Anal. Chem. 1989, 6 1 , 275-279
chromatography with ICP-AES allows for the separation of the sulfite from the oxidative product of sulfite, sulfate, as well as from any other sulfur-bearing species in more complex matrices. The wide linear response range as well as the low detection limit indicates that this method meets the requirements for sulfite determination in beverages and foods, as well as for other types of sulfite analysis. ACKNOWLEDGMENT The author thanks James Harley and Harry W. Wonders of Applied Research Laboratories (Dearborn, MI), James Benson of Benson Polymeric, Inc. (Reno, NV), and my colleague, Vipin Agarwal, for assistance in this investigation. Registry No. Sulfite, 14265-45-3;sulfate, 14808-79-8. LITERATURE CITED (1) Maynard, D. 0.; Kaira, Y. P.; Radford, F. G. SoilSci. SOC.Am. 1987, 57, 801-806.
275
(2) Novozamsky, I.; Van Eck, R.; Van der Lee, J. J.; Houba, V. J. G.; Temminghoff, E. Commun. Soil Sci. Plant Anal. 1986, 1 7 , 1147-1157. (3) Caroll, S.; Farina Mazzeo, A.; Laurenzi, A.; Senofonter, 0.; Violante, N. J . Anal. At. Spectrom. 1988, 3 , 245-248. (4) Irgolic, K. J.; Hobill, J. E. Spectrochim.Acta, Part 8 1987. 428, 1-2. (5) Sunga, H.; Kobayashi, E.; Shimojo, N.; Suzuki, K. T. Anal. Biochem. 1987, 160. 160-168. (6) Gardiner, P. E. J . Anal. At. Spectrom. 1988, 3 , 163-168. (7) Casetta, 6.; DiPasquale, G.; Sofflentinl, A. A t . Spectrosc. 1985, 6 , 62-64. (8) Lewin. K.; Walsh, J. N.; Miles, D. L. J. J. Anal. At. Spectrom. 1987, 2 . 249-250. (9) Russ, G. P., 111; Bazan, J. M.; Date, A. R. Anal. Chem. 1987, 59, 984-989. (10) LaFreniere, K. E.; Fassel, V. A.; Eckels, D. E. Anal. Chem. 1987, 59, 879-887.
Dennis R. Migneault The Connecticut Agricultural Experiment Station New Haven, Connecticut 06504 RECEIVED for review July 25,1988. Accepted October 24,1988.
TECHNICAL NOTES Solubilization of Cyclodextrins for Analytical Applications Daniel Y.Pharr,’ Zheng Sheng FU? T h u y K. Smith, and Willie L. Hinze* Department of Chemistry, Laboratory for Analytical Micellar Chemistry, Wake Forest University, P.O.Box 7486, Winston-Salem, North Carolina 27109 INTRODUCTION During the past decade, there have been increasing accounts of the utilization of cyclodextrins (CDs) in chemical analysis (1-26). Cyclodextrins (also referred to as cycloamyloses or Schardinger dextrins) are cyclic, homologous oligosaccharides containing rings of a-(1,4)-linked D-glucose moieties (27,28). The three most commonly employed CDs contain six, seven, and eight such glucose residues and are referred to as CY-,0-, and y-cyclodextrin, respectively. Applications include their use as mobile phase additives in liquid chromatography (1-9), fluorescence and chemiluminescence enhancement agents (1&18,26), a medium for the observation of room temperature liquid phosphorescence (19-21 ), and reagents in isotachophoresis (22,23),spectrophotometric assays (241, and NMR optical purity measurements (25) among others. In many of these applications, the CD concentration is very important and dictates the maximum analytical response obtainable. For instance, chromatographic retention (k’and R, values) of solutes as well as the magnitude of luminescence intensity enhancement9 observed for analytes can strongly depend upon the CD concentration (1-23, 26). While the aqueous solubilities of a-and y C D at 25 “C are 0.121 and 0.168 M, respectively, that of O-CD is roughly a factor of 10 less, Le. 0.0163 (29,30). Consequently, this can create problems in those applications involving 0-CD and limita the beneficial effects possible. Previously, mixed solvent systems (3) or water-soluble /3-CD derivatives (i.e. hydroxyethyl or methylated CDs (31-34) and polymeric CDs (28,35, 36)) have been employed in attempts to overcome such solubility problems. However, such soluble @-CDderivatives are Present address: Department of Chemistry, Virginia Military Institute, Lexington, VA 24450. *Present address: Chemistry Department, Northwestern Teachers’ College, Lanzhou City, Gansu Province, People’s Republic of China. 0003-2700/89/0361-0275$01.50/0
much more expensive than 0-CD itself (37)and their use (as well as use of mixed solvent systems) can alter the CD-solute binding interaction (1,31,38). In addition, we and others had previously mentioned that base and/or urea can be used to increase the water solubility of cyclodextrins, particularly p-CD, for use in some analytical procedures (13,15,39-41). Aside from two preliminary reports concerning p-CD in urea (13,41),no detailed quantitative investigation of the ability of such media to solubilize the different CDs is reported in the literature. In this note, we detail the results of our quantitative solubility study and demonstrate that either base or urea can be utilized to easily solubilize appreciable quantities of p-CD in aqueous solution. Solubility data for a-and y C D in these media are also presented. Lastly, preliminary results that illustrate the viability of using such solubilized 0-CD in selected analytical applications are summarized. EXPERIMENTAL SECTION Apparatus. A Model 7637 “Roto Torque” heavy duty rotator (Cole-Parmer Instrument Co., Chicago, IL) was used to agitate and mix the solutions in the cyclodextrin solubility studies. Thermal analysis studies on selected samples were made by using a Mettler system that consisted of a TG-50 thermogravimetric unit, M3 microbalance, and a TC-10 controller T A processor equipped with TA 3000 version 3-1 software attached to an IBM PC. The chemiluminescence measurements were made on a Turner Designs 20-000 luminometer. This latter system is described in more detail elsewhere (13). Materials. CY-, p-, and y-Cyclodextrins (CD) were obtained from Advanced Separation Technologies, Inc. (Whippany, NJ), Sigma Chemical Co. (St. Louis, MO), or Aldrich Chemical Co. (Milwaukee, WI). Certified ACS or A grade urea was obtained from Fisher Scientific Co. (Raleigh, NC) and CalBiochem (San Diego, CA). Basic solutions were prepared from certified standard 0.20, 1.00, 2.50, 5.00, or 10.0 M solutions of sodium hydroxide (Fisher). Lucigenin (lO,lO’-dimethyl-9,9’-biacridinium dinitrate) 0 1989 American Chemical Society
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(Sigma) was employed as the chemiluminescent (CL) reagent. Hydrogen peroxide solutions were prepared by dilution of a 30% hydrogen peroxide stock solution (Sigma). All aqueous solutions were prepared with HPLC grade water (Fisher). Polyamide-TLC 6 UV,, 20 X 20 cm X 0.1 mm sheets were obtained from Brinkmann Instruments, Inc. (Westbury, NY).All other chemicals and reagents employed in this study were of the best commercial grade available. Procedures. Solubility Measurements. The solubility of the different cyclodextrins in aqueous urea solutions was determined in the following manner. First, excess amounts of the appropriate solid CD were placed in screw-top vials containing aqueous urea solutions. The vials were initially agitated by manual shaking and then by rotation using the Roto Torque rotator at room temperature (25.5 f 1.5 "C) for a minimum of 60 h. The solutions were then processed in the following manner in order to determine the amount of CD solubilized at a particular urea concentration. The excess CD solid phase was initially separated from the solution phase by filtration. Next, a measured portion (typically 5.0 mL) of this solution was transferred to a preweighed 10-mL beaker and placed in a ca. 65 "C oven until apparently dry (about 50 h) after which they were subsequently dried for 4 h more in an oven at ca. 108 "C prior to being weighed. This mass corresponds to both the CD solubilized and the urea present in the original solution. A parallel series of "blank" solutions (Le. aqueous urea solutions containing specified urea concentrations to which no solid CD had been added) were treated in exactly the same manner. The mass of cyclodextrin solubilized by a particular urea-containing aqueous solution was given by the mass difference between the sample (urea + CD solubilized) and the blank (urea). Since it had been reported that aqueous urea solutions, upon heating, can decompose to liberate ammonia (42),thermal analysis was performed on some sample solutions in order to verify that urea did not undergo decomposition under the mentioned experimental conditions. The solubility of CDs in aqueous basic sodium hydroxide solutions was determined as follows. First, to a series of 10.0-mL volumetric flasks was added increasing, known amounts of the solid cyclodextrin followed by addition of an appropriate aliquot of a sodium hydroxide stock solution and dilution to final volume with HPLC grade water. These flasks were then Vigorously shaken manually and then agitated over a 24-h period by using the Roto Torque rotator. After equilibration, the series of solutions was visually inspected to determine the highest amount of CD that had been solubilized by the particular sodium hydroxide concentration level being examined. The CD solubility studies in urea were carried out in triplicate while those in sodium hydroxide were carried out in duplicate. Thin-Layer Chromatographic Experiments. Aqueous urea or sodium hydroxide solutions of varying P-CD concentration were prepared and utilized as the mobile phase in TLC experiments with polyamide stationary phase sheets. The standard aromatic test solutes were prepared by dissolving 1-10 mg of the solid in about 10 mL of spectral grade methanol, acetone, or dimethyl sulfoxide. Typically, about 0.5-1.0 pL of the standard solution was spotted on the polyamide sheet with a Hamilton microsyringe and, after drying, the sheets were placed in an airtight rectangular development chamber (Analtech), which was lined with solvent-soaked saturation pads (Analtech). After development, the components were detected as fluorescence-quenched dark spots against the brightly fluorescencing background when illuminated at either 254 or 366 nm with a Model UVSL-25 Mineralight handlamp. Viscosity Measurements. Relative viscosity measurements (not corrected for flow effects) were made with an Oswald viscometer and water as the reference liquid in the usual manner. Chemiluminescence Experiments. The lucigenin-hydrogen peroxide CL reaction was conducted in aqueous sodium hydroxide solubilized P-CD solutions by using the following procedure. First, 50 pL of hydrogen peroxide and 200 pL of the appropriate aqueous sodium hydroxide P-CD-containing solution (or just sodium hydroxide alone for the water reference system) were pipetted (using Hamilton microsyringes) into a polypropylene cuvette (8 X 50 mm, 1.6 mL volume, Evergreen Scientific), mixed for 5 s on a Vortex-Genie mixer (Fisher), and placed into the cuvette holder of the luminometer. After an incubation period of 10 s, 50 p L
8
2.0
4.0
I
I
8.0
10.0
I
6.0
I
12.0
IUREAI, M
Flgwe 1. Plots of the molar SdUbilRy of aCD (O),@CD(V), and yCD (W) as a function of the urea concentration in water at 25.5 "C.
of the lucigenin CL reagent was injected into the cuvette by using the instrument's automatic dispensing syringe. The injection process simultaneously initiated the CL reaction and activated the data acquisition system. In all of the CL experiments,a delay time of 10 s and run time of 10 s were employed. The CL signal (peak) as a function of P-CD concentration was determined in this fashion. RESULTS AND DISCUSSION Solubilization of Cyclodextrins. Urea, an achiral planar molecule which itself is soluble to the extent of 16.65 M in water (42),is well-known for its ability to improve the water solubility of a variety of nonpolar as well as polar organic solutes (43). Table I summarizes some of the solubility data obtained for the CDs in water alone as well as in 4.0and 8.0 M urea. The solubility of the CDs in aqueous solution alone agrees very well with those reported in the literature (Table I). The solubility of the three cyclodextrins in water as a function of the urea concentration is given in Figure 1. As can be seen, the solubility of @- and y C D increases steadily with increasing urea concentration up to 10 M, which was the highest concentration examined. The solubility enhancement (i.e. ratio of the molar solubility of CD in the presence of urea to that in its absence) in the presence of 7.0 M urea solutions is ca. 11 and 2.3, respectively, for 0- and y C D . In contrast, it is observed in Figure 1 that the solubility of a-CD is significantly diminished by the addition of a small amount of urea, after which further increases in urea concentration cause a linear increase in its solubility. However, even in 10 M urea, the solubility of a-CD is still somewhat less than that observed in water alone. The use of urea is thus recommended for
ANALYTICAL CHEMISTRY, VOL. 81, NO. 3, FEBRUARY 1, 1989
Table I. Solubility of Cyclodextrins in Selected Aqueous Media ~~
cyclo-
dextrin CY-CD
0-CD 7-CD
molar solubilitV of indicated cvclodextrin in water 4.0 M 8.0 M 0.20 M 0.50 M 1.0 M aloneb urea urea NaOH NaOH NaOH
277
Table 11. Relative Viscosity of Solubilized &Cyclodextrin Solutions
solution composition
relative viscosity, CP (20“C)
~
0.1197 0.056 0.086 (0.1211) 0.0168 0.089 0.226 (0.0163) (0.18)c 0.180 0.370 (0.168)
20.20
10.38
10.55
-0.15
20.29
20.40
20.20
20.37
OMolar solubility of indicated cyclodextrin at 25.5 i 1.5 O C . The values in urea or base are good to within i10% depending upon the source and lot number of the cyclodextrin examined. bLiteraturevalues for the solubility of the CDs in water alone at 25 “C are given in parentheses (taken from ref 30). cLiteraturevalue for the solubility (based on a fluorescence enhancement assay) of 0-CD in aqueous 8.0 M urea at 24.0 O C was estimated from data presented in ref 41. enhancing the aqueous solubility of 0- and yCD, but not that of a-CD. On the basis of discussions in the literature (43-45),the increased solubility of organic molecules (such as @- and yCD) in aqueous urea is thought to result primarily from a smaller free energy of cavity formation in the mixed solvent due to the replacement of water by the larger urea molecule in the solvation shell. According to the theory of Melander and Horvath, urea can also serve as a “salting-in”type of molecule (43,44). It is not clear why the presence of urea depressed the solubility of a-CD. The only reported prior study on the interaction of cyclodextrin and urea concerned a-CD (46).In that work, a calorimetric study indicated that urea does not bind with a-CD. Each glucose moiety of the cyclodextrin molecule possesses two secondary and one primary hydroxyl group. The pK, of the secondary hydroxy proton is 12.3 f 0.2 (27,47) while that of the primary hydroxyl proton is ca. 15-16 (27). Thus, it should be possible to solubilize CDs (i.e. their conjugate base form) by addition of sufficient base to deprotonate the CDs secondary hydroxyl proton. The literature also reports that CDs are “fairly stable in alkaline solutions” (27,28).As shown by the data in Table I, for each of the three CDs examined, their molar solubility in water increased as the concentration of base increased. The solubility enhancements in aqueous 0.75 M NaOH were 3.7, 21.0, and 2.6 for a-, @-, and y C D , respectively. Consequently, the use of base can enhance the solubility of all three CDs. In summary, both base and urea can solubilize substantial amounts of the CDs in aqueous solution (Table I). For comparison, hydroxyethyl-@-CD,which was synthesized specifically to help overcome the inherent limited solubility of 0-CD in water, has an aqueous solubility of ca. 0.46 M (33). By use of 1.00 M NaOH, one can solubilize roughly this same amount of the parent @-CDwhile the use of 10 M urea allows one to solubilize approximately 56% of this value. Use of more basic solutions allows one to exceed the solubility exhibited by hydroxyethyl-@-CD. For instance, use of 2.25 M NaOH solutions allows for the easy preparation of 0.57 M @-CD.Thus, use of either base or urea to solubilize parent CDs should allow their use in lieu of the specially synthesized and more expensive derivatized or polymeric CDs in many analytical applications. Lastly, some general comments on these solubilized CD solutions are in order. First, the preparation of such solubilized CD solutions is facilitated by slight heating (ca. 55 OC or so). The urea-CD solutions appear to be more stable than those solubilized in base. Such urea-CD solutions ([urea] = 8 M) gave the same analytical performance (TLC) after 1year
aqueous 4.0 M urea + 0.00 M b-CD + 0.03 M 0-CD + 0.06M 0-CD + 0.10 M 0-CD aqueous 8.0 M urea + 0.00 M 0-CD + 0.05M 0-CD + 0.10 M 0-CD + 0.15 M 0-CD + 0.20M 0-CD aqueous 1.0 M NaOH + 0.40 M B-CD
1.16 1.26 1.39 1.64 1.60 1.9 2.3 2.9 3.5 8.6
and exhibited no mold growth during this time. Even though the literature suggests that CDs are stable in the presence of basic media (27,28), we observed an initial slight yellowish coloration upon solubilization of a-and @-CD,which intensified with aging. This was not observed in the case of yCD. Preliminary data reveal only a very slow rate for the alkaline hydrolysis of the 1,4-ether linkage of the CD molecule. The urea-CD solutions are somewhat more convenient to handle compared to the base-CD counterparts. This is due mainly to their lower viscosity. Th’e relative viscosities for aqueous solutions of solubilized @-CDat 20 “C are listed in Table 11. As can be seen, the viscosity increases as the CD and/or urea (or base) concentration in the solution increases. In general, the viscosity is much greater for the base-solubilized systems. The viscosity of aqueous urea (48)and aqueous @-CD(49) solutions has been previously reported in the literature. Selected Analytical Applications. In order to demonstrate the analytical viability of such solubilized CD systems, we briefly examined their effectivenessas a TLC mobile phase and as a chemiluminescent enhancement agent. We had previously reported that aqueous solutions of a-CD could function as a TLC mobile phase for the separation of positional isomers on polyamide stationary phases (1,2,5).No analogous separations could be performed with @-CDdue to its limited solubility in water. However, as is evident from the data presented in Table 111, the urea-@-CD solution can be utilized as a mobile phase in such TLC separations. With this mobile phase, the separation of different para-substituted phenolic components as well as 1-naphthol from 2-naphthol was possible. In addition, the ortho, meta, and para isomers of nitrophenol could be separated by using 8 M urea/0.20 @-CD(Table 111). As had been previously observed with a-CD mobile phases (1,2), the retardation factors for the test solutes were found to also increase with increasing @-CDconcentration in the mobile phase (Table 111). It is also apparent that the presence of the solubilizing reagent, urea, influences these Rf values. For instance, examination of the Rf values for m- and p nitrophenol (Table 111) reveals that they somewhat increase as the urea concentration is increased from 4 to 8 M (at fixed @-CDconcentrations). This is hardly surprising since it is known that urea can form complexes with suitable solute molecules (50).Thus, for such solutes, the quantitative description of the separation process will need to account for this effect and be more complicated (5).The development times are somewhat longer when using these solubilized 0-CD mobile phase systems compared to that previously noted with the aqueous a-CD mobile phases. This arises from the greater solution viscosity of the solubilized @-CDmobile phase solutions (Table 11). Despite these complications, the solubilized @-CDmobile phase solutions can be utilized for TLC separations. A detailed comparative study of these solubilized CD
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 3, FEBRUARY 1, 1989
Table 111. Summary of Retardation Factors for Phenolic Compounds with Solubilized 6-Cyclodextrin-ContainingMobile and Polyamide Stationary Phases
test solute 1,2-benzenediol 1,3-benzenediol
1,4-benzenediol o-nitrophenol m-nitrophenol p-nitrophenol 1-naphthol 2-naphthol phenol p-phenylphenol p-iodophenol p-methoxyphenol p-tert-butylphenol phenanthrene
0.00 0.30 0.19 0.29 0.00 0.11 (0.19)O 0.10 (0.18Y 0.02 0.03 0.20 0.02 0.03 0.26 0.04 0.00 [O.OO]b
R, value using aqueous 4.0 M urea mobile phase and the indicated fl-CD concn 0.03 0.05 0.06 0.10 0.15 0.20 0.38 0.33 0.41 0.02 0.24
0.28
0.28 (0.34) 0.31 (0.40) 0.13 0.34 0.47 0.24 0.26 0.42
0.80 0.04
0.40
0.47 0.44
0.55 0.02 0.31
(0.51) 0.34
(0.05)"
0.03 0.43
(0.58)
(0.62)
(0.64)
(0.74)
0.49
(0.53) 0.19 0.43
0.59 0.30 0.37 0.59 0.89 0.06 [0.03]
[0.13]
" Values in parentheses were obtained by using an aqueous 8.0 M urea solution containing the indicated molar concentrations of fl-CD as the mobile phase. bValuesin brackets for phenanthrene were obtained by using an aqueous 1.0 M sodium hydroxide solution containing the indicated molar concentrations of fl-CD as the mobile phase. systems and that of the different water-soluble derivatives with respect to their ability to function as chromatographic mobile phases is under way in our laboratory and will be communicated in due course. It has been reported that the presence of cyclodextrins can enhance the light output from the lucigenin-hydrogen peroxide CL reaction and that the magnitude of the enhancement depends directly upon the CD concentration (11-13). With @-CD,its limited aqueous solubility restricts the maximum output obtainable. We attempted to overcome this problem by using urea-solubilized @-CDbut found no enhancement in the CL intensity relative to that observed in aqueous media alone (13). In this work, we investigated the effect of the base-solubilized @-CDupon this CL reaction system. Since many of the commonly employed analytical CL reaction systems require very basic conditions, the use of such basesolubilized CD media as described in this work (which also function as efficient buffers around pH 12) may prove to be quite useful. The preliminary data indicate that the presence of increasing amounts of solubilized @-CDin the reaction medium increases the CL peak intensity in a linear fashion. Regression analysis of the CL peak intensity observed from the lucigenin-hydrogen peroxide CL reaction vs P-CD concentration data (conditions: [Luc] = 3.2 X lo4 M, [H20,] = 1.0 X M, [NaOH] = 1.50 M, 23.5 "C) yielded the following equation: CL intensity = 7042[@-CD] + 10.8 (correlation coefficient 0.9961, n = 6). Compared to the peak intensity observed in water alone under the same experimental conditions, the presence of 0.099 and 0.378 M @-CDenhanced the CL intensity by factors of 9.5 and 32.5, respectively. More work is required, but it appears that such solubilized CD media may be very beneficial in CL assays. There is one natural potential application of the basesolubilized CD media. Namely, to function as an inclusion complexing additive in electrokinetic chromatography requires that the CD be ionized. Previously, 2-0-carboxymethyl-P-CD was synthesized to fulfill this requirement and successfully utilized in electrokinetic separations (51). However, use of the very basic media mentioned in this work for solubilizing the CDs means that the secondary hydroxyl proton is dissociated allowing for the use of such ionized parent cyclodextrins in electrokinetic capillary separations. Aside from the two examples mentioned in this work, there are two previous reports in which solubilized CDs have been
utilized in analytical applications. First, Kinoshita et al. reported that the degree of labeling of proteins using dansylation in urea-@-CD media was greater than that observed in aqueous P-CD alone (40).In addition, Armstrong and Jin utilized solubilized CD media as the selective transport vehicle in liquid membranes for the separation of optical and other isomeric species (39). Taken together, it appears that there will probably be many instances in which base or urea solubilized CDs can be utilized to overcome aqueous solubility problems encountered in the attempted use of such cyclodextrins in analytical chemistry.
ACKNOWLEDGMENT We are grateful to Dr. Fawzy S. Sadek (Department of Chemistry, Winston-Salem State University) for allowing us to use his thermal analysis system. LITERATURE CITED Hinze, W. L.; Armstrong, D. W. Anal. Lett. 1980, 13, 1093-1104. Hinze, W. L. Sep. Purlf. hfsethodp 1881, 10, 159-237. Debowski, J.; Sybilska, D. J . Chromatogr. 1888, 353, 409-416. Zukowski, J.; Sybllska, D.; Jurczak, J. Anal. Chem. 1985, 5 7 , 22 15-22 19. Armstrong. D. W.; Nome, F.; Spino, L. A.; Golden, T. D. J . Am. Chem. Soc. 1988. 108, 1418-1421. Fujima, K.; Ueda, T.; Kitagawa. M.; Takayanagl, H.; Ando, T. Anal. Chem. 1988, 56, 2660-2674. Debowski, J.; Grassini-Strazza, G.; Sybilska, D. J . Chromatogr. 1985, 349, 131-136. Uekama, K.; Hirayama, F.; Nasu, S.; Matsuo, N.; Irie, T. Chem. Pharm. Bull. 1878, 26, 3477-3484. Cline Love, L. J.; Arunyanart, M. In Chrometcgraphy and Separathm Chemlshy: Advances and Developments; Ahuja, S., Ed.; American Chemlcai Society: Washington, DC, 1986; pp 226-243. Kondo, H.;Nakatani, H.; Kiromi. K. J . Blochem. 1978, 79, 393-405. Grayeski, M. L.; Woolf, E. J. J . Lumin. 1885. 33, 115-119. Grayeski, M. L.; Woolf, E. J. J . Lumln. 1987. 3 9 , 19-27. Malehorn. C. L.; Riehi, T. E.; Hinze, W. L. Ana/yst 1888, 1 7 7 , 931-947. Kinoshita. T.: Iinuma.. F.:. Tsuii. ,. A. Blochem. Bioohvs. , . Res. Commun 1973, 5 f , 868-671. Kinoshita, T.; Ilnuma, F.; Akio, T. Chem. Pharm. Bull. 1974, 22, 2413-2420 - - -_ k k , A.; Heiiweii, E.; Hinze, W. L.: Oh, H.; Armstrong, D. W. J . Liq. Chromatoor. 1984. 7 . 1273-1280. Cline Love, L. J.; &ayeski, M. L.; Noroski, J.; Welnberger, R. Anal. Chlm. Acta 1985, 169, 355-360. Hoshino, M.; Immure, M.; Ikehara. K.; Hama, Y. J . fhys. Chem. 1881, 85, 1820-1624. Scypinski, S.; Cline Love, L. J. Anal. Chem. 1884. 56, 322-327. DeLuccia, F. J.; Cline Love, L. J. Taknta 1985, 32, 665-667. Femia, R. A.; Cline Love, L. J. Spectrmhim. Acta, Part A 1988, 42A, 1239-1246. Tazaki, M.; Hayashita, T.; Fujlno, Y.; Takagi. M. Bull. Chem. SOC. Jpn. 1888, 59, 3459-3464.
.
Anal. Chem. 1989, 6 1 , 279-282 (23) Snopek, J.; Jeiinek, 1.; Smolkova-Keuiemansova, E. J . Chromatogr. 1888, 438, 211-218. (24) Qi, W. 6.; Zhu, L. Huaxue ShJi 1887, 9 . 208-211. Chem. Abstr. 1888, 708, 1423042. (25) Greatbanks, D.; Pickford, R. M a g . Reson. Chem. 1887, 2 5 , 208-21 5. (26) Francis, 0:J.; Kirschenheuter, 0.P.; Ware, G. M.; Carman, A. S.; Juan, S. S. J . Assoc. Off. Anal. Chem. 1888, 77, 725-728. (27) Bender, M. L.; Komlyama. M. C y c M x t d n Chemlshy; Springer-Verlag: New York, 1978; pp 2-9, and references therein. (28) Szejtli, J. Cyclodextrlns and Their Inclusion Complexes ; Akademiai Klado: Budapest, 1982; Chapter 1, and references therein. (29) Wiedenhof, N.; Lammers, J. N. J. J. Carbohyd. Res. 1888, 7 , 1-6. (30) Jorqlakowski, M. J.; Connors, K. A. Carbohyd. Res. 1885, 743, 51-59. (31) Cserhatl, T.; BoJarski, J.; Fenyvesi, E.; Szejtli, J. J . Chromatogr. 1888, 357, 356-362. (32) Tanaka, M.; Miki, T.; Shono, T. J . Chromatogr. 1885. 330, 253-281. (33) Friedman. R. B. Ger. Offen DE Patent 3,712,246, Oct., 1987. Chem. Abstr. 1888, 108, 96538d. (34) Imamura, T.; Kamiya, H.; Kurosaki, T. Jpn. Kokai Tokkyo Koho JP Patent 62,220,501, Sept 1987. Chem. Abstr. 1888, 709, 110838f. (35) Szefli, J.: Hdtilts, I.; Keszler. 6.; Govacs, G.; Fenyvesi, E. Hung. Teijes HU Patent 41,824, May 1987. Chem. Absb. 1888, 708, 77531e. (36) Szepli, J. J . Incl. Phenom. 1883, 7 , 135-150. (37) Pagington, J. S. Chem. Brn. 1887, 23, 455-458. (38) Harata, K.; Uekama, K.; Otagiri, M.; Hlrayama, F. J . Inclusion Phenom. 1884, 7 , 279-293. (39) Armstrong. D. W.; Jin, H. L. Anal. Chem. 1887, 5 9 , 2237-2241. (40) Kinoshlta, T.; Iinuma, F.; Tsujl, A. Chem. h r m . Bull. 1874, 22, 242 1-2426. (41) Kinoshita, T.; Iinuma, F.; Tsuji, A. Chem. Pharm. Bull. 1874, 22(11), 2755-2758, (42) The Merck Index, 9th ed., Windhoiz, M., Ed.; Merck 8 Co., Inc.: Rahway, NJ, 1976; p 1266.
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(43) Nakai, S.; Li-Chan, E. Hycl-ophoblc Interactlms in Food Systems; CRC Press: Boca Raton, FL, 1988; Chapter 1, pp 1-21, and references therein. (44) Melander, W.; Horvath, C. Arch. Biochem. Biophys. 1877, 783, 200-213. (45) Kuharski, R. A.; Rossky, P. J. J . Am. Chem. SOC. 1884, 106, 5794-5800. (46) Barone, G.; Castronuovo, G.; Eiia, V.; Muscetta, M. J . Solution Chem. 1888, 75, 129-140. (47) Van Etten, R. L.; Ciowes, G. A.; Sebastin, J. F.; Bender, M. L. J . Am. Chem. SOC. 1887, 8 9 , 3253-3262. (48) Herskovits, T. T.; Kelly, T. M. J . Phys. Chem. 1973, 77, 381-388. (49) Flohr, K.; Paton, R. M.; Kaiser, E. T. J . Am. Chem. SOC. 1875, 97, 1209-1 2 18. (50) Takemoto, K.; Sonoda, N. In Inclusion Compounds; Atwocd, J. L., Davles, J. E. D., MacNicoi, D. D., Eds.; Academic Press: New York, 1984: VOi. 2. ChaDter 2. DD 47-67. (51) Terabe, S.; Ozakl H.; Oiiuka, K.; Ando, T. J . Chromatogr. 1885, 332, 211-217.
RECEIVED for review July 8,1988. Accepted October 31,1988. This work was supported in part by the National Science Foundation (CHE-8215508), a Susan Greenwall Foundation, Inc., Grant of the Research Corporation, and Wake Forest University. Z.S.F. acknowledges support in the form of a fellowship from the American Chinese Education Foundation of Indiana. This work was presented in part a t the 1982 Pittsburgh Conference, Atlantic City, NJ, March 1982 (Abstr. No. 265).
Chemically Modified Carbon Paste Electrode for Chronoamperometric Studies. Reduction of Oxygen by Tetrakis(p-2-anilinopyridinato)dirhodium( I I , I I I ) Chloride Chao-Liang Yao, Kwang Ha Park, and John L. Bear* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 We recently reported that dioxygen could be catalytically reduced by tetrakis(pl-2-anilinopyridinato)dirhodium(II) at an applied potential of -0.48 V in aprotic solvents (I). In order to extend these studies to aqueous solutions, a working electrode material was needed to disperse the water-insoluble dirhodium catalyst for electrochemical measurements. In this regard, carbon paste has several distinct advantages. Carbon paste electrodes containing various electroactive compounds have been extensively characterized in the literature (2,3). Their use in evaluating electrochemical processes ( 4 , 5 ) and, more specifically, the electrocatalytic mechanism (5-7) has been frequently reported. The most common preparation of paste electrodes is thoroughly mixing graphite powder and Nujol in a 5 g to 3 mL ratio followed by blending the desired weight of electroactive reactants. The paste is then packed into an electrode assembly consisting of two concentric lengths of glass tubing in a pistonlike configuration (8). All carbon paste electrodes described in the literature have been designed for voltammetric studies and therefore have a relatively small electrode surface area. To our knowledge, no studies on the electrochemical behavior of large surface area paste electrodes have been reported. In order to conveniently analyze the bulk solution components generated by the electrode reaction, and at the same time investigate the mechanism of O2reduction by the dirhodium catalyst, a paste electrode with a large surface area is desirable. This is difficult to achieve by simply enlarging the cross section diameter of the electrode due to the weak mechanical strength of the pasting material. Even though electrodes with a large surface 0003-2700/89/0361-0279$01.50/0
area may be prepared by hand-coating paste material directly on a graphite plate of the desired surface area, this type of design is not convenient for chronoamperometric studies because of the work required to prepare a fresh surface and poor reproducibility of the electrode behavior. In this study, we report the design and characterization of a chemically modified carbon paste electrode with a large geometric surface area that is easy to construct and convient to use for chronoamperometric studies. The electrode is used to investigate the catalytic reduction of O2in aqueous solution by the electroactive dirhodium complex and allows the easy analysis of the generated H20z in aqueous solution.
EXPERIMENTAL SECTION Electrode Design. The design of the carbon paste electrode for controlled potential electrolysis is shown in Figure 1. The electrode consists of a 60-mL disposable syringe cylinder (Plastipak)with the needle end cut off leaving a 3 mm wide edge. A Teflon disk of 0.5-cm thickness that contained 44 holes of 0.235-cm diameter is then sealed to the open end of the syringe with epoxy glue. The electrode has a calculated geometric area of 1.91 cm2. Close attention should be given to the size of the holes and the thickness of the Teflon disk in order to maintain the machanical strength of the electrode surface and allow easy extrusion of fresh carbon paste. The paste is then packed into the syringe cylinder and extruded through the honeycomb holes. The carbon paste was prepared by thoroughly mixing 5 g of graphite powder (UPS grade, Ultra Carbon, Inc.) and 3 mL of Nujol oil (Aldrich). A fresh surface can be obtained quickly by extruding fresh carbon past followed by polishing on wax paper. The carbon paste electrode without electroactive reactanta is used 0 1989 American Chemical Society
