Anal. Chem. 1988, 6 0 , 2013-2016
hydroxide, respectively, and they are independent of the added cosolvent. Table V compares the experimental values of the slopes of Figures 1-4 (C and C? with the theoretical values predicted by eq 10 and 13 (assuming e = el in the last expression). The experimental absolute values for alcohols are markedly higher than the theoretical ones, showing that the mixtures of alcohols present higher dielectric constants than those predicted by the theory. The current theories about computation of dielectric constant values predict lower ones than those observed experimentally for pure alcohols. Onsager (16) explains that this is because of the formation of hydrogen bonds in these solvents, which increases the electric moment of the hydrogen bond donor group and thus the dielectric constant of the medium. The increase in the dielectric constant value in the alcohol mixtures can be explained by the formation of hydrogen bonds between the main solvent and cosolvents. In summary, the proposed equations are simple, but they allow for a good estimate of the dissociation constant values of electrolytes in mixed media up to about 10-15% cosolvent. This range is approximately the percentage of cosolvent added with the titrant in everyday nonaqueous titrations. Registry No. tert-Butyl alcohol, 75-65-0.
2013
(2) Fritz, J. S. Acid-Base Tnrations in Nonaqueous Solvents; Allyn and Bacon: Boston, 1973. Marple, L. W.; Fritz, J. S.Anal. Chem. 1982, 3 4 , 796-800. Fritz, J. S.; Marple, L. W. Anal. Chem. 1982, 34, 921-924. Marple, L. W.; Fritz, S.J. Anal. Chem. 1983, 35, 1223-1227. Marple. L. W.; Fritz, S.J. Anal. C h m . 1083, 3 5 , 1431-1434. Marple. L. W.; Scheppers, G. J. Anal. Chem. 1988,3 8 , 553-557. Chantooni, M. K.; Kokhoff, I. M. J . Phys. Chem. 1978. 8 2 , 994-1000. Chantooni, M. K.; Kokhoff, I . M. Anal. Chem. 1979, 57, 133-140. Barbosa, J.; Bosch, E.; Rosbs, M. Analyst (London) 1987, 772,
179-184.
Bosch, E.; Rosbs, M., submitted for publication in Talenta . Sucha, L.; Kotrly. S. Solution Equilibria in Analytical Chemistry; Van Nostrand Reinhold: London, 1972. Budevsky, 0. Foundations of Chemical Analysis; Ellis Horwood: ChiChester, -1979. (14) Bishop, E. Indicators; Pergamon: Oxford, England, 1972. (15) Debye, P. Physlk 2. 1912, 13, 97. (16) Onsager, L. J. Am. Chem. Soc. 1938, 58, 1486-1493. (17) Wlnkelmann, J.; Quitzsch, K. 2. Phys. Chem. (Leipig) 1972, 2550,
355-366. (18) Abboud, J. L. M.; Guiheneu, J. G.; Essfar, M. M’h; Taft, R. W.; Kamlet, M. J. J . Phys. Chem. 1984, 88, 4414-4420. (19) Akerlof, G.J. Am. Chem. SOC. 1932. 5 4 , 4125-4139. (20) Kolllng, 0.W. Anal. Chem. 1985, 5 7 , 1721-1725. (21) Kolling, 0.W. Anal. Chem. 1988, 58, 870-873. (22) . . ACS Analytical Reagents Committee Anal. Chem. 1986, 5 8 , 1276-1280. (23) Fuoss, R. M.; Shedlovsky, T. J. Am. Chem. SOC. 1949, 7 7 , 1496-1498. (24) Conway, B. E.; Bockris, J. O’M.; Linton, H. J. Chem. Phys. 1958, 2 4 , 834-850.
LITERATURE CITED (1) Treatise on Analytical Chemistry, 2nd ed.;Kolthoff, I. M., Elving, P. J., Eds.; Wiley: New York, 1979;Part I, Vol. 2.
RECEIVED for review May 27,1987. Resubmitted January 1, 1988. Accepted April 25, 1988.
Neutral-Carrier-Based Magnesium-Selective Electrode Marizel V. Rouilly, Martin Badertscher, Ern6 Pretsch, Gabriela Suter, and Wilhelm Simon* Department of Organic Chemistr.y, Swiss Federal Institute of Technology (ETH), Universitatstrasse 16, CH-8092 Zurich, Switzerland
Through methods of molecular modeling, N,N,N’,N’-tetramethylaspartamlde (2) was designed. It Is supposed to complex Mg2+ In a 1:2 1on:llgand complex. Solvent polymeric membranes doped with the more llpophlllc N,N’-dlheptylN,N’-dlmethylaspartamlde (ETH 2220) (3) reject all alkallmetal and all other alkaline-earth-metal cations, preferring Mg2+over Ca2+,Na’, and K+ by factors of 300,400, and 200, respectively. There Is a heavy Interference by hydrogen Ions (log p w = 10.8). An application of such Ion sensors Is therefore limited to hydrogen Ion buffered solutions at relatively high pH values (pH 8-9).
A wide variety of ion-selective electrodes (ISE’s) with a preference of a given alkali-metal or alkaline-earth-metal cation over all the other group IA/IIA (group 1/2 in 1985 notation) cations is known (1-3). So far, however, no Mg2+-selectiveelectrode with a high rejection of alkali-metal as well as the other alkaline-earth-metal cations has been described (1-3). To circumvent this problem, chemometric approaches have been suggested for an assay of Mg2+by ISE technology (4). Even in ISE’s with equal selectivity for Mg2+ and Ca2+designed for the measurement of water hardness, the poor rejection of Na+ and K+ is a severe source of error
* Author to whom correspondence should be addressed.
(for a review see ref 5 and 6). Several attempts to design Mg2+-selectiveISEs led to sensors with a preference for Ca2+ over Mg2+or only a modest rejection of Ca2+relative to Mg2+ (7-11).cyclo(-L-Pro-D-Leu-)5behaves as an ionophore that selects Mg2+over Ca2+by a factor of 100 (7). Unfortunately, K+, Sr2+,and Ba2+are favored over Mg2+(7). So far, only the neutral ionophore N,”-diheptyl-N,”-dimethylsuccinamide (ETH 1117) (1) (9)has led to Mg2+ sensors of analytically relevant application (12). Since microelectrodes based on ETH 1117 select Ca2+by a factor of about 12 over Mg+, they have only been used for an intracellular assay of M$+ where the interference by Ca2+is negligible because of its low concentration (13-16). Through a substitution of the skeleton of ETH 1117 by an amino group we obtained a neutral carrier inducing interesting Mg2+selectivities in membranes. Here we report on the preparation of this ionophore and on the electromotive behavior of sensors based on it.
EXPERIMENTAL SECTION emf Measurements. The membrane preparation and measuring technique are described in detail elsewhere (17). Membrane composition: 1 wt % ligand, 33 wt % poly(viny1 chloride), and 64-66 wt % o-nitrophenyl octyl ether (0-NPOE). Potassium tetrakis(pchloropheny1)borate(KTpCLPB) was added in molar ratios of 0-120 relative to the ligand. Because of the high selectivity of the sensor for H30+ (see Table I), a tris(hydroxymethy1)aminomethane (Tris) buffer of pH 8.8 (0.01 M Tris and hydrochloric acid) has been used for the sample solutions.
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Table I. Selectivities of Mglt-Selective Electrodes for Different Ions M
M
ETH 1117 (1,50 mol % KTpClPB) (9)
Mg2+ Ca2+
log Kf&, for the following ionophores ETH 2220 (3, 73 mol cyclo(-L-Pro-L-Leu-)6 CyClO(-L-PrO-D-LeU-)6 % KTpClPB) (pH (58 mol % KTpClPB) (82 mol % KTpClPB)
0 +1.5 f0.3 +0.3 -0.9 -2.3 -1.2 -0.6 +0.3 +6.5
Sr2+
Ba2+ Lit Nat K+ Rb+ csc H30+
8.8)
(7)
(7)
0 -2.5 -3.2 -3.1 -2.6 -2.6 -2.3 -2.0 -1.3 +10.8"
0 -0.1 +o. 1
0 -2.1 +2.6 +3.5 -1.6 -1.2 t1.2 +2.1
+1.1 -2.6 -2.3 -1.0 -1.3 -0.4 +0.4
+3.1
+0.7
Obtained through emf measurements with cell assemblies in 0.1 M Mg2+ at pH 8.8 as well as in different buffer solutions (pH 8, 7, and 6, 0.1 M KzHP04and HCl) and extrapolation to Mg2+ as well as H30t activities of 1, respectively. a
was introduced (for details see ref 22). The methanol was evaporated, and the residue was purified by chromatography (silica gel, ethanol/ethyl acetate 1:4) to yield 0.80 g (53.1%)of pure ionophore 3. The 'H NMR, IR, and mass spectra correspond to the molecule 3. Anal. Calcd for 3: C, 67.56; H, 11.62; N, 11.82. Found: C, 67.40; H, 11.66; N, 11.57.
Figure 1.
3
2 -
1
(
(ETH 1 1 1 7 )
ETH2220)
Structure of ligands discussed.
To keep the transmembrane electric potential difference small and to increase the stability in the cell emf, 0.01 M MgCl, in the same buffer was used as the internal filling solution as well. The tx, were obtained by the separate soselectivity factors, log lution method (SSM, 0.1 metal chloride solutions of pH 8.8) (18). The activity coefficientsused are described in detail in ref 19 and 20. Experimental data were corrected for changes in the liquid-junctionpotential by using the Henderson formalism (20). Membrane Resistance. Resistance measurements were carried out as mentioned in ref 21. Synthesis of the Ionophore N,N'-Diheptyl-N,N'-dimethylAspartamide (3). A 7.62-g (29.94 mmol) sample bis(2oxo-3-oxazolidiny1)ph~phinicchloride (Fluka, purum) was slowly added to a solution of 4.0 g (14.97 mmol) of N-carbobenzoxy+ aspartic acid (Fluka, puriss), 3.02 g (29.94 mmol) of triethylamine (Fluka, puriss p.a.), and 3.87 g (29.94 mmol) of N-heptyl-Nmethylamine (Fluka, purum) in 200 mL of THF. The reaction mixture was stirred at room temperature overnight, the THF was evaporated, and the residue was dissolved in CHzClzand washed with a solution of 50% NaHC03 in water and again with HzO. The CH2C12phase was dried over MgS04, and the solvent was evaporated. The product was purified on a silica gel column (silica gel 60, Fluka No. 60738, hexane/ethyl acetate 2%). Yield 4.34 g (59.2%). A 2.1-g aliquot (4.28 mmol) of this product was dissolved in 80 mL of methanol, and 0.2 g of 10% palladium-oncharcoal catalyst was added. During 2 h a slow stream of hydrogen
%
Figure 2.
Hypothetical complex of Mg2+ with two molecules of
2,
RESULTS AND DISCUSSION The ionophore N,N'-diheptyl-N,N'-dimethylsuccinamide (ETH 1117, see Figure 1) can be assumed to form 1:3 Mg2+:ligand complexes with an octahedral coordination of Mg2+by the ligand 0 atoms (9). Since hydrates of magnesium aspartate exhibit an almost perfect octahedral coordination sphere with five oxygen atoms and one nitrogen atom (23), we studied the 1:2 Mg2+:ligand complex formation by using the hypothetical ionophore 2 and molecular modeling procedures as described recently (24). Figure 2 indeed corroborates that a structure can be found that corresponds to an almost perfect octahedral coordination of Mg2+. It corresponds to a local energy minimum of the interaction of Mg2+with two molecules of 2 (24). The more lipophilic ionophore 3 (ETH 2220) was therefore prepared (see Experimental Section) and studied in solvent polymeric membrane electrode cell assemblies. The addition of mobile cation-exchange sites to neutralcarrier-based liquid membranes was shown to be favorable in many respects, producing reduction of interferences by lipophilic sample anions, increase of the potentiometric selectivity for divalent over monovalent cations, reduction of the response time, reduction of the electric membrane resistence, and reduction of the activation barrier for the cationexchange reaction a t the membrane/solution interface (25). Several neutral carriers are known that do not induce any selectivity in membranes in the absence of anionic sites (25).
obtained by molecular modeling techniques.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
79 mol-%
2015
120 mol-%
88 mol-'/.
_I- 7.2 c s' 6.1 K+
lLcs'
1 4 . 8I R b '
3'91NH; Na' I2*
-IBa 2+
L-Mg 1-1
-JCa I2+
2+
-Mg
-- Li'
LBa2'
Rb+
c s' K+ Li +
K+ Sa2'
K'
Na' Li+
Li+
Sr2*
MEMBRANES
:
ETH 2220 I o-NPOE I PVC I KTpClPB (mol-%)
SAMPLE
:
TRlS I HCI (PH
8.8)
Flgure 3. Selectivity factors, log Gx, for solvent polymeric membranes with the ionophore 3, o-nitrophenyl octyl ether (o-NPOE) as membrane solvent, and incorporated lipophilic anionic sites (potassium tetrakisb -chlorophenyl)borate(KTp CIPB), mol % given). Membranes without lipophilic anionic sites (column 1) are compared with membranes containing different concentrations of lipophilic anionic sites (separate solution method, 0.1 M solutions of the chiorides and Tris buffer of pH 8.8, 20-22 "C). EMF [mvl
A 73 mol-% KTpClPB
30 mV
o NO INTERFERING IONS 0
-7
-6
-5
-4
-3
CaCI2 1.1 mM KCI 4.0mM NaCl 140.0mM -2
loga,,
Flgure 4. emf response of me solvent polymeric membrane electrode cell assembly to different Mg2+ activities in the buffered (pH 8.8. Tris and HCI) sample solution. Response was in MgCI, solutions without interfering ions and Mg2+ response was at constant ion background.
In some cases carrier-induced selectivity may be observed in membranes only after adding anionic sites to the membrane phase doped with such ionophores. Assuming the same 1:n cation:ionophore stoichiometry for all cationic complexes, electrodes for Mg2+are predicted to exhibit optimal rejection of monovalent interfering ions if 1.62, 0.73, and 0.46 mol % anionic sites are added to the membrane phase relative to the neutral carrier present for n = 1, 2, and 3, respectively (25). The selectivities presented in Figure 3 indeed corroborate the formation of 1:2 Mg2+:ETH 2220 complexes. Because of the amino group of relatively high basicity in 2, heavy interference by hydrogen ions has to be expected (I7). The measurements of the ion selectivities have therefore been performed at pH 8.8 of the sample solution. Under these conditions, the sensor described here exhibits surprisingly high selectivities over all alkali-metal and alkaline-earth-metalcations, by far surpassing values reported for other Mg2+-selectiveelectrodes (Table I). In the absence of interfering ions, the slope of the electrode response (10-3-10-1M) is 32 f 1mV. The detection limit is at a pMg of about 5 (Figure 4). The slopes of the electrode response might be above 29.1 mV (25 "C) because of the permeation of MgCl+- or MgOH+-ionophore complexes instead of Mg2+-ionophore complexes only (see ref 7 and 23). Because of the relatively large concentration of MgOH+ at pH values above 8, the sensor presented should be used in pH buffered solutions at pH values below about 9 (7). Because of interference by H30+the sample pH should be higher than about 8. The optimized electrode has a resistance of about 2 X 10sQ. By transfer of the cell assembly from a stirred 0.03 M to a 0.1 M MgC12 solution (pH 8.8) response times tw2 and tg5%
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Anal. Chem. 1988, 60,2016-2020
of 2.4 and 3.8 s, respectively, were found. The corresponding figures for the transfer from a 0.003 to a 0.01 M solution are 3.7 and 8.1 s. The measured emf of the ETH 2220 based macroelectrode cell assembly shows a stability of f0.06 mV (standard deviation, 8 h, IO4 M MgC12, pH 8.8).
LITERATURE CITED (1) Ion-Selective Electrodes in Analyflcal Chemistry; Freiser, H., Ed.; Plenum: New York, London, 1978, Vol. 1; 1980, Vol. 2. (2) Haras, J. Ion- and Molecule-selective Electrodes in Biologkal Systems ; Springer-Verlag: Berlin, Heidelberg, New York, Tokyo, 1985. (3) I?n-gdeCtiVe Electrodes ; Pungor. E., Buzls, I.. Eds.; Akademiai Klado: Budapest, 1985. (4) Otto, M.; Thomas, J. D. R. Anal. Chem. 1965, 5 7 , 2647. (5) Meier, P. C . ; Erne, D.; Cimerman, 2.; Ammann, D.; Simon, W. Mikroh i m . Acta 1980, I , 317 (6) Hassan, S.K. A. G.; Moody, G. J.; Thomas, J. D. R. Analyst (London) 1980, 105, 147. (7) Behm, F.; Ammann, D.; Simon, W.; Brunfeldt, K.; Halstrom, J. Helv. Chlm. Acta 1985, 6 8 , 110. (8) Erne, D.; Stojanac, N.; Ammann, D.; Pretsch, E.; Simon, W. Helv. Chlm. Acta 1980, 6 3 , 2264. (9) Erne, D.: Stojanac, N., Ammann, D.; Hofstetter, P.; Pretsch, E.; Simon, W. Helv. Chlm. Acta 1980, 63, 2271. (IO) Erne, D.; Morf, W. E.; Arvanitis, S.;Cimerman, 2.; Ammann, D.; Simon, W. Helv. Chim. Acta 1979, 62,994. (11) Maj-Zurawska. M.; Erne, D.; Ammann, D.; Simon, W. Helv. Chim. Acta 1982, 65,55. (12) Lanter, F.; Erne, D.; Ammann, D.; Simon, W. Anal. Chem. 1980, 52, 2400.
(13) Alvarez-Leefmans, F. J.; Gamitio, S. M.; Giraldez, F.; Gonzilez-Serratos, H. J. Physiol. (London) 1988, 378, 461. (14) Blatter, L. A.; McGuigan, J. A. S. J. Exp. Physiol. 1988, 77, 467. (15) Rink, T. J.; Tsien, R. Y.; Pozzan, T. J. CellBbl. 1982, 9 5 , 189. f 16) Ammann, D. Ion-Selective Mlcroelectrodes:Springer-Verlag: Berlin, Heidelberg, New Ywk, Tokyo, 1986. (17) Oesch, U.;Brzbzka, 2.; Xu, A.; Rusterholz, B.; Suter, G.; Pham, H. V.; Welti, D. H.; Ammann. D.; Pretsch. E.; Simon, W. Anal. Chem. 1986, 5 8 , 2285. (18) Guilbault, G. G.; Durst, R. A.; Frant, M. S.; Freiser, H.; Hansen, E. H.; Light, T. S.; Pungor, E.; Rechnitz, G.; Rice, N. M.; Rohm, T. J.; Simon, W.; Thomas, J. D. R. Pure Appl. Chem. 1976, 4 8 , 127. (19) Meier, P. C.; Ammann, D.; Osswald, H. F.; Simon, W. M e d . Prog. Techno/. 1977, 5, 1. (20) Meier. P. C.; Ammann, D.; Morf, W. E.; Simon, W. MedicalandBbbgical Applications of Electrochemical Devices ; Koryta, J., Ed.; Wiley: Chichester. New York, Brisbane, Toronto, 1980; p 13. (21) Oesch, U.;Simon, W. Anal. Chem. 1980, 52, 692. (22) Bodansky, M.; Bodansky, A. The Practice of Peptide Synthesis; Springer-Verlag: Berlin, Heidelberg, New York, Tokyo, 1984; Vol. 21. (23) Schmidbaur, H.; Muller, G.; Riede, J.; Manninger, G.; Helbig, J. Angew. Chem. 1986, 9 8 , 1014. (24) Pretsch, E.; Badertscher, M.; Welti. M.; Maruizumi, T.; Morf, W. E.; Simon, W. Pure Appl. Chem. 1988, 6 0 , 567. (25) Meier, P. C.; Morf, W. E.; Laubl, M.; Simon, W. Anal. Chim. Acta 1984, 158, 1.
RECEIWDfor review January 20,1988. Accepted May 6,1988. This work was partly supported by the Swiss National Science Foundation and by Eppendorf Geratebau, Hamburg.
Electron Transfer Reactions of Catechols at Ultrasmall Carbon Ring Electrodes Reginald0 A. Saracen0 and Andrew G. Ewing*
Department of Chemistry, Penn State University, University Park, Pennsylvania 16802
Ultrasmall carbon rlng electrodes exhibit charge-selective enhancement of oxidatlon rates for a serles of catechols following anodlc electrochemicaltreatment. Voltammetry Is more Nernstlan for catlonlc catechols and less Nernstlan for anlonlc catechols after Identical oxldatlve cycllng of the electrode. This behavlor Is not observed for voltammetry obtained at dlsk-shaped carbon fiber electrodes. Double layer effects do not appear to be the domtnant mechanlsm reSpOnaiMe for charge-selecthre electron transfer at carbon rlng electrodes. The unlque vonammetry for catechols at carbon rlng electrodes suggests that the carbon formed by the pyrolysls step contalns charge-selective sites that affect the electron transfer process. These dtes may Involve selective catechol adsorpHon on the electrode prior to electron transfer.
Since the idea of using carbon as an electrode material was first entertained, many advances in electrode materials and construction have been made. Throughout the development of different types of carbon electrodes, there has been a sustained interest by many investigators in determining which properties of carbon electrodes are responsible for their electron transfer characteristics. Various types of carbon have been examined with respect to the crystalline morphology of the carbon (i.e., basal versus edge plane orientations) ( 1 4 , surface smoothness/roughness ( 1 , 2, 6), analysis of surface atoms and functional groups (7-11), and voltammetric response after different polishing protocol (7,9, 12-14). The
* A u t h o r t o whom correspondence should be addressed.
nature of the solution adjacent to the electrode can also determine the characteristics of electron transfer a t carbon surfaces (15,16). Attempts have been made to correlate one or more of these characteristics of carbon electrodes with an enhanced or diminished rate of charge transfer. For example, Deakin et al. (15) have shown, in specific cases, that a change in solution pH can alter the apparent rate constant for charge transfer at glassy carbon electrodes while no change is observed at noble metal electrodes (Pt and Au). Physical and chemical treatments of the surfaces of carbon electrodes prior to studying charge transfer have also been widely examined. These treatments have taken on many forms including heating of the electrode (16-19), laser irradiation of the electrode surface (ZO),immersion in solutions of chemical oxidizing/reducing agents (21),and application of constant or cyclic potential for electrochemical treatment (2, 8-10, 18, 22-26). The end result of most of these treatments is that an acceleration of the apparent rate of charge transfer is observed following the activation step. Many explanations have been presented to rationalize the observed behavior. Engstrom ( 4 9 )and others (10,14-16) have reported on the production of surface-bound functional groups following electrochemical treatment. These moieties are often electroactive and their production is dependent on the range of potentials applied during treatment. Many attributes have been assigned to the presence of these functional groups on the surface of carbon including adsorption sites for specific molecules (8,9,27-30) and catalytic charge transfer mediators (31-34).
It is apparent that no single property of carbon surfaces can explain all of the observed voltammetric behavior. Rather,
0003-2700/88/0360-2016$01.50/00 1986 American Chemical Society
