Anal. Chem. 1994,66, 3188-3192
Use of a Guanidinium Ionophore in a Hydrogen Sulf ite-Selective Electrode Richard S. Hutchins,t Pedro Mollna,* Mateo Aiajarh,* Angel Vidai,* and Leonidas G. Bachas'J Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506-0055, and Departamento de Quimica Orghnica, Facultad de Quimica, Universidad de Murcia, Campus de Espinardo E-30077, Murcia, Spain
A liquid membranebasedanion-selectiveelectrodefor hydrogen sulfite has been developed using a guanidinium ionophore in a poly(viny1 chloride) membrane. At pH 6.00, a logarithmic to 0.500 response to hydrogen sulfite in the range 5.0 X M is observed with a detection limit of 3.9 (f0.3) X M ( D = 6). The electrode demonstrated minimal response to other anions, with perchlorate (W$.ISQ-,~O,= 7.0 X and salicylate ( W ~ i ~ c =y4.6 ~ Xt e being the two strongest interferents. 'H-NMR complexation was used to study the interactionbetween hydrogensulfite and the ionophore in this novel HSO3- sensor. It has been demonstrated that chemical recognition principles can be useful in the design of anion-selective Most ion-selective electrode (ISE) membranes containing quaternary ammonium salts as the charged ion carrier (ionophore) have no selective recognition properties to enable a selective response to anions. Instead, a selectivity pattern is obtained simply based on the lipophilicities of the anions, with the more lipophilic anions producing the greatest response. This response pattern, known as the Hofmeister series,3can be altered by using an anion-selective recognition site built into the ionophore to yield selective anion ISEs. Several ISEs based on such chemical recognition principles have been developed. One class of ISEs that has shown selective responses to various anions uses metallomacrocyclic ionophores. A lipophilic vitamin B12 derivative has been used successfully to develop an ISE that demonstrated high selectivity for iodidea4Salicylate-, chloride-, and thiocyanateselective electrodes have been developed by employing Sn(IV), Mn(III), In(III), and Co(II1) metalloporphyrins as the ion-complexing specie^.^-^ Additionally, hydrophobic coby* Phone: (606) 257-6350; Fax: (606) 323-1069; E-mail address: chm148 @ukcc.uky.edu. + University of Kentucky. Universidad de Murcia. (1) Wotring,V. J.; Johnson,D. M.;Daunert,S.;Bachas,L.G.InZmmunochemical Assaysand Biosensor Technologyfor the 1990's;Nakamura.R. M., Kasahara, Y ., Rechnitz, G. A., Eds.; American Society of Microbiology: Washington, DC, 1992; pp 355-376. (2) Pretsch, E.; Badertscher, M.; Welti, M.; Maruizumi, T.; Morf, W. E.; Simon, W. Pure Appl. Chem. 1988, 60, 567-574. (3) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247-260. (4) Daunert, S.; Bachas, L. G. Anal. Chem. 1989,61,499-503. ( 5 ) Chaniotakis, N . A.; Park, S. B.; Meyerhoff, M. E. Anal. Chem. 1989, 61, 566-570. (6) Chaniotakis, N. A,; Chasser, A. M.; Meyerhoff, M. E.; Groves, J. T. Anal. Chem. 1988, 60, 188-191. (7) Ammann, D.; Huser, M.; Krlutler, B.; Rusterholtz, B.; Schulthess, P.; Lindemann, B.; Halder, E.; Simon, W. Helu. Chim. Acta 1986,69,849-854. ( 8 ) HodinBr, A.; Jyo, A. Chem. Lett. 1988, 993-996. (9) Park, S.B.; Matuszewski, W.; Meyerhoff, M. E.; Liu, Y. H.; Kadish, K. M. Electroanalysis 1991, 3, 909-916. 3188
AnalyticalChemistry, Vol. 66, No. 19, October 1, 1994
rinates have demonstrated an affinity for complexing nitrite and have been useful in the development of ISEs for this anion.10-13 Other classesof ionophores include organometallic tin compounds,l"16 macrocyclic polyamines,17diquaternary ammonium salts,18 bis(dialkyldithiocarbamat~)mercury,~~ trifluoroacetyl-p-butylbenzene?O and Schiff base complexes of C O ( I I ) . ~The ~ chemical recognition by the ionophore for the given anion in each of these ISEs is responsible for the observed deviation from the Hofmeister series and the selective sensors that have resulted. With the goal of developing anion-selective electrodes for oxyanions, the chemical recognition provided by the guanidinium moiety was chosen as a base structure in the design of an ion carrier. There have been numerous examples of the affinity that the guanidinium moiety of arginyl side chains in proteins has toward carboxylates and other oxyanions.22-25 Additionally, binding studies performed using acyclic and cyclic guanidinium compounds demonstrated that, by incorporating the functional group into a rigid framework, the structural integrity of the guanidinium group was maintained and enhanced binding r e ~ u l t e d . Crystallographic ~~*~~ evidence has also been obtained using synthetic bicyclic guanidinium receptors that demonstratesthat these receptors bind oxyanions in an analogous fashion to that found in protein^.^^.^^ (IO) Stephek. R.; Kriiutler, B.; Schulthes, P.; Lindemann, B.; Ammann, D.;Simon, W. Anal. Chim. Acta 1986, 182, 83-90. (1 1) Schulthess, P.; Ammann. D.; Krlutler, B.; Caderas, C.; Steplnek, R.; Simon, W. Anal. Chem. 1985, 57, 1397-1401. (12) Schulthess, P.; Ammann, D.; Simon, W.; Caderas, C.; Steplnek, R.; Kriiutler, B. Helu. Chim. Acta 1984, 67, 1026-1032. (13) Daunert, S.; Witkowski, A.; Bachas, L. G. Prog. Clin. Biol. Res. 1989, 292, 2 I 5-225. (14) Glazier, S.A.; Arnold, M. A. Anal. Chem. 1991, 63, 754-759. (15) Chaniotakis, N . A.; Jurkschat, K.; RBhlemann, A. Anal. Chim. Acra 1993, 282, 345-352. (16) Fluri, K.; Koudelka, J.; Simon, W. Helu. Chim. Acta 1992, 75, 1012-1021. (17) Umezawa, Y.; Kataoka, M.; Takami, W . Anal. Chem. 1988,60,2392-2396. (18) Wotring, V. J.; Johnson, D. M.; Bachas, L. G. Anal. Chem. 1990,62, 15061510.
(19) Pranitis, D. M.; Meyerhoff, M. E. Anal. Chim. Acta 1989, 217, 123-133. (20) Meyerhoff, M. E.; Pretsch, E.; Weltis, D. H.; Simon, W. Anal. Chem. 1987, 59, 144-150. (21) Yuan,R.;Chai,Y.Q.;Lin,D.;Gao,D.;Li,J.Z.;Yu,R.Q.Ana/.Chem.1993, 65, 2572-2575. (22) Luecke, H.; Quiocho, F. A. Nature 1990, 347, 402406. (23) Yokomori, Y.; Hodgson, D. J. Znt. J. Pept. Protein Res. 1988, 31, 289-298. Reddy, P.; Sutrina, S.;Saier, M. H. Jr.; Reizer, J.; Kapadia, (24) Herzberg, 0.; G . Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 2499-2503. (25) Riordan, J. F.; McElvany, K. D.; Borders, C. L. Science 1977,195,884-886. (26) Dietrich, B.; Fyles, D. L.; Fyles, T.M.; Lehn, J. M. Helu. Chim. Acta 1979, 62, 2763-2787. (27) Dietrich, B.; Fyles, T. M.; Lehn, J. M.; Pease, L. G.; Fyles, D. L. J. Chem. SOC.,Chem. Commun. 1978, 934-936. (28) Muller, G.; Riede, J.; Schmidtchen, F. P. Angew. Chem., Znf.Ed. Engl. 1988, 27, 1516-1518. (29) Gleich, A.; Schmidtchen, F. P.; Mikulcik, P.; Muller, G. J . Chem. Soc., Chem. Commun. 1990, 55-57. 0003-2700/94/03663188$04.50/0
0 1994 American Chemical Society
HsC\
Flguro 1. Structure of the guanidinium ionophore incorporated in the PVC membranes.
The structurally rigid guanidiniumcompound investigated herein (Figure 1) was prepared by a unique synthetic approach that involved annealing two C,C-bis(iminophosphoranes) attached to different heterocyclic ringsa30It was demonstrated that ionophore 1has a remarkable affinity for hydrogen sulfite and gave a unique selectivity pattern compared to the Hofmeister series: HSO3- >> C104- > salicylate > benzoate > SCN- > I- > CH3COO. > N02- > s~03~> Nos- > SO4*-, HzPO4-. Using an electrode based on a quaternary ammonium salt, tridodecylmethylammonium chloride, HSO3was determined to follow nitrite in the Hofmeister series: C104> SCN- > salicylate > I- > B r , NO2- > HSO3- > CHsCOO-, H2P04-. Unlike the previously reported hydrogen sulfite electrode, which employs a mercury(I1) complex,19 the guanidiniumbased electrode is not subject to interference from the many halides and sulfur-containing species that complex with the mercury center. Wereport here thedevelopment of a hydrogen sulfite-selective electrode based on the tetrafluoroborate salt of the guanidinium compound 1.
EXPERIMENTAL SECTION Reagents. 2-Nitrophenyl octyl ether (NPOE), dibutyl phthalate (DBP), diethyl suberate (DES), tris(2-ethylhexyl) phosphate (TOP), bis(Zethylhexy1) sebacate (DOS), and bis(1-butylpentyl) adipate (BBPA) were purchased from Fluka (Ronkonkoma, NY). [Bis(2-hydroxyethyl)amino] tris(hydroxymethy1)methane (bis-tris) and 1,3-bis[tris(hydroxymethy1)methylaminolpropane (bis-tris propane) were obtained from Research Organics (Cleveland, OH). Poly(viny1 chloride) (PVC) was acquired through Polysciences (Warrington, PA), ethanol (200 proof) was supplied by Aaper Alcohol and Chemical (Shelbyville, KY), and sodium thiocyanate was procured through Matheson Coleman & Bell (Cincinnati, OH). The sodium salts of hydrogen sulfite, sulfite, dibasic phosphate, and thiosulfate were obtained from Mallinckrodt Chemical Works (St. Louis, MO), while those of acetate, iodide, and nitrate were purchased from J. T. Baker (Phillipsburg, NJ). Sodium nitrite and potassium chloride were from Fisher Scientific (Cincinnati, OH). The sodium salts of benzoate, perchlorate, monobasicphosphate,and salicylate along with dibutyl sebacate (DBS) were provided by Sigma (St. Louis, MO). The deuterated solvents (D2O and DMSO&) were purchased from Aldrich (Milwaukee, WI), along (30) Molina, P.; Alajarln, M.; Vidal, A. J. Org. Chem. 1993, 58, 1687-1695.
with sodium sulfate and tetrahydrofuran (THF). All of the aqueous solutions were prepared using deionized (Milli-Q Water Purification System; Millipore, Bedford, MA) distilled water. Apparatus. Voltages were monitored using Fisher Accumet 8 10or 825 digital pH/mV meters. The potential was recorded using a Fisher Recordall (Series 5000) strip-chart recorder. The electrode cell was maintained at 25.0 OC for all experiments using a Fisher Isotemp circulator bath (Model 9500). The reference electrodes used were Fisher Ag/AgCl double-junctionelectrodes. IH-NMR spectra were obtained using a 200-MHz Gemini spectrometer from Varian (Palo Alto, CA) with tetramethylsilane (TMS) as reference. Membranes and Cell Assembly. The membranes were prepared by dissolving 1.O mg of the BF4- salt of 1,66.0 mg of the plasticizer, and 33.0 mg of PVC in 0.500 mL of THF. This compositionwas used for all experiments, unless otherwise stated. The solution was vortexed until the contents were dissolved, and then it was poured into a 16-mm4.d. glass ring located on a glass slide, and the solvent was allowed toevaporate at room temperature overnight.” The membrane was then cut into small disks, which were mounted onto a Philips IS561 (Glasblaserei Miiller, Zurich) electrode body. All potentiometric measurements were performed using the following cell assembly:
Ag-AgCllKCl(satd)llbufferllbufferedsample solution1 membranelo.100 M KClJAg-AgC1 Bis-tris HCl, pH 6.00, was the buffer used throughout the experiments, unless otherwise indicated. Procedure. The potentiometric response of the prepared membranes was observed by adding incrementally sized aliquots of a given anion solution to a stirred initial buffer solution of 5.00 mL. The buffers used were bis-tris HCl of 0.100,0.250, and 0.500 M at a pH of 6.00; bis-tris HCl, pH 6.00, at a fixed ionic strength of 0.500 M; and bis-tris propane HC1, pH 6.50, 7.00, and 8.00, at a fixed ionic strength of 0.500 M. After each addition, the response of the electrodes was recorded on the strip-chart recorder. Each membrane was conditioned overnight before use in the buffer to be used in the upcoming experiment. Between experiments, the electrodes were conditioned for at least 1 h in buffer. The sulfite and hydrogen sulfite solutions were prepared fresh before use. To obtain the calibration curve of the electrodes, the steadystate potential changes (AE) were plotted vs the logarithm of the concentrationof the anion present in the buffered solution; A E refers to the potential change with respect to the baseline potential. The selectivity coefficients were determined using the fixed interference method.32 Lipophilicity Determination. The lipophilicity study was performed using reverse-phase TLC plates and a mobile phase consisting of a 7:3 ethano1:HzO mixture. The reverse-phase TLC plates used were KC18F silica plates obtained from Whatman (Hillsboro, OR). The lipophilicity of a compound (31) Moody. G. J.; Thomas, J. D. R. In Ion-Selective Electrode Methodology; Covington, A. K., Ed.; CRC Rw: Boca Raton, FL, 1980; pp 111-130. (32) Commission on Andytical Nomenclature.Pure Appl. Chem. 1975,18, 129132.
Analytical Chemis&y, Voi. 88, No. 19, October 1, 1994
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F
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.-0
%
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3 ~ 1 0 ~ ~
c
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6,O
6,5
7,O
7,5
8,O
-2251 5
8.5
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"
"
7
8
PH Flgure 2. Effects of pH on the detection llmlt (total sulflte) for an electrode containing 1.O% (w/w) l.BF4, 66% (wlw) NPOE plasticizer, and 33% (w/w) PVC in a constant ionic strength buffer prepared as explained In the text.
is defined as the partition coefficient, log P, between water and 1-octanol. Using thesystem described above, theretention factors, k, for a series of standards with known lipophilicities were determined and used to make a linear calibration plot (log P vs log k ) ; the lipophilicity of the ionophore was then obtained from this calibration plot and its k
RESULTS AND DISCUSSION Membranes were prepared with the BF4-salt of compound 1 and tested for response to various anions. This study indicated that the ISEs had a strong response to sulfite species. In order to identify whether this response was due to sulfite, hydrogen sulfite, or both, the ISEs were calibrated in fixed ionic strength buffers prepared with 0.500 M HCl at several pHs (see Experimental Section). The electrodes were first tested at pH 6.00, where the sulfite species present is predominantly hydrogen sulfite (94%HSOs- and 6% S032-). The slopes of the calibration plot, -47 f 2 mV/decade, were sub-Nernstian for a monovalent anion response. As the pH was increased from pH 6.00 to pH 8.00, the detection limit of the ISE worsened (as seen in Figure 2), but the slopes of the calibration plots did not change significantly (f5 mV/ decade). At pH 8.00, S032-is the dominant sulfite species present (87%), and the theoretical Nernstian slope for a divalent anion is -29.6 mV/decade. The slopes obtained by calibrating the ISE in buffers of different pH are evidence that the electrode membranes respond only to hydrogen sulfite, as response to both sulfite species would result in a lowering of the slopes at higher pHs. The worsening of the detection limit at high pHs can be explained by the pH dependence of the membrane potential (Figure 3). As shown in this figure, the hydroxide activity strongly affects the response of the ISE. By working at a low pH (6.00) and buffering the sample solution, the detection limit for hydrogen sulfite was improved (Figure 2), and potential changes due to hydroxide interference were mini(33) Oesch, U.;Simon, W. Anal. Chem. 1980,52, 692-700. (34) Dinten, 0.; Spichiger, U. E.; Chaniotakis, N.; Gehrig, P.; Rusterholz, B.; Morf, W. E.; Simon, W. Anal. Chem. 1991, 63, 596-603.
3190
AnalyticalChemistry, Vol. 66, No. 79, October 1, 1994
"
9
'
I
'
I
1 0 1 1
" 12
PH Flgure 3. Response of an ISE containing 1.O % 1.BF4, 33 % PVC, and 66% NPOE plastlcizer (w/w) to sample pH In the absence of hydrogen sulflte.
-40
>
-
E -80
-
-120
-
-160 I -4
I
I
-3 -2 log [hydrogen sulfite]
-1
Flgure 4. Potentiometric response of electrodesemploying 1 % (wlw) l-BF, to sodlum hydrogen sulflte In 0.25 M bls-trls HCI, pH 6.00 buffer. The plasticizers(66% w/w) used were DOS (a),TOP (O),BBPA (X), DBP (+), and NPOE (A).
mized. The effect of the buffer strength was also evaluated. It was found that when the molarity of the bis-tris HCl (pH 6.00) was less than 0.500 M, the buffering capacity for some strongly basic anions was exceeded, and the resulting pH change influenced the slopes at high analyte concentrations. All remaining experiments were performed using a bis-tris HCL, pH 6.00, buffer of 0.500 M. Additionally, the analyte standard solutions were also prepared in the same buffer. Several plasticizers were examined in membranes containing (w/w) 1.0% bBF4, 33.0% PVC, and 66.0% plasticizer, and the electrodes were evaluated for response toward hydrogen sulfite at pH 6.00. The hydrogen sulfite calibration curves in Figure 4 were averages of three or more trials for each membrane. The guanidinium salt was not completely soluble in any of the plasticizers except DBP and NPOE. These two plasticizers elicited the best responses of all the ISEs tested. Membranes prepared using NPOE demonstrated the best linear range, most Nernstian-like slopes, and best detection limits for hydrogen sulfite. For these reasons, NPOE was selected as the optimal plasticizer for use in the ISEs.
Table 1. Effect of Ionophore Concentration on Detection Lknns and Slope8 % ionophore detection limit (pM)" slope (mV/decade)o.*
0.0 0.1 0.5 1 .o 1.5 2.0
80 f 20 42 f 3 39 f 3 30f6 35 f 7
-8 f 4 -18 f 6 -48f2 47f2 -35 f 3 41f9
Data are averages fl SD (n = 6,except for 1% ionophore where n = 12). * Response was linear over the concentration range studied in this
experiment (1Wto 10-1 M), except for the 0 and 0.1% ionophore membranes, which demonstrated a worse limit of linear response, only down to 5.0 X M.
E -100 J -150 -175
1
-200' -6
The percentage of l.BF4 used in the membranes was optimized by preparing membranes with NPOE as the plasticizer and differing percentages of the guanidinium salt, ranging from 0 to 2.0% (w/w). Six or more hydrogen sulfite calibrations were performed using each membrane, the results of which are summarized in Table 1. The limited solubility of the ionophore in NPOE resulted in visibly grainy, nonhomogeneous membranes when greater than 1.0% l.BF4 was used (i.e., 1.5% and 2.0%). Both the slope of the calibration curve and the reproducibility of the response worsened for membranes prepared with more than 1.0% l.BF4. At 0.1%, the response of the ISE in terms of detection limit and slope of the calibration curve fell off considerably. A percentage of 1.0% l.BF4 was selected for use in the preparation of all further membranes. The membranes exhibited a response to hydrogen sulfite with a detection limit of 3.9 (f0.3) X M ( n = 12) and a slope of -47 f 2 mV/decade. This detection limit is equivalent to 3.5 mg/L hydrogen sulfite. The response time of the guanidinium-based membrane electrode ranged from less than 1 min at concentrations of hydrogen sulfite higher than 10-4M to as long as 2 min at lower concentrations; the response time is defined as the length of time from the initial change in the analyte concentration to the point at which less than 1 mV/min change in potential occurs. The responseof themembranesto 11 potentially interfering anions was tested under the determined optimal conditions. The selectivity pattern can be seen in Figure 5 and reflects the average response of the electrodes to six or more tests with each anion. With the exception of hydrogen sulfite, the remainder of the anions responded based on their hydrophobicity, with perchlorate, salicylate, and thiocyanate being the strongest interferents. These guanidinium-based ISEs exhibited excellent selectivity for hydrogen sulfite over every anion tested. Selectivity coefficients (Gt) were measured using the fixed interference method with an interferent concentration of 5.0 X M. The more significant interferents for the guanidinium-based membranes were perchlorate and salicylate with selectivity coefficients of 7.0 X 10-3 and 4.6 X l e 3 , respectively. The rest of the anions had selectivity coefficients of 1.O X le3 or smaller, including C1- (data not shown), which is why HC1 was used to adjust the pH of the buffer used. The high degree of selectivity and linear range of response over four decades of concentration (5.00 X le5to 0.50 M, see Figure 5 ) make this ISE of great value for applications requiring the determination of hydrogen sulfite.
I
1
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-2
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log (concentration, M 1 Figure 5. Anion seiecttvityof an ISE basedon 14F, In 0.500 M bis-tris HCI, pH 8.00 buffer. The tested anions were prepared using the same buffer, andthe calibrationcurvesshown representthe average response of the electrodeto six or more trials. The anions tested were hydrogen sulfite ( l ) , perchlorate(2), salicylate (3),benzoate (4), thiocyanate (5), iodide (8), acetate (7), nitrke (8), thiosulfate (g), nitrate (lo), sulfate ( l l ) , and phosphate (12).
Three of the most lipophilic anions (perchlorate, salicylate, and thiocyanate) were also tested with each of the membranes containing the various plasticizers. These anions respond preferentially over hydrogen sulfite when membranes are prepared using nonselective ionophores, such as quaternary ammonium salts. Despite the weak response to hydrogen sulfite observed using membranes prepared with the plasticizers DOS and TOP (see Figure 4), all five plasticizers gave electrodes that responded preferentially to hydrogen sulfite over perchlorate, salicylate, and thiocyanate. This result demonstrates that the observed selectivity is not due to the plasticizer, but instead results from a selective interaction taking place between the hydrogen sulfite anion and the guanidinium moiety of 1. Lifetime studies were performed to examine the stability of these electrodes. The electrodes were stored in buffer when not in use, consisting of 0.500 M bis-tris HCl (pH 6.00). It has been shown that membrane components in PVC-based ISEs leach into solution over time, resulting in a degradation of the performance of the electrode^.^^-^^ It was observed that while the starting potentials (potential of the cell before any addition of anions) remained constant (f5 mV) over the time period studied, there was a worsening of response after several days. This degradation was evidenced in both the slope of the calibration curve and in the detection limits. After 1 week, the electrodes were only responding at 80% of their initial capacity (based on the slopes observed). By the end of 16 days, the response observed was less than half of what was observed on day 1. This limited lifetime of the hydrogen sulfite sensor is attributed in large degree to the relatively low lipophilicityof the ionophore. The less lipophilicthe membrane components, the faster the leaching process occur^.^^.^^ The lipophilicity of the ionophore was determined using reverse-phase thin-layer chromatography. This method for elucidating the lipophilicity value, log P,is a well-established (35) Moody, G. J.; Saad, E. E.; Thomas,J. D. R.Sel. Electrode Rev. 1988, 10, 71-106. (36) Dauncrt, S.; Bachas, L. G. Anal. Chem. 1990.62, 1428-1431.
AnaWcalChemistry, Vol. 66, No. 19, October 1, 1994
3191
t e c h n i q ~ e . ~Several ~ ? ~ ~standards of known lipophilicity were used to make a linear calibration plot of log P vs log k.33,34 It should be noted that the capacity factor k' referred to in refs 33 and 34 has since been changed to k and relabeled as the retention factor by IUPAC.39 These standards were plasticizers used in the membrane composition studies described earlier. The value of log P for l.BF4 was 3.9, the exact value for the plasticizer DBP (log P = 3.9) and not too far from that of NPOE (log P = 5.4). The solubility of l.BF4 was best in these two plasticizers. TOP (log P = 7.9) and DOS (log P = 10.0) were 4 or more orders of magnitude more lipophilic than l.BF4. The degradation of the ISE response may be explained by the limited lipophilicity of the ionophore, which can result in the ionophore leaching from the membrane. Currently work is underway to extend the lifetime of this ISE. Although the guanidinium compound was selected as an ionophore for, among other reasons, its known ability to complex with oxyanions,22-25,29~4~41 the specific mechanism for hydrogen sulfite complexation is not known. A 'H-NMR titration was performed using a fixed ionophore concentration and varying amounts of the hydrogen sulfite anion. This titration was carried out by dissolving a fixed amount of ionophore in DMSO-&, taking the 'H-NMR spectrum, and then adding aliquots of NaHSO3 (in D20) and recording the lH-NMR spectrum following each addition. As the anion was added, most of the proton chemical shifts were observed to move upfield, until a 1:l ratio [HSO3-]/[ionophore] was reached, after which the addition of hydrogen sulfite no longer caused a change in the proton signals. The effect of combining two solvents, DMSO-da and D20, on the 'H-NMR spectrum of l.BF4 was investigated by making additions of D20 without hydrogen sulfite to a DMSO-& solution containing l.BF4. No significant change (>0.01 ppm) in the chemical shifts could be attributed to the varying solvent composition. The effect on the proton chemical shift from the aromatic region of l.BF4 is shown in Figure 6. This representative signal arises from the proton located in the para position in the pendant phenyl ring, having 6 = 7.39 ppm in the absence of hydrogen sulfite. This data demonstrates that a 1:l complexation between the ionophore and hydrogen sulfite takes place. In conclusion, a new ISE has been developed for hydrogen sulfite that contains a guanidinium compound as the ion carrier. (37) Ellgehausen, H.; D'Hondt, C.; Fuerer, R. Pesric. Sei. 1981, 12, 219-227. (38) Leo,A.; Hansch, C.; Elkins, D. Chem. Reo. 1971, 71, 525-616. (39) Ettre, L. S. Pure Appl. Chem. 1993, 65. 819-872. (40) Schmidtchen, F. P. Tefrahedron Leu. 1989, 30, 4493-4496. (41) Echavarren, A,; Galan, A.; Mendoza, J.; Salmeron, A,; Lehn, J. M. Helv. Chim. Acra 1988, 71, 685-693. (42) Furia,Th.E.,Ed.HandbookofFdAdditiues, Znded.;CRCPress: Cleveland, OH, 1972; pp 142-147. (43) Dibbelt, L.; Kuss, E. Biol. Chem. Hoppe-Seyler 1991, 372 (3). 173-185.
3192 Analytical Chemistty, Voi. 66,No. 19, October 1, 1994
0.2 E
Q Q
a" 0.1 nn "I"
0.0
1.0 1.5 2.0 (moles HSO;) / (moles ionophore) 0.5
2.5
Flgure 6. Changes in the aromatic region of the lH-NwI chemical shlft observed uponincreaslngthesodium hydrogensulfite Concentration in the presence of a fixed concentration of 1-BF4 (the proton in the para positionof the pendant phenylrlng, 6 = 7.39 ppm at zero hydrogen sulfite, is the representative signal shown). The ionophore complexes in a 1:l fashion wlth hydrogen sulfite.
The selectivity for hydrogen sulfite over other anions and the linear response range over four decades of concentration make this sensor an important addition to the methods presently available for monitoring this analyte. Such a hydrogen sulfiteselective electrode would be of use in the food and beverage industry where sulfite species are used for their antimicrobial, preservative, and dehydrating effects.42 The paper industry also produces large amounts of sulfite wastewater that can be harmful to the environment and which mandates the need to monitor sulfite levels in streams and wastewater treatment plants. Additionally, sulfite species have been found to inhibit biological activities, such as the enzyme steryl sulfatase, which catalyzes the hydrolysis of such steroids as cholesterol and pregnen~lone.~~ Thus, there are a variety of potential applications for hydrogen sulfite-selective electrodes. Strategies for improving the lifetime of this ISE are currently being investigated in our laboratories.
ACKNOWLEDGMENT The authors would like to thank the Kentucky Space Grant Consortium, the National Aeronautics and Space Administration, and the National Science Foundation (EHR-9108764) for financial support of this work. The help of Li J. Sun in conducting preliminary studies for this work is also gratefully appreciated. Received for review April 12, 1994. Accepted June 10, 1994." a
Abstract published in Aduance ACS Absrracrs, August 1 , 1994.
