Anal. Chem. 1998, 70, 5252-5258
Polymeric Membrane pH Electrodes Based on Electrically Charged Ionophores Yanming Mi, Christopher Green, and Eric Bakker*
Department of Chemistry, Auburn University, Auburn, Alabama 36849
Solvent polymeric membranes containing electrically charged H+ carriers show dramatic variations in their pH response behavior depending on the charge type of lipophilic ionic sites present in the membrane. These shifts are predicted with a simplified phase boundary potential model. Membranes without ion-exchanger sites are expected to show half-Nernstian electrode slopes with extended measuring ranges if the ionophore has acidic as well as basic properties. Relevant experiments with membrane electrodes on the basis of the electrically charged carriers 4′,5′-dibromofluorescein octadecyl ester (ETH 7075) and 5-(octadecanoyloxy)-2-(4-nitrophenylazo)phenol (ETH 2412), lipophilic indicators formerly used in optode films, confirm these predictions. It is shown that the incorporation of tridodecylmethylammonium chloride into NPOE-PVC membranes with ETH 7075 induces a dynamic range from 4 to 10.4 in 0.1 M KCl while membranes with ETH 2412 show a dynamic range that is shifted by ∼4 orders of magnitude toward higher pH values. Experimental EMF responses correlate satisfactorily to analogous absorbance experiments on thin polymeric films, although deviations in the ion-exchange region at high sample pH are larger than ordinarily observed with neutral carrier-based systems. Electrode membranes based on ETH 7075 are characterized and optimized. Anion interferences quantitatively follow the Hofmeister selectivity pattern of ionophore-free membranes. These electrodes show no discrimination among the tested alkali metal ions, indicating that the lipophilicity of cations is not a major factor governing the lower detection limit. The electrodes are stable and exhibit lifetimes of over half a year without appreciable loss in analytical performance after continuous storage in electrolyte solutions and may in some cases be a viable alternative to electrode membranes containing neutral carriers. For in vivo blood pH determinations, solvent polymeric membrane pH electrodes are often preferred to classical glass electrodes.1,2 On the basis of their ease of preparation, low electrical resistance, and handling safety, there is still considerable interest in developing such pH electrodes for clinical intravascular and intraluminal applications. Historically, solvent polymeric (1) Pucacco, L. R.; Carter, N. W. Anal. Biochem. 1978, 89, 151. (2) Ammann, D. Ion-Selective Microelectrodes; Springer: Berlin, 1986.
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membrane pH electrodes have been developed on the basis of electrically neutral carriers such as tridodecylamine with excellent selectivities over potentially interfering cations such as sodium, potassium, and calcium.3,4 The response mechanism of polymer membrane pH electrodes containing such neutral carriers is now well established.5,6 Indeed, owing to the extremely high binding selectivity of these compounds, these are unique model systems that may be used to understand more fully how potentiometric sensors function. Much of this development was initiated by the availability of lipophilic pH indicators that are used in optode films.7 The group of Buck had first shown that such compounds are also functional in potentiometric systems.8 Subsequent works have revealed that absorbance measurements on thin optode films may be correlated to the observed EMF response of the respective ion-selective electrode.9 These observations have allowed us to formulate simplified response models to optimize membrane compositions and more fully understand the practical limits of ionselective electrodes.6 In other studies, lipophilic phenoxazine derivatives as H+ ionophores have been found to be extremely selective, so that they may be used as reference ionophores in polymer membranes to allow for the optical10 and potentiometric11 determination of effective stability constants of alkali metal ionophores within solvent polymeric films. In recent years, it has been realized that membranes containing electrically charged carriers (relating to the uncomplexed form) require lipophilic ionic additives with the same charge as the analyte ion for optimum functioning.12,13 These added sites ensure that a given concentration of ionophore remains uncomplexed in the membrane. Without such sites, this concentration would be dictated by the dissociation constant of the ionophore-analyte ion complex in the membrane. As sample interferences partially (3) Ammann, D.; Lanter, F.; Steiner, R. A.; Schulthess, P.; Shijo, Y.; Simon, W. Anal. Chem. 1981, 53, 2267. (4) Anker, P.; Ammann, D.; Simon, W. Mikrochim. Acta 1983, I, 237. (5) Egorov, V. V.; Lushchik, Y. F. Talanta 1990, 37, 461. (6) Bakker, E.; Xu, A.; Pretsch, E. Anal. Chim. Acta 1994, 295, 253. (7) Bakker, E.; Lerchi, M.; Rosatzin, T.; Rusterholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211. (8) Cosofret, V. V.; Nahir, T. M.; Lindner, E.; Buck, R. P. J. Electroanal. Chem. 1992, 327, 137. (9) Bakker, E.; Na¨gele, M.; Schaller, U.; Pretsch, E. Electroanalysis 1995, 7, 817. (10) Bakker, E.; Willer, M.; Lerchi, M.; Seiler, K.; Pretsch, E. Anal. Chem. 1994, 66, 516. (11) Bakker, E.; Pretsch, E. Anal. Chem. 1998, 70, 295. (12) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66, 391. (13) Bakker, E.; Malinowska, E.; Schiller, R. D.; Meyerhoff, M. E. Talanta 1994, 41, 881. 10.1021/ac980678l CCC: $15.00
© 1998 American Chemical Society Published on Web 11/11/1998
exchange with primary ions in the membrane, they will increase the concentration of noncomplexed ionophore in the membrane if they form weaker complexes. It can be predicted theoretically with the phase boundary potential model12,13 that this leads to less than ideal membrane selectivities. Adding appropriate ionic sites will alleviate this problem. Subsequent works in this direction have included the characterization of so-called acidic ionophores, i.e., ionophores that preferably remain protonated when not complexed.14,15 These systems may show twice-Nernstian electrode slopes that have been successfully modeled theoretically.15 Interestingly, the very first reported solvent polymeric pH electrode membranes were apparently based on electrically charged ionophores.16,17 Since that time, neutral ionophores have completely replaced these early systems. In the past years, due to the development of bulk optodes, a number of electrically charged H+ ionophores have been successfully used and characterized in optode films.18 We have recently reported in a short communication on the potentiometric response of pH electrodes based on 4′,5′-dibromofluorescein octadecyl ester (ETH 7075).19 The incorporation of ionic sites of different charge types dramatically alters the response properties of the respective ISE. These experiments have initiated a more thorough experimental and theoretical investigation of the response behavior of such charged carrier-based pH electrodes. The results of this study are presented here. THEORETICAL SECTION EMF Functions of Membranes Containing a H+-Ionophore with Basic and Acidic Properties. Some H+-ionophores may have the capability of acting as an electrically charged or electrically neutral ionophore since functional groups may be present that can undergo acid dissociation or hydrogen ion addition reactions. The effect of the concentration and charge type of added ionic sites to membranes containing such an ionophore is described theoretically with a simple model. As in earlier treatments,6,20,21 ion pair formation processes within the membrane are assumed to be insignificant. The membrane potential EM is described in general terms as a function of the sample-membrane phase boundary potential:
EM ≈ K +
RT kHaH ln + F [H ]
(1)
[H+]
where aH is the sample hydrogen ion activity, is the so-called free hydrogen ion concentration in the organic-phase boundary, kH is a function of the standard chemical potentials of the hydrogen ion in the aqueous and membrane phases, K is a constant, and R, T, and F are the gas constant, the absolute temperature, and the Faraday constant, respectively. As is well-known, a Nernstian response function of the electrode is expected where [H+] is (14) Schaller, U.; Bakker, E.; Pretsch, E. Anal. Chem. 1995, 67, 3123. (15) Amemiya, S.; Bu ¨ hlmann, P.; Umezawa, Y. Anal. Chem. 1998, 70, 445. (16) LeBlanc, O. H.; Brown, J. F.; Klebe, J. F.; Niedrach, L. W.; Slusarczuk, G. M. J.; Stoddard, W. H. J. Appl. Physiol. 1976, 40, 644. (17) Erne, D.; Ammann, D.; Simon, W. Chimia 1979, 33, 88. (18) Tan, S. S. S. Thesis, ETH Zurich, No. 9552, 1991. (19) Mi, Y.; Bakker, E. J. Electrochem. Soc. 1997, 144, L27. (20) Bakker, E.; Meruva, R. K.; Pretsch, E.; Meyerhoff, M. E. Anal. Chem. 1994, 66, 3021. (21) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083.
relatively sample independent.21 Since ion-selective electrodes are measured under zero current conditions, sample-dependent concentration changes are typically induced by ion-exchange or coextraction processes with other sample ions. Since a lipophilic ionophore CH is incorporated into the membrane that may show acidic and basic properties, the respective acid dissociation constants within the membrane are formulated for the dissociation of the positively charged form as
Ka1 ) [CH][H+]/[CH2+]
(2)
where species in brackets are concentrations in the membrane phase. The dissociation equilibrium of the neutral form is described as
Ka2 ) [C-][H+]/[CH]
(3)
The ion-exchange equilibrium with other sample cations J+ may be described in most general terms with the ion-exchange constant:
Kexch ) aH[J+]/aJ[H+]
(4)
where aspecies are the respective sample activities. The coextraction process of sample anions A- and protons may be described with the coextraction constant:
Kcoex ) [A-][H+]/aAaH
(5)
The mass balance equation in the membrane is
CT ) [CH2+] + [CH] + [C-]
(6)
where CT is the total concentration of H+ ionophore in the membrane, and the charge balance is written as
RT- + [A-] + [C-] ) RT+ + [CH2+] + [J+]
(7)
where RT- and RT+ are the membrane concentrations of lipophilic anionic and cationic sites. It is assumed in eq 7 that the H+ ionophore is sufficiently basic so that [H+] can be neglected. The last six equations contain a total of six unknowns. Elimination of [C-], [CH], [CH2+], [A-], and [J+] yields the following implicit equation:
aHCT[H+] (-[H+]2 + Ka1Ka2) ) ([H+]2 + [H+]Ka1 + Ka1Ka2)(-(aAaH2Kcoex) + aJ[H+]2Kexch + aH[H+](RT+ - RT-)) (8) Equation 8 may be solved exactly for [H+] by use of suitable software such as Mathematica. The results are subsequently inserted into eq 1 for predicting specific electrode response functions. Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
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Correlation of EMF and Optode Film Experiments. Experimental membrane potentials can sometimes be correlated to spectroscopic measurements on thin polymeric films having the same composition.9 Since the electrode membrane-phase boundary is assumed to equilibrate with the contacting sample, its composition is ideally identical to the thin polymeric film observed spectroscopically under the same conditions. Consequently, the simplified assumptions used in predicting the EMF response may be ascertained experimentally to some extent. For a polymeric membrane containing an electrically charged ionophore CH and cationic sites only, the phase boundary potential according to eq 1 may be further simplified by inserting the respective acid dissociation constant Ka2 (see eq 3) to obtain
EM ≈ K +
RT kH[C ]aH ln F Ka2[CH]
(9)
If ion-pairing processes within the membrane are not considered, the mole fraction of nonprotonated ionophore may be expressed as R:
R ) [C-]/CT; 1 - R ) [CH]/CT
(10)
This mole fraction is conveniently obtained from absorbance measurements as described.22 Consequently, eq 10 is inserted into eq 9 to obtain a relationship between absorbance data and measured EMF:
EM ≈ K′ +
RaH RT ln F 1-R
(11)
This eq 11 allows one to predict EMF values from optical experiments on thin films having the same composition. To predict R values from potentiometric data, on the other hand, eq 11 may be solved for R to obtain
{RTF (E
R ) exp
M
}(
- K′) aH + exp
{RTF (E
M
})
- K′)
-1
(12)
The constant K′ may be determined from the boundary condition that, in the ideal Nernstian EMF range, R ) RT+/CT on the basis of the simplified charge balance (see eq 7). The expected optode film response function may be calculated by combining eqs 3-7 and 10 (by neglecting RT- and [CH2+] in eqs 6 and 7) to obtain
aH2R2aAKcoex/Ka2 + aHR(1 - R){(RCT - RT+} (1 - R)2aJKa2Kexch ) 0 (13) EXPERIMENTAL SECTION Reagents. All salts and the membrane components 4′,5′dibromofluorescein octadecyl ester (chromoionophore VI, ETH 7075), 5-(octadecanoyloxy)-2-(4-nitrophenylazo)phenol (chromoionophore IV, ETH 2412), tridodecylmethylammonium chloride (22) Lerchi, M.; Bakker, E.; Rusterholz, B.; Simon, W. Anal. Chem. 1992, 64, 1534.
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(TDDMACl), tetradodecylammonium chloride (TDDACl), potassium tetrakis(p-chlorophenyl)borate (KTpClPB), o-nitrophenyl octyl ether (NPOE), bis(2-ethylhexyl) sebacate (DOS), highmolecular-weight poly(vinyl chloride) (PVC), and tetrahydrofuran (THF) were purchased in the highest quality available from Fluka Chemika-Biochemika (Ronkonkoma, NY). Aqueous solutions were prepared by dissolving the appropriate salts or diluting standard solutions as specified in Nanopure-purified distilled water. Membrane Preparation and EMF Measurements. Ionselective electrode membranes, containing 9.6 mmol/kg carrier and, if applicable, 4.8 mmol/kg TDDMACl, TDDACl, or KTpClPB (concentrations are given relative to total membrane mass) were otherwise prepared as described.17 At least two electrodes were prepared from each membrane composition. A 0.01 M KCl solution in 10 mM KCl + 25 mM potassium phosphate adjusted to pH 7.0 with NaOH served as the internal filling solution of the assembled electrodes. After conditioning the electrodes overnight in a solution identical to the inner filling solution, the EMF measurements were performed as reported elsewhere23 with 1 M KCl as the bridge electrolyte of the reference electrode. The sample solution, containing 0.1 M salt solution (NaCl, KCl, LiCl, NaNO3, NaClO4), 10 mM boric acid, and 10 mM citric acid (adjusted to pH 1 with HCl) was dropwise titrated with 0.1 M KOH, NaOH, or LiOH, with the cation matching the one contained in the sample electrolyte, while the pH was monitored with a calibrated glass electrode (HA glass, Mettler-Toledo, Wilmington, MA). Selectivities. Selectivity coefficients were calculated according to IUPAC recommendations using the fixed interference method24 but by determining the cross section of the Nernst pH function and the observed EMF value at the potential dip of the response curve and taking into account the activity coefficient for potassium as γ( ) 0.77, sodium as γ( ) 0.78, and lithium as γ( ) 0.79.25 Optical Measurements. The optical film consisted of 9.6 mmol/kg ETH 7075 and 4.8 mmol/kg TDDMACl, PVC, and NPOE (1:2). The film components were weighed out and dissolved in 1.5 mL of THF. By using a spin coater, two identical membranes, ∼2 µm thick, were cast onto two quartz glass plates that were subsequently mounted into a custom-built flow cell. The cell was placed in a single-beam computer-controlled diode array spectrophotometer (HP 8452A, Hewlett-Packard) and connected to a peristaltic pump. The flow was kept constant at ∼2 mL/min while the feeding/receiving electrolyte solution, 100 mL in volume, was dropwise titrated with a strong base as described above while simultaneously monitoring pH. Three membrane electrodes identical in composition to the optode film were measured as above in the feeding/receiving solution at the same time. The absorption spectra were obtained in transmission mode by measuring the absorbance of the optical sensing film contacting the flowing electrolyte solution. The absorbances were recorded from 300 to 800 nm every 5 min after sample additions. The R values were obtained from experimental absorbances at 440 nm as described.22 (23) Bakker, E. J. Electrochem. Soc. 1996, 143, L83. (24) Buck, R. P.; Lindner, E. Pure Appl. Chem. 1995, 66, 2527. (25) Meier, P. C. Anal. Chim. Acta 1982, 136, 363.
Figure 1. Calculated pH response functions of a membrane containing an ionophore with acidic and basic properties (see text). Addition of cationic additives forces the membrane to respond according to a charged carrier, while addition of anionic additives induces the neutral carrier mechanism. The shift in the response functions corresponds to the difference in the two pKa values of the ionophore. Membranes without ion-exchanger properties are expected to show half-Nernstian electrode slopes over both response ranges of the two other electrode formulations.
RESULTS AND DISCUSSION Influence of Electrically Charged Additives on EMF Response. Effect of Charge Type of Added Ionic Sites. In the theoretical section, a model was developed that allows one to predict electrode response functions of membranes containing an ionophore with acidic and basic properties and respective pKa values that are widely different. To mimic the experimental results obtained previously,19 the parameters in eq 8 were chosen as CT ) 0.01 mol kg-1, Ka1 ) 10-3 kg mol-1, Ka2 ) 10-12 kg mol-1, aA ) aJ ) 0.075 M, Kexch ) 1, and Kcoex ) 10-9. The concentration of added ionic sites were (a) RT- ) 0, RT+ ) 0.005 mol kg-1; (b) RT+ ) 0, RT- ) 0.005 mol kg-1; and (c) RT+ ) RT- ) 0. Figure 1 shows calculated electrode functions for these three systems. According to Figure 1, addition of anionic or cationic sites to the membrane dramatically changes the pH response behavior of the membrane. While the overall pH range remains unaltered, it is shifted by 9 orders of magnitude to higher sample hydrogen ion activities if anionic instead of cationic sites are incorporated. This shift corresponds exactly to the pKa difference of the ionophores, indicating that the charge type of added ionic sites dictates which hydrogen ion dissociation equilibrium is relevant for the electrode response. Perhaps surprisingly, a membrane electrode containing no ionic sites at all is expected to show a half-Nernstian electrode slope with an extended pH response range of ∼16 pH units. To our knowledge, this is the first theoretically predicted example of a polymer membrane pH electrode system where the pH response range is dramatically improved beyond traditional limits. While the overall potential window remains about the same and is dictated by anion and cation interferences to the same extent as the other two examples, the apparent half Nernstian response slope enables the electrode to cover a much larger pH range. These are important considerations that may be analytically exploited to fabricate massively improved pH electrode systems.
Figure 2. pH response functions of NPOE-PVC (2:1) membranes containing cationic sites (TDDMACl) and either ETH 7075 or ETH 2412 as charged H+ carrier. Electrolyte solution: 0.1 M KCl buffered in 10 mM citric acid and 10 mM boric acid. Dotted lines with Nernstian electrode slope.
These predictions correlate quite well with the experimental results on ETH 7075-containing membranes reported previously.19 In particular, a sub-Nernstian electrode slope of ∼46 mV/decade was indeed observed with site-free membranes containing this ionophore. Since a low concentration of intrinsic anionic impurities is always present in PVC membranes,26 it is not overly surprising that this system shows somewhat less than ideal response properties. Evidently, other ionophores with different pKa values would need to be characterized in more defined membrane systems to shift the overall pH response range to practically relevant values. As it stands, ETH 7075 membranes seem to show optimum behavior with incorporated cationic sites only. In this paper, membranes containing the charged H+ carrier ETH 2412 were also characterized. This compound has served as H+ chromoionophore in optical sensing films for the determination of anions in bulk optodes.27 Figure 2 shows the potentiometric characteristics of NPOE-PVC membranes containing ETH 7075 or ETH 2412. It is shown that the presence of ETH 2412 shifts the useful dynamic pH range to as much as 4 orders of magnitude toward higher pH values as compared to electrodes containing ETH 7075 as charged H+ carrier. According to theory, this indicates that ETH 2412 shows a pKa2 value (see eq 3) that is ∼4 units larger than for ETH 7075. After several days of conditioning in electrolyte solution, ETH 2412 started to crystallize in the membrane, and the electrodes containing ETH 2412 deteriorated in function owing to this effect. This effect was not observed with electrodes containing ETH 7075. Correlation to Optical Film Experiments. In the theoretical section, it is explained how absorbance measurements on thin polymeric films containing a H+ ionophore and lipophilic ionic (26) Na¨gele, M.; Pretsch, E. Mikrochim. Acta 1995, 121, 269. (27) Hauser, P. C.; Pe´risset, P. M. J.; Tan, S. S. S.; Simon, W. Anal. Chem. 1990, 62, 1919.
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Figure 3. Limiting experimental absorbance spectra of an optical sensing film containing ETH 7075 (membrane composition as in Figure 2) in an acidic and basic 0.1 M NaCl sample.
sites may be correlated to the EMF function of conventional electrode membranes of identical composition. Such experiments are very valuable in order to understand how bulk extraction properties relate to potentiometric behavior. A similar experiment was reported earlier for a neutral H+ ionophore system, and an excellent correlation was found.9 For the particular case discussed here, the main assumptions to predict EMF response functions from spectroscopic experiments on thin optical films are that (1) membrane internal diffusion potentials are neglected, (2) ion pair formation within the membrane is not considered, (3) rapid equilibration across the sample-membrane interface is assumed, (4) no concentration polarization within the aqueous diffusion layer adjacent to the membrane is allowed, (5) concentrations of total ionophore and ionic sites within the membrane are uniform and identical to the concentrations in the optical film, and (6) extraction properties of the supported thin polymer film are assumed to be identical to the phase boundary layer of the electrode membrane. In addition, it is assumed that Beer’s law is fulfilled for the optical measurement. A typical absorption spectrum of an optical sensing film containing ETH 7075 and TDDMACl in NPOE-PVC (2:1) in contact with 0.1 M NaCl is shown in Figure 3. As the sample pH is increased, the absorbance at 440 nm, attributable to the protonated form of ETH 7075, decreases and the absorbance at 530 nm, arising from the deprotonated form of ETH 7075, increases. Figures 4 and 5 show measured and predicted EMF response functions and normalized optical film absorbances for polymeric films of identical composition in contact with KCl solutions at different pH values. The theoretical curve was obtained according to eq 13 with log(Kcoex/Ka2) ) 1.98 and log(KexchKa2) ) -11.99. Evidently, the overall response behavior is consistent with neutral carrier-based systems studied previously.9 A Nernst function is observed for the electrode where concentrations within the membrane remain relatively sampleindependent. In this region, the concentration of nonprotonated ionophore is given by the concentration of incorporated ionic sites. Deviations at low pH occurs when the concentration of nonprotonated ionophore starts to decrease owing to the uptake of sample 5256 Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
Figure 4. Potentiometric response behavior of NPOE-PVC membranes containing ETH 7075 and TDDMACl in buffered 0.1 M KCl samples: (O) experimental values; (b) predicted according to absorbance experiments on thin polymeric films of the same composition (see Figure 5 and eq 11).
Figure 5. Normalized absorbances of protonated ionophore in thin NPOE-PVC films containing ETH 7075 and TDDMACl in buffered 0.1 M KCl samples. The variable (1 - R) is the mole fraction of protonated ionophore as obtained from absorbance measurements: (O) experimental values; (b) predicted according to potentiometric experiments on thick polymeric membranes of the same composition (see Figure 4 and eq 12). Theoretical curve according to eq 13.
anions and protons. At high pH, sample cations start to exchange with protons from the membrane, thereby increasing the concentration of nonprotonated ionophore in the membrane. In the cation interference region, interestingly, the predicted concentration change occurs much more suddenly, and at higher pH values than observed optically. This leads in practice to apparent potential dips that are not predicted by the extraction experiment. This effect may be transient in nature, i.e., involving bulk membrane diffusion processes that cannot be observed with the optical film experiments. Ion pair formation processes may also be likely reasons for this behavior, although separate efforts to consider ion pair formation with such correlations did not yield better matches. A possible explanation might be the development of a membrane internal diffusion potential28 that assumes more (28) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: New York, 1981.
positivevalues as more sample cations exchange with membrane protons. Naturally, such a diffusion potential is not visible with the optical experiment, conveniently explaining the discrepancy. More experimental studies will be necessary to more clearly elucidate the reasons for this interesting behavior. The anion coextraction range correlates very well with the predicted values (see Figure 4). These findings are somewhat complementary to neutral carrier membrane cases where the ion-exchange region can be quantitatively predicted from optical experiments and the anion interference range correlates more poorly. Indeed, in the example discussed here, the total number of membrane ions does not appreciably change in the coextraction range since uptake of anions leads to concentration decrease of the negatively charged nonprotonated ionophore. A similar situation is observed with neutral carrier-based systems in the ion-exchange region: deprotonation of the positively charged ionophore is accompanied with the uptake of a sample cation, again leaving the number of membrane ions constant. These two activity ranges apparently conform most closely to the model assumptions used here. Overall, the simple extraction model used here may account satisfactorily for the majority of effects observed in practice. A quantitative correspondence is not always observed in the cation interference region. Analogous experiments with different sample electrolytes were performed (data not shown), and similar trends were obtained. Consistent with the potentiometric experiments commented on below, different cations did exchange with membrane protons at practically the same sample pH, while more lipophilic anions such as perchlorate shifted the coextraction range to higher pH values. Specific Characterization of ETH 7075-Based pH Electrodes. Effects of Structure of Cationic Lipophilic Additives and Plasticizers. Membranes containing ETH 7075 and TDDMACl in DOS-PVC instead of NPOE-PVC showed virtually the same pH response behavior. With neutral carrier membranes, a distinct improvement at high pH is usually observed with NPOEplasticized membranes. This has been explained by the lack of cation-binding functional groups relative to the lipophilic diester DOS.6 With charged carrier-based systems, it can be expected that extracted sample cations are not bound by the plasticizer but by the negatively charged ionophore, thereby forming relatively strong ion pairs. Consequently, a smaller plasticizer effect is expected with such systems. The addition of a more shielded cationic additive such as TDDACl might be beneficial to charged carrier-based systems relative to the more polar TDDMACl owing to its likely decreased tendency to form ion pairs within the membrane. Indeed, DOSPVC membranes containing TDDACl with ETH 7075 showed an anion interference range that was shifted by half a pH unit toward lower sample pH values. However, TDDACl membranes showed a somewhat deteriorating influence on the lower detection limit, shifting it by ∼1 order of magnitude. In addition, NPOE-PVC membranes containing this ionic additive showed no useful pH response at all, indicating that this additive shows solubility limitations in some membrane formulations. Cation and Anion Interferences. Cation selectivities of NPOEPVC membranes containing ETH 7075/TDDMACl were determined according to the fixed interference method. Owing to the observed potential dip at high pH, selectivity coefficients were
Figure 6. Anion interference for electrode membranes containing ETH 7075 (see Figure 5) in buffered 0.1 M NaCl, 0.1 M NaNO3, and 0.1 M NaClO4 solutions. Dotted line with Nernstian electrode slope.
somewhat difficult to determine consistently. In a previous short communication,19 potential values for the interference were taken classically, beyond the potential dip in the completely pHindependent activity region. In this work, potential readings were used at the bottom of the potential dip, reflecting the close adherence of the pH response function until that situation. Selectivity coefficients thus determined were found to be logKHLipot ) -9.6, logKHNapot ) -9.5, and logKHKpot ) -9.4, indicating no significant discrimination among these cations. It is expected that the extent of ion pair formation between the nonprotonated ionophore and the extracted cation increases with decreasing ionic radius. This counteracts the increased ionic hydration energy and leads to less spread in the observed selectivities than typically observed with neutral carrier pH electrodes. Anion interferences should quantitatively follow the Hofmeister selectivity sequence observed with an analogous ionophore-free TDDMACl membrane because the only positively charged species that may bind to the extracted anion is the cationic additive. Indeed, Figure 6 shows typical Hofmeister anion interference characteristics, indicating that samples containing a high concentration of highly lipophilic anions may deteriorate the pH response behavior of the electrode. Lifetime. Ion-selective electrodes containing ETH 7075 and TDDMACl in NPOE-PVC were stored in electrolyte solution for half a year. After two months, the electrode function reproduced exactly the one of a freshly conditioned electrode. After six months, minor deviations from the original curve for sample pH values above 8 were observed, indicating a long useful shelf life of the electrode. CONCLUSIONS Solvent polymeric membrane pH electrodes based on electrically charged carriers are interesting and analytically robust systems that may offer a viable alternative to neutral carrier pH electrodes in some cases. Theoretical predictions that the Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
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observed linear pH response range may be shifted dramatically by adding a different charge type of ionic additive have been confirmed experimentally. Theory also expects that dual-function ionophores, i.e., ionophores with acidic and basic properties, will show a half-Nernstian electrode slope and a dramatically extended measuring range relative to established electrode systems. Comparative experiments between potentiometric and absorbance experiments show qualitative agreement, although too many assumptions are involved for a quantitative EMF prediction from optical extraction experiments. Nonetheless, these experiments confirm that the models used to describe the general response behavior of such potentiometric membranes appear to be appropriate. Differences in response behavior of charged carrier systems relative to established neutral carrier electrodes provide
5258 Analytical Chemistry, Vol. 70, No. 24, December 15, 1998
valuable insights on the role of the different membrane components on potentiometric performance. Selectivity, response range, and lifetime of NPOE-PVC membranes containing ETH 7075 as ionophore and TDDMACl as cationic additive are suitable for practical applications. ACKNOWLEDGMENT The authors thank the Petroleum Research Fund (administered by the American Chemical Society) for financial support.
Received for review June 23, 1998. Accepted October 12, 1998. AC980678L
