMW-PVC PVC-COOH PVC-COOH PVC-COOH PVC-COOH PVC-COOH PVC-HMW PVC-COOH PVC-COOH PVC-COOH PVC-COOH PVC-COOH PVC-COOH HMW-PVC PVC-COOH PVC-COOH PVC-COOH PVC-COOH PVC-COOH PVC-COOH
Q
R S T U V W
content of ionophore/ion ionophore/ion exchanger plasticizer exchanger (W,w/w) o-NPOE DOS o-NPOE 0-NPOE o-NPOE o-NPOE 0-NPOE 0-NPOE 0-NPOE o-NPOE o-NPOE o-NPOE 0-NPOE 0-NPOE 0-NPOE DOS 0-NPOE o-NPOE 0-NPOE 0-NPOE 0-NPOE 0-NPOE o-NPOE
DNNS DNNS ETH 5294 ETH 5294 ETH 5294 ETH 5294 ETH 5294 VAL VAL VAL VAL VAL VAL VAL TDA-Br TDA-Br TDA-Br TDA-Br TDA-Br TDA-Br
2.5 2.5 0.3 0.7 1.3 2.7 0.3 0.15 0.3
0.6 1 .o 1.3 0.6 0.6 0.3 0.6 0.9
1.2 1.5 1.75
All membranes contain one part polymer and two parts plasticizer (WIW).
used. Most of the products were supplied by Fluka AG (Buchs, Switzerland): ETH 5294 proton carrier (Chromoionophore I, No. 27086); valinomycin potassium carrier (VAL, No. 94675); tridodecylmethylammonium chloride (TDMA-C1, No. 9 1661); tetradecylammonium bromide (TDA-Br, No. 87580); 2-nitrophenyl octyl ether (o-NPOE, No. 73732); bis(2ethylhexyl) sebacate (DOS, No. 848 18). Dinonylnaphthalenesulfonic acid (DNNS) was purchased from Pfaltz and Bauer (Waterbury, CT) as a 50% solution in kerosene, and it was used as received. As polymeric materials, carboxylated poly(viny1 chloride) (PVC-COOH with 1.8%, w/w, COOH groups, No. 81395) and high molecular weight poly(viny1 chloride) (HMW-PVC, No. 8 1392) were also purchased from Fluka. Membranes and Electrodes. The solvent polymeric membranes were prepared according to the classical procedure,l2 and their compositions are listed in Table 1. In all cases, no lipophilic salt additive was added to the membranes. All studied membranes were incorporated into Philips IS-560 liquid membrane electrode bodies (Moller Glasblaserei, Zurich, Switzerland) and checked for their electroanalytical properties. The inner filling solution varied, depending on which carrier/ion-exchanger system was evaluated, but generally a Tris buffer of pH 7.0 was used with pH electrodes and M KCl in pH 7.0 Tris buffer was used with potassium electrodes. For electrodes designed to respond to Na+ and/ or C1-, a M NaCl solution was used as an internal filling solution. emf Measurements. Measurements of emf were carried out at room temperature in an air-conditioned laboratory at 22.5 f 0.5 OC with an Orion pH/mV meter (Model 701A) (12) Moody, G. J.; Oke, R. B.; Thomas, J. D. R. Analyst 1970, 95, 910-918.
Analytical Chemistty, Vot. 66, No. 21, November 1, 1994
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connected to an Orion Model 605 electrode switch box. As a reference electrode, an Orion Model 90-02 Ag/AgCl double junction was used throughout. The solution in the outer compartment of the reference electrode consisted of 1.O M lithium acetate solution. The measured emf values were corrected for changes in the liquid junction potential according to the Henderson e q ~ a t i 0 n . l ~ Determination of the Internal Resistance of the Electrochemical Cells. The internal resistances of the potentiometric cells were determined by the voltage divider method using known shunts. For all electrodes, the cell voltage was first measured in lo-’ M KC1 buffered with pH 7.0 Tris buffer and constant ionic background (0.14 MNa+). Next, thecelloutput was shorted for 15 s by high-resistance shunts, and the internal resistance of the potentiometric cell was calculated from the original voltage, the voltage drop, and the shunt resistance. If the original voltage is halved, the shunt resistance is equal to the internal resistance of the potentiometric Selectivity Coefficients. The selectivity coefficients, KH,M or KK,M were determined by the separate solution method at a 0.1 M concentration level.16 The mean activity coefficients were calculated with the extended Debye-Huckel equation,13 and the measured emf values were corrected for changes in the liquid junction potential according to the Henderson equation. 13
RESULTS AND DISCUSSION It has been a nagging problem to explain why PVC-COOH did not show normal pH responses in earlier studies.’y2 There is an overwhelming theoretical basis for predicting pH responses, assuming the COOH groups can ionize to some albeit small extent and can exchange protons between COOand bathing electrolyte solutions. As long as the exchange current density of protons is large to permit electrochemical reversibility, the interfacial Nernst-Donnan potential difference arising from space charge and dipole alignment at each interface is expressed by equating the electrochemical potential of protons at each abutting phase and solving for A4.l’ When the phase is homogeneous, proton partition coefficients (expressing the energy difference between protons in two phases) are the same at each interface and then the total membrane potential difference should be Nernstian. Even when the membrane’s site distribution is not homogeneous, the response can have a Nernstian slope. A special complexity occurs from the ion pairing in the membrane bulk that arises from the low dielectric constant of the plasticized membrane. Given the Kf for the reaction in the ion exchanger
the membrane potential difference can be expressed in terms (13) Meier, P. C.; Ammann, D.; Morf, W. E.; Simon, W. In Medicaland Biological Applicationr ofBlectrochemica1Devices;Koryta, J., Ed.; J. Wiley: Chichester, England, 1980; pp 13-91. (14) Oesch, U.; Simon, W. Anal. Chem. 1980, 52, 692-700. (15) Ammann, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A,; Pungor, E. Anal. Chim. Acta 1985, 171, 119-129. (16) Pungor, E.; Toth, K.; Hrabeczy-Pall, A. Pure Appl. Chem. 1979, 51, 19131980. (17) Buck, R. P.; Vanysek, P. J. Electround. Chem. 1990, 292, 73-91.
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Analyiical Chemistry, Vol. 66, No. 21, November 1, 1994
of bound or free protons. However, Nernstian response is always predicted as long as Kfis not infinite. In that case the membrane becomes an insulator. When other monovalent ions are present and permeable, the diffusion/migration potential difference is an integral of an exact differential,I8 that gives the well-known simple form of theNicolsky equation for constant ion charge. It is not often pointed out that the uniformity of site and counterion concentration assumption remains inside the membrane, regardless of ion pairing. If there were an asymmetric differential uptakeof water, or protein adsorption at oneinterface but not at theother, then theinterior membrane proton activities may not cancel. This result is a so-called “asymmetry” potential difference that will be measured when the bathing proton activity is the same in each of the “inner” and “outer” bathing solutions, but is not the same just inside the two membrane surfaces at x = d and at -d. Thus, we expect the plasticized membrane without added proton carriers to be a pH sensor by application of classical ion-exchanger principles regardless of the itemized imperfections, unless cation interferences are excessive. The ion selectivities of the PVC-COOH membranes using high and low dielectric plasticizers have been measured. The membranes are found to be generally cation selective, but considerably more selective to larger monovalent cations. In fact, plasticized PVC-COOH is surprisingly responsive toNa+ and other alkaline and alkaline-earth cations. These results contrast with neutral carrier pH sensors based on ordinary high molecular weight (HMW) PVC or based on aminated PVCs. Selectivity of proton detection of both neutral carrierHMW-PVC and aminated PVC for protons is remarkably large, and ordinary buffers using monovalent cation salts can be tolerated for calibration.1° We show in this paper that conventional monovalent metal cation buffers cannot be used to demonstrate pH response of PVC-COOH membranes. pH and pM Responses of PVC-COOM Membranes. Ionselective electrode membranes are typically based on HMWPVC matrix loaded with specific carriers, selective for ions of interest. Our previous works on planar microchemical sensors for biomedical application^^^^^^ demonstrated that modified PVCs, e.g., aminated or carboxylated PVC, showed good adhesive properties to polyimide-coated Kapton, a polymeric material used as a substrate for our planar microsensors. The adhesive strength, measured by peel-off forces, was found to be practically the same for both PVCNH2 and PVC-COOH. Also, we demonstrated the advantages and disadvantages of using PVC-COOH in fabrication of pH and potassium membrane sensors.8 We reported earlier that PVC-COOH was dissociated in solvent polymeric membranes and the degree of dissociation depended on the nature of the solvent plasticizer.8 However, a pH sensitivity of only 6.3 mV/pH unit was reported when pH standard solutions used for calibration were buffered with Tris containing 0.14 M Na’ ionic background. Subsequently, in the course of this work, we found that the (18) Buck, R. P. In Ion Selecriue Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum Press: New York, 1978; Vol. 1, pp 1-141. (19) Lindner, E.; Cosofret, V. V.; Ufer, S.;Kusy, R. P.; Buck, R. P.; Ash, R. B.; Nagle, H. T. J. Chem. SOC..Faraday Trans. 1 1993,89,361-367. (20) Lindner, E.; Cosofret, V. V.; Ufer, S.;Buck, R. P.; Kao, W. J.; Neuman, M. R.; Anderson, J. M. J . Biomed. Mater. Res. 1994, 28, 591-601.
Table 2. Selectlvlty Coeffklents, -log KH,y,for Carboxylated PVC Membranes Containing Two Different Plartklzen
I
-150
-200
I
K+
c
I
\\ \\
I
-50
membrane plasticizer ion (M*) H+ Li+ Na+ CaZ+ NH4+
%carrier (w/w) mmol of carrier/ 100 g of membrane pH range slope (mV/pH)
t 1
I -150
-200
1 I
I
\ \
\ \
1
~
1
2
"
' 3
' 4
'
' 5
'
DOS
0 1.56 1.32 1.46 1.13 1.13
0 1.08 0.79 2.13 0.46 0.33
Table 3. pH Ranges and Slopes of PVCCOOH Membranes Doped wlth Different Concentrations of ETH 5204 Proton Carrier
g. -100 w
B
A 0-NPOE
'
~
6
-1% %a+ Flgure 1. Potentiometric pH (a, top) and pNa (b, bottom) responses of carboxylated PVC membranesplasticized wlth o-NPOE (membrane A) and DOS (membrane B). Response to Na+ Ions of a HMW-PVC membrane plasticized with o-NPOE (membrane C) is shown for comparison (b). Pure HCI solutions and pure NaCl soiutlons were used for pH and Na+calibrations, respectively.
carboxylated PVC membranes, plasticized with either a polar solvent (o-NPOE; cco = 23.9) or a nonpolar solvent (DOS; ceo = 3.9), showed near-Nernstian response to H+ in the range of pH 2-5 with slopes of 63.6 and 54.3 mV/pH, respectively (Figure la), using dilution of a strong acid and absence of monovalent metal cations. As expected, membranes A and B (see Table 1) respond also to Na+ (Figure 1b) and to other monovalent and divalent ions (Table 2). As shown in Figure lb, the sodium ion calibration curves are nearly linear in the range 10-1-104 M Na+ with slopes of 55.7 (membrane A) and 58.0 mV/decade (membrane B). In both cases, the calculated detection limits were 6.3 X M. In contrast to these responses, HMW-PVC membranes, plasticized with o-NPOE (membrane C) showed negligible response to Na+ (see Figure 1b). From the selectivity coefficients values listed in Table 2 we can conclude that PVC-COOH responds as a permselective ion exchanger with only fair selectivity for H+ when a polar solvent is used as plasticizer. The interferences by monovalent metal cations of group I are great, and so pure
membrane H
F
G
I
0.3 0.5
0.7 1.2
1.3 2.2
2.7 4.6
4-8 20.8
4-9 36.4
4-10 45.9
4-12 51.0
pH response was overlooked previously.2 When a nonpolar solvent is used as plasticizer, Ca2+ is not a preferred ion and the membrane shows poor divalent response. Modification of pH Responses in PVC-COOHby Added Basic H+Neutral Carrier. It is known that small concentrations of a selective carrier in a plasticized polymeric matrix2' can induce a selective potentiometric response to the selected ion. This is a general rule which applies to inert matrices such HMW-PVC. A concentration of 0.3% (w/w) ETH 5294 proton carrier in HMW-PVC matrix, equivalent to 0.5 mmol/ 100 g of membrane (o-NPOE as plasticizer) induces a nearNernstian pH response in the range of pH 4-12 (slope 54.6 mV/pH) (membrane J). By increasing the concentration of the carrier in the membrane, the pH response becomes more nearly ideal as more of the negative sites of the matrix are electrically compensated by carrierH+ species. At low concentrations of ETH 5294 in PVC-COOH (membranes F, G, and H), the respective membranes are still barely permselective, but with poor selectivity to H+. When the concentration of the proton carrier in the membrane reaches a value of 2.7% (w/w), Le., 4.6 mmo1/100 g of membrane, the pH response is about 94% when compared with the response of a membrane containing HMW-PVC as a matrix. At this level of carrier concentration, we can calculate that about 140 mmol/kg PVC-COOH is dissociated, which corresponds to 35% dissociation yield of RCOO-. We assume that the base from the carrier reacts with PVC-COOH to form carrierH+ species. This is consistent with previously observed conductivity increases8 and with reaction of inorganic bases, e.g., LiOH. These results are in agreement with the conventional wisdom: carriers must be in an excess of sites to achieve a selective Nernst response. In contrast with ordinary ion-exchanger m e m b r a n e ~ ~ ~ J ~ , ~ ~ - ~ ~ showing the Hofmeister-like pattern, addition of carriers into the membrane confers special selectivity that forces a nonHofmeister response and can even partially reverse the (21) Ammann, D.; Morf, W. E.; Anker, P.; Meier, P. C.; Pretsch, E.; Simon, W. Ion-Sel. Electrode Rev. 1983,5, 3-92.
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
3595
electrostatic response sequence. However, the compounding of the sensor components requires careful consideration. The normal composition: 66 wt % ’ inert plasticizer, 33 wt ’% PVC, 1 wt % carrier, and mobile sites is only an approximation to the optimal. The high plasticizer content can be lowered and adjusted to accommodate the precise amounts of carrier and site-containing salt. For example, in fixed site PVC membranes, the native negative site concentration is about 0.05-1 mM.’ The mole ratio of carrier to site concentration must exceed 1. Otherwise, the special selectivity of the carrier cannot be achieved, because there must be one positively charged carrier-counterion complex to electrostatically balance each negative site. A 2:l ratio with excess free carrier is thought to be ideal. For membranes with added TPB-type mobile sites, the excess of carrier must be reckoned with respect to total sites. For pH responses, and for K+ responses (below) we show that, for PVC-COOH membranes, the excess of carrier must be calculated from the total ionizable RCOOH groups and any added lipophilic anion salt (e.g., KTpClPB). Modification of pK Responses in PVC-COOH by Added K+ Neutral Carrier (Valinomycin). The data in Table 2 show that PVC-COOH also behaves as an ion exchanger for free potassium ions at a controlled low H+ concentration.. The selectivity coefficients, -log KH,M,,for membranes A and B, respectively, indicate the permselectivity character of this ion exchanger. When PVC-COOH membranes are loaded with valinomycin, a neutral carrier for K+, the potentiometric response to K+ ions is significantly improved with respect to slope and linear range. This improvement depends, as for pH sensors, on the concentration of the neutral carrier added to the matrix as well as on the polarity of the plasticizer. A membrane with 0.15% (w/w) valinomycin (0.13 mmol of carrier/ 100 g of membrane; membrane K) shows a very good response to K+ ions in pure KCl solutions: linear range, 10-llo4 M; slope 56.0 mV/decade. However, the response in buffered potassium standard solutions with 0.14 M Na+ ionic background shows a very poor slope of only 6.5 mV/decade (-log K K , N= ~ 0.21). At this low level of carrier concentration in the membrane (0.135 mmol/lOOgmembrane), therearemany moreavailable negative sitesdue to PVC-COOH than positive sites (KVAL+). The potentiometric response to K+ ions is generated mainly by the ion-exchanger (PVC-COO-H+) response: K+ + H+ + H+ K+ rather than by K+ VAL F= KVAL+. By doubling the concentration of the carrier in the membrane [0.3% (w/w) VAL, Le., 0.27 mmol of VAL/100 g of membrane; membrane L] the response is not significantly improved. (slope, 7.3 mV/decade; -log K K , N=~ 0.26). By further increasing the valinomycin concentration, the K+ response becomes significantly better. Practically, at concentrations of 2 1.O% (w/w) valinomycin, corresponding to 0.9 mmol/ 100 g membrane) the ideal Nernstian response to K+ is reached and remains the same for any higher carrier loading. Now, we can presume to have enough positive sites (KVAL+) in the membrane and the electrode response is
+
+
(22) Morf, W. E.; Simon,W. In IonSelectiue Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum Press: New York, 1978; Vol. I, pp 211-286. (23) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: Amsterdam, 198 1. (24) Buck, R. P. In Comprehensive Treatise of Electrochemistry; White, R. E., Bockris, B., Conway, B., Yeager, E., Eds.;Plenum Press: New York, 1984; Vol. 8.
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Analytical Chemistry. Vol. 66,No. 21, November 1, 1994
100 r
t
I
1
.
I
2
.
I
.
3
I
4
.
1
5
-log + Figure 2. Potassium potentiometrlc response of carboxylated PVC membranes doped with the same concentration of vallnomycln (0.6 %, wlw) and plasticlzed with a polar solvent, eNPOE (membrane M) and a nonpolar solvent, DOS (membrane P). The response of a HMW-PVC membrane doped with 0.6% (wlw) valinomycin and plastlcized with eNPOE (membrane Q ) is shown for comparison (pure potassium chloride solutions were used for calibrations).
governed by the carrier mechanism. These results are direct evidence of the often quoted rule that ideal response is achieved when the carrier/site ratio exceeds unity. The extent of PVC-COO- formation is less for KVAL+ membranes than in the previous case for H+ basic carrier added to the PVC-COOH matrix. For KVAL+ membranes, the free anion sites concentration corresponds to 27 mmol/kg PVC-COO-, Le., 6.75% dissociation yield (PVC-COO-). This difference can be explained by the fact that in the former H+ sensing case the dissociation process is favored by the acidbase reaction between COOH groups and the basic amine carrier in the membrane. The degree of dissociation is again a function of the plasticizer polarity. For the samevalinomycin concentration in the carboxylated PVC membrane (0.6%, w/w) but for different plasticizer polarities, the K+ potentiometric response is different (see Figure 2). In a DOSplasticized carboxylated PVC membrane, the response is almost the same as in HMW-PVC plasticized with o-NPOE (membrane Q; slope 54.6 mV/decade). However, in a o-NPOE plasticized carboxylated PVC membrane (membrane M), the response is worse (slope of only 29.1 mV/decade). This result again gives an indication that the degree of dissociation is different in two plasticizers with different polarities, despite the same amount of plasticizer being present. The effect may be correlated with a different number of counterions in the membrane. In o-NPOE-plasticized carboxylated PVC membranes, there seem to be more negative sites generated from PVC-COOH and the K+ potentiometric response is governed by the simple, nonselective ion-exchange process. In DOS-plasticized membranes, the degree of dissociation is smaller and the K+ potentiometric response may be governed by the selective ion carrier mechanism. Carboxylated PVC membranes doped with different carriers (basic, ETH 5294, and neutral, VAL) at the same
0.6r
:;!
200
0.4 h
-5
z
0.3
a:
0.2
W
9w
0.1
0
-50
0 0 . 5 1
1 . 5 2
3
4
5
6
7
8
Time (h) Figure 3. Variation of internal membrane resistance for u-NPOE-
plasticized carboxylate PVC membranes doped with two different carriers at the same level of concentration (1.2 "ole of carrierll00 g of membrane). CH1 refers to chromoionophore 1 (ETH 5294) and VAL to valinomycin.
level of concentrations (e.g., 1.2 mmol of carrier/100 g membrane) show different internal membrane resistances, measured in the same electrolyte, Le., in M KCl solution buffered with pH 7.0 Tris buffer containing 0.14 M Na+ ionic background. As shown in Figure 3, the resistance of a membrane doped with a basic carrier (ETH 5294) is smaller by about half, in the first hours of soaking solution contact, that of a membrane doped with VAL. This supports the supposition that the presence of a basic carrier in the PVCCOOH matrix forces dissociation by acid-base interactions.8 This deduction follows from the slow response of the K+ electrode in a Na+/K+ bathing solution. Steady state response requires penetration of the membrane by Na+ and K+ to replace H+ and form a distribution of KVAL+, K+, and Na+ counterions. The slow diffusing species, KVAL+ (D 1 0-8 cm2/s), controls thecoupled motion. The typical timeconstant is T = d2/4D when d is the membrane thickness of 150 pm. The time constant is about 1.5 h in agreement with the data. The constancy in the membrane resistance of the pH electrode means that the ionization process in the membrane was complete during the electrode preparation, as expected. Response of PVC-COOH Membranes Doped with MobileSite Anion Exchanger: Competitive Responses to Cations and to Anions. A mobile-site anion exchanger (positive charged trapped mobile site) such as tridodecylmethylammonium chloride (TDMA-C1) or tetradecylammoniumbromide (TDABr), loaded in an inert polymeric matrix (e.g., HMW-PVC), induces an anionic potentiometric response with the classical order of anion selectivity, the Hofmeister series. By incorporating TDMA-Cl or TDA-Br into a plasticized carboxylated PVC membrane, the potentiometric response can be determined either by the fixed native negative sites (PVC-COO-) or by the added positive sites (R4N+), depending on their relative concentrations in the membrane. Our experimental results show that when the concentration of R4N+ in the membrane is not high enough to exceed the concentration of PVC-COO- negative sites due to the dissociation of PVCCOOH, the potentiometric response is cationic. There is a response to Na+, for example, shown in Figure 4. Upon increasing the concentration of R4N+ in the membrane, the cationic response gradually deteriorates (see Figure 4,
-
-
-100
-200'
I
1
'
I
2
'
I
3
'
I
"
4
-log aNa+or -log
5
'
I
6
acr
Figure 4. Potentiometricresponse of carboxylated PVC membranes doped with increasingconcentrationsof tetradecylammoniumbromide (eNPOE as plasticizer): 0.3% (membrane R), 0.6% (membrane S), 0.9% (membrane T), 1.2% (membrane U), 1.5% (membrane V), and 1.75 YO (membrane W). For membrane identification, see Table 1. Curves S, T, and W were shifted, for clarity, with -30, -10, and +70
mV, respectively (pure sodium chloride solutions were used for calibrations).
membranes S and T). At a concentration of 1.8 mmol of TDA-Br/lOO g membrane (membrane U), the negative sites in PVC-COOH are electrically compensated by the positive sites of quaternary ammonium derivative and the membrane does not respond to either Na+ or C1-. This condition corresponds to 18 mmol/kg dissociated PVC-COOH, i.e., 4.5% dissociation). With continuing increase of the concentration of R4N+ in the membrane, its response becomes anionic (curves corresponding to membranes V and W in Figure 4). Here we have an excess of positive charged carrier in the membrane, and the response is governed by the excess of R4N+ positive sites to form an anion sensor: once again the crucial figure of merit is the ratio of trapped sites to fixed sites. This is the vaguely related analogue of the neutral carrier/fixed site ratio rule. The measured selectivity coefficients of a PVC-COOH membrane containing low level of TDA-Br (0.3%, membrane R) for the ions listed in Table 2 show an improvement in cation selectivitywhen compared with a carrier free membrane, Le., membrane A (e.g., -log KH,K= 1.62; -log KH,Na = 1.81; -log KH,NH~ = 1.58). Membrane R shows a better selectivity for H+ as well. These results are in agreement with the theoretical predictions published recently by Schaller et al.25 These authors showed that the incorporation of ionic sites into the PVC membrane is beneficial for charged carrierbased ion-selectiveelectrodesas well, and in contrast to neutral carrier-based electrodes, cation exchangers are needed for anion-selectiveelectrodes and vice versa.25 By increasing the concentration of quaternary ammonium salt additive in the membrane to 0.6%(membrane S), an electrode with a smaller slope (see Figure 4) and with reduced selectivity for H+ is obtained. (25) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994,66, 391-398.
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-log $a+ Figure 5. Potentiometric pH (a, top) and pNa (b, bottom) response of DNNS ion exchanger loaded into HMW-PVC matrix (membrane D) and PVC-COOH matrix (membrane E). For membrane identification, see Table 1 (different pH solutions were prepared by serial dilutions of 0.1 M HCI while the Na+ solutions were obtained by serial dilutions of 0.1 M NaCI).
Response of a Stronger Acidic Carrier (Dinonylnaphthalenesulfonic Acid, DNNS) to H+and MH. Harrell et a1.26 showed that a liquid ion-exchange membrane based on dinonylnaphthalenesulfonicacid (DNNS) is suitable for ions of high valence (tri- and tetravalent cations). DNNS acts as a cation exchanger (a trapped mobile anion site) because of the similarity of its exchange properties to those of Dowex-50 resin.27 It was also used as an ion-pairing agent for many lipophilic organic cations in order to fabricate drug membrane sensor^.^^-^^ When incorporated into a plasticized HMWPVC (membrane D), DNNS shows both pH and pNa responses (Figure 5a,b). The potentiometric responses are improved by incorporating DNNS into a carboxylated PVC (26) Harrell, J . 8.; Jones,A. D.; Choppin, G. R. Anal. Chem. 1969,41,1459-1462. (27) White. J. M.; Kelley, P.; Li, N . C. J . Inorg. Nucl. Chem. 1961, 16, 337-344. (28) Cosofret, V. V.; Buck, R. P. Ion Sel. Electrode Rev. 19f44, 6 , 59-121. (29) Zhang, Z. R.; Cosofret, V. V. Sel. Electrode Rev. 1990, 12, 35-135. (30) Cosofret,V. V.; Buck, R. P. Pharmaceutical Applications of MembraneSensors; CRC Press, Boca Raton, FL, 1992.
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matrix (membrane E; see Figure 5a,b); the slopes of the respective electrodes are very close to the theoretical (59.2 and 56.8 mV/ ApNa, respectively). The electrode responses are, in the last two cases, dictated by both ion exchangers in the membrane (PVC-COOH and DNNS). Unfortunately, these membranes are not selective enough for use in selective electrodes. The measured selectivity coefficients (-log K H , M ) of membrane E showed the following values: 1.77,1.38,1.36, 0.52, and 0.19 for Li+, Na+, Ca2+,NH4+, and K+, respectively. These results satisfy the expectation of the ion-exchange theory of the membrane potential. It is simply surprising that both the weak RCOOH and strong RSO3H acids are so sensitive to monovalent metal ion interferences. Control of Potentiometric Response by Mobile or Fixed Cation/Anion Ratios. The basic discovery of a pure H+ response by plasticized PVC-COOH membranes required use of pure strong acid standards, with dilutions. It is understandable from the high selectivities of the membranes for monovalent metal cations that the pure H+ response could be missed. However, the elementary significance of this result is as follows: (1) evidence for the presence of fixed anionic sites RCOO- in these membranes is provided; (2) the simple ion-exchange responses depend on the site concentrations; (3) site concentrations are determined mainly by the plasticizer dielectric constant. A more substantial result is the ability to interpret the selectivity dependence rule: carrier concentration should exceed total available site concentration. We have shown how selectivity varies systematically with this ratio. Furthermore, the data suggest a way of determining fixed-site concentrations by observing membrane compositions where selectivities change dramatically, corresponding to the approximately equal concentration of carrier and sites. The extension of this concept is a new rule for application to charged, trapped, positive sites added to fixed negative or mobile negative site membranes. As the ratio varies over the range (sites+)/(sites-) > I , or I , or < I the membrane response selectivity passes from anionic, to inert, to cationic. This has major results on interpretetation. For example, it is common to add a hydrophobic salt3' to a nonsite or low site density membrane to reduce resistance, but with no effect of intrinsic selectivities. This observation can be understood in detail.
CONCLUSIONS In this paper we could demonstrate by theory and experiments the ion-exchange properties of plasticized carboxylated poly(viny1 chloride) used in fixed matrix for fabrication of of chemically selective ion sensors. The ion selectivities of the PVC-COOH membranes using high and low dielectric plasticizers have been evaluated. The membranes were found to be generally cation selective, but considerably more selective to larger monovalent cations. Carboxylated PVC membranes doped with different carriers (basic, ETH 5294, and neutral, VAL) at the same level of (31) Horvai, G.; Nieman, T. A,; Pungor, E. In Ion-Selective Electrodes; Pungor, E., Ed.; Akademiai Kiado: Budapest, 1985; pp 439-448.
concentrations show different internal membrane resistances, measured in the same electrolyte solution. This result confirmed the supposition that the presence of a basic carrier in the PVC-COOH matrix forces dissociation by acid-base interactions. We showed that a mobile-site anion exchanger (positive charged trapped mobile site) such as tetradecylammonium bromide, incorporated into a plasticized carboxylated PVC membrane, induces a potentiometric response, determined either by the fixed negative sites (PVC-COO-) or by the added positive sites (R4N+), depending on their relative concentrations in the membrane. The crucial figure of merit is the ratio
of trapped sites to fixed sites. This is the related analogue of the neutral carrier/fixed site ratio rule.
ACKNOWLEDGMENT This work was supported by N S F Engineering Research Center, Grant CDR-8622201. Scientific Parentage of the Author. R. P. Buck, Ph.D. under D. N. Hume, Ph. D. under I. M. Kolthoff. Received for review March 31, 1994. Accepted July 29, 1994." e Abstract
published in Advance ACS Abstracts, September 15, 1994.
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