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Anal. Chem. 1987, 5 9 , 1766-1770
LITERATURE CITED (1) Mlkkers. F. E. P.; Everaerts, F. M.; Verheggen, Th. P. E. M. J. Chromatogr. 1979, 769, 11-20, (2) Jorgenson, J. W.; Lukacs, K. D. J. Chromatogr. 1981, 218,209-216. (3) Jorgenson, J. W.; Lukacs, K. D. Clln. Chem. (Winston-Salem,N . C . ) 1981, 27. 1551-1553. (4) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53, 1298-1302. (5) Lauer, H. H: McManigill, D. Anal. Chem. 1986, 58, 165-170. (6) Tsuda. A.; Kazuhiro. N.; Nakagawa. G. J. Chromatogr. 1983, 264, 385-392. (7) Green, J. S.;Jorgenson, J. W. J. Chromatogr. 1986, 352, 337-343. (8) Walbroehl, Y.; Jorgenson, J. W. Anal. Chem. 1988, 5 8 , 479-481. (9) Terabe, S.;Otsuka, K.; Ichikawa, K.; Tsuchlya, A; Ando, T. Anal. Chem. 1984, 5 6 , 113-116. (IO) Otsuka, K.; Terabe, S.;Ando, T. J. Chromatogr. 1985, 348, 39-47. (11) Terabe, S.;Otsuka, K.; Ando T. Anal. Chem. 1985, 5 7 , 834-841. (12) Burton, D. E.; Sepaniak, M. J.; Maskarinec, M. P. J. Chromatogr. Sci. 1986, 24, 347-351. (13) Jorgenson, J. W.; Lukacs, K. D. Science 1983, 222. 266-272. (14) Green, J. S.;Jorgenson, J. W. HRC CC. J . High Resolut. Chromatogr. Chromatogr. Common. 1984, 7, 529-531. (15) Gassman, E.; Kuo, J. E.; Zare, R. N. Science 1985, 230. 813-814. (16) Gozel, P.; Gassmann, E.; Michelsen, H.; Zare, R. N. Anal. Chem. 1987. 59,44-49. (17) Tsuda, S.;Nakagawa, G.; Sato, M.; Yagi, K. J . Appl. Biochem. 1983, 5 , 530-336. (18) Walbroel, Y.; Jorgenson, J. W. J. Chromatogr. 1984, 375, 135-143. (19) Fujiwara, S.;Honda, S.Anal. Chem. 1986, 58, 1811-1814.
(20) Waliingford, R. A.; Ewing, A. G. Anal. Chem. 1987, 5 9 , 678-681. (21) Knecht, L. A.: Guthrie, E. J.; Jorgenson, J. W. Anal. Chern. 1964, 56, 479-482. (22) St. Claire, R. L., 111; Jorgenson, J. W. J. Chromatogr. Sci. 1985, 23, 186-191. (23) White, J. G.; st. Claire, R. L., 111; Jorgenson, J. W. Anal. Chem. 1986, 58,293-298. (24) Kaniansky, D.;Havasi, J.; Marak. J.; Sokoiik, R. J. Chromatogr. 1988, 366, 153-160. (25) Hood, H. P.; Nordberg, M. E. US. Patent 2 106744, Feb. 1, 1938. (26) Nordberg, M. E. J. Am. Ceram. SOC. 1944, 27(10), 299-305. (27) Elmer, T. H.; Nordberg, M. E.; Carrier, G. B.; Korda, E. J. J. Am. Ceram. SOC.1970, 53(4), 171-175. (28) Elmer, T. H. J. Am. Ceram. SOC. 1983, 62(4), 513-516. (29) Pretorius, V.; Hopkins, B. J.; Shieke. J. D.J. Chromatogr. 1974, 132. 23-30. (30) Lukacs. K. D.; Jorgenson, J. W. HRC CC, J . High Resolut. Chromatogr . Chromatogr Commun . 1985, 8 , 407-4 11. (31) Lauer, H. H.; McManigill, D. TrAC, Trends Anal. Chem. (Pers. Ed.) 1986, 5 , 11-15.
.
RECEIVED for review January 20,1987. Accepted April 1,1987. This material is based upon work supported by the National Science Foundation under Grant No. BNS-8504292 and the National Institutes of Health under Grant No. 1 R 0 1 GM37621-01.
Liquid Chromatography/Electrochemical Detection of Carbohydrates at a Cobalt Phthalocyanine Containing Chemically Modified Electrode Leone1 M. Santos and Richard P. Baldwin* Department of Chemistry, University of Louisville, Louisville, Kentucky 40292
Numerous carbohydrates can be oxldlzed at low positive POtentlals at chemically modified carbon paste electrodes containing added cobalt phthaiocyanlne (CoPC). Although no response is observed at plain carbon paste electrodes, a diverse group of carbohydrates including mono- and disaccharides, pyranose and furanose rings, and reduclng and nonreducing sugars are readily oxldlzed at the modlfled electrode surface. I n 0.15 M NaOH, the oxldatlons exhlblt a cyclic voltammetric peak potential of 4-0.40 V vs. Ag/AgCI, the waves decreasing In magnltude and shiftlng to more posltlve potentials at less baslc pH. The CoPC electrodes can be used for electrochemical detectlon of the carbohydrates In liquid chromatography as long as the applled potentlal Is regularly pulsed to -0.3 V or lower. Detection Umlts obtained in this manner range from 100 pmol injected for glucose and maltose to 500 pmoi Injected for fructose and sucrose.
In recent years, several electrochemical approaches have been proposed for use in the flow injection or high-performance liquid chromatographic (HPLC) analysis of carbohydrates (1-15). These approaches are of particular interest because carbohydrates do not exhibit significant absorption a t wavelengths above 210 nm and thus are not well suited for the absorption and fluorescence detection methods most commonly employed in HPLC. As a consequence, monitoring of sugars has ordinarily been performed either by refractive index detection of the intact carbohydrates or by chemical derivatization with strongly absorbing or fluorescing groups. Many carbohydrates-most notably, the reducing sugars-are known for the ease with which their chemical 0003-2700/87/0359-1766$01.50/0
oxidation can be made to take place (16). Thus, it might be expected that electrochemical detection following liquid chromatography (LCEC) should provide a relatively straightforward monitoring approach. Unfortunately, utilization of such an approach has been stymied by the fact that carbohydrates, including the reducing sugars, have a large overpotential toward electrooxidation at the glassy carbon or carbon paste electrodes most commonly used in LCEC. As a consequence, inordinately high detector potentials are required for the redox processes to occur to an appreciable extent. Thus, direct electrochemical detection is not a viable option for these compounds when carried out at conventional electrodes in the ordinary manner. Alternatively, several new electrochemical detection schemes have been developed for carbohydrates. These schemes have been of two varieties. In the first, metallic sensing electrodes such as platinum (1-3))gold (4-61,and nickel (7-9) have been used in place of the usual glassy carbon or carbon paste. Although the mechanism involved in carbohydrate oxidation appears to be somewhat different a t each of these surfaces, each permits the oxidation to occur at modest potentials (-0.2 to -0.8 V vs. Ag/AgCl for Pt, +0.15 V for Au, and +0.45 V for Ni) and thereby provides very sensitive LCEC detection of these compounds. However, with Pt and Au, where the electrocatalysis proceeds with adsorption of the starting carbohydrate (Pt) or of resulting oxidation products (Pt and Au), stable response is obtained only if appropriate cycles of oxidative cleaning of adsorbed material and reductive removal of the resulting oxide layer are applied between detection intervals. Thus, the use of dual- or triple-pulse potential waveforms is generally required for operation of these electrode materials to be practical (3, 6). With Ni, where the 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
electrocatalysis proceeds through the formation of an active nickel(II1) oxide (NiOOH) formed in situ on the electrode a t the detection potential (9),simple constant potential amperometric operation is possible. In the second approach, LCEC of carbohydrates has been accomplished by intentionally adding an oxidizing agent to the mobile phase so that the sugar oxidation occurs in solution and the products of this reaction can be monitored downstream at conventional carbon electrodes. The additives that have been employed range from the enzyme glucose oxidase (10) which selectively generates HzOzfrom glucose to general oxidants such as Fe(CN),* (12-14) and Cu(phen):+ (15) (phen = phenanthroline) whose reactions are quantitated by electrochemically monitoring the Fe(CN)64- and Cu(phen),+ generated respectively for each. In this work, we describe an electrocatalytic chemically modified electrode (CME) system that we have developed for carbohydrates and have started to apply for their quantitation via LCEC. In this approach, an otherwise ordinary electrode substrate is modified by attachment of a reversible redox molecule capable of chemically oxidizing the carbohydrates; carbohydrate detection occurs indirectly by measuring the reoxidation of the modifier following exposure of the CME to the sugar-containing sample. In principle, any of a number of general oxidants (e.g., Fe(CN)c- or Cu(phen):+) could be employed for modification purposes. However, practical requirements relating to ease of CME fabrication and subsequent CME stability and reproducibility place additional constraints on the selection of modifier and electrode modification method. The specific CME that was utilized here consisted of a conventional carbon paste mixture to which cobalt(I1) phthalocyanine (CoPC) was added as modifier. CoPC itself is well-known for its electrocatalytic capabilities in numerous organic oxidations, including those of sugars (17). As had been seen in several earlier studies (18-20), the carbon paste CME approach can provide an extremely simple means for incorporation of CoPC in a stable and highly responsive form. EXPERIMENTAL SECTION Reagents. CoPC was obtained from Eastman Kodak Co. and was used as received without further purification. Stock solutions of D-glucose, D-fmCtOSe, D-leVUlOSe, D-gdaCtoSe, D-ribose, maltose, and lactose, purchased from various vendors, were made fresh daily. Electrodes. Plain carbon paste for unmodified electrodes was made by thoroughly hand-mixing 5 g of graphite powder (Fisher Scientific Co.) with 3 mL of Nujol oil (McCarthy Scientific Co., Fullerton, CA) in a mortar and pestle. Modified carbon pastes were prepared similarly except that 0.10 g of CoPC was initially added to the graphite to give a mixture that had a CoPC loading of 2.0% by weight; the Nujol oil was then added and ground with the graphite/CoPC mixtures for at least 30 min. Apparatus. Cyclic voltammetry was performed with an IBM Model EC/225 voltammetric analyzer. A carbon paste working electrode (with or without CoPC), a saturated Ag/AgCl reference electrode, and a platinum wire auxiliary electrode were used for all experiments. Liquid chromatography experiments were performed with a Perkin-Elmer Series 10 pump, a Rheodyne (Berkeley, CA) Model 7125 injector with a 50-pL sample loop, and the same IBM instrument as above but operated in either the constant potential or the normal pulse mode. All chromatographic separations utilized a 25 cm X 4 mm (i.d.) Dionex HPIC-AS6 anion exchange column that was thermostated at 36 OC with a water bath. The mobile phase used was 0.15 M NaOH, and the flow rate was 1.0 mL/min. R E S U L T S A N D DISCUSSION Cyclic Voltammetry. CoPC-containing carbon paste electrodes have previously been shown to permit the electrooxidation of hydrazine (I@, sulfhydryl compounds (19), and a-keto acids (20) at potentials substantially lower than
I
+0.6
1
I
I
+0.3
1
I
1
I
0
1
1767
1
-0.3
POTENTIAL in v o l t s vs. AglAgCl
Flgure 1. Cyclic voltammograms of 1.0 X M glucose in 0.15 M NaOH at (A) 2 % CoPC carbon paste electrode and (B) unmodified carbon paste electrode. Scan rate was 64 mV/s.
what is required at conventional carbon electrodes. At the CMEs, each of these oxidations occurred a t an applied potential that closely matched that of the Co(II)/Co(III) oxidation of the phthalocyanine and thus has been explained by a mechanism involving mediation or catalysis by the Co(II1) state of the modifier. In this work, a similar electrocatalysis was also seen for carbohydrate oxidation. The observed behavior is illustrated in Figure l which shows cyclic voltamM glucose solution mograms (CVs) obtained for a 1.0 x in 0.15 M NaOH a t both plain and CoPC-containing carbon paste electrodes. The sugar oxidation at the CME consisted of an irreversible anodic wave with a peak potential E , of +0.42 V vs. Ag/AgCl. As would be expected for an electrode process limited by solute diffusion, the height of this wave was directly proportional to the glucose concentration employed and to the square root of the potential scan rate. No activity was observed for glucose at the unmodified carbon paste surface under these conditions or under any others employed during this work. Continued cycling of the CoPC CME in glucose solution in the positive potential range resulted in such a rapid decrease in electrocatalytic response that, after two or three cycles, current attributable to the sugar oxidation could no longer be seen. This phenomenon is shown by the dashed traces in Figure 1 which represent the eventual voltammograms obtained after repetitive scanning between 0.0 and +0.6 V vs. Ag/AgCl. Only two measures were found to be effective in restoring CME activity. Either the CME surface could be removed and replaced by a fresh layer of modified carbon paste, or a potential of -0.3 V or lower could be momentarily applied to the passivated surface. In practice, the latter approach was much more convenient and could be applied to CV studies of the CME by simply adjusting the cathodic switching potential to -0.3 V or lower. This procedure, which succeeded in producing indefinitely stable CVs after the first few scans, was employed for all subsequent CV investigations. The mechanism responsible for the initial passivation of the CME by the glucose oxidation process and its subsequent cathodic activation still remains to be determined. However, in view of the fact that Co(II)/Co(I) reduction of the phthalocyanine modifier occurs in the -0.3 to -0.5 V region (21), it appears likely that this reduction process may be involved in the restoration of the CME response. The CME activity for glucose oxidation was highly dependent on the pH at which the electrode process was carried out, with optimum response obtained above pH 13. At lower
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
pH values, two separate effects were observed. First, the glucose wave shifted to more positive potentials, the extent of the shift being nearly 60 mV per pH unit. Second, the wave decreased considerably in amplitude: 31.3 pA at pH 12,13.1 pA at pH 11,7.5 pA at p H 10, and 5.0 pA a t pH 9. No wave at all was observed for glucose a t lower pH values. Both of these effects were easily understood on the basis of previously reported information concerning (1)the electrochemical behavior of CoPC and (2) the oxidation behavior of glucose. As far as the former is concerned, an identical pH/potential shift has been reported for the Co(II)/Co(III) wave of both CoPC and its tetrasulfonate derivative a t unmodified graphite electrodes (21). Thus, any electrocatalytic process associated with formation of the Co(II1)species would also be expected to exhibit the same shift. As far as the latter is concerned, it is well-known that the chemical oxidation of carbohydrates by standard oxidants such as Fehling’s and Tollens’ reagents is most facile under very basic conditions (22). In addition, a similar pH requirement has been observed for the electrocatalytic oxidation of sugars by Cu(phen)?+ (14). Thus, it is not surprising that the extent-Le., peak amplitude-of glucose oxidation by CoPC should display an analogous pH dependence. One explanation is that the equilibrium between the usual cyclic structure of glucose and its more easily oxidized acyclic aldehyde form is more rapid in basic solution (23). Other as yet undetermined considerations may also be involved in the p H effect on the electrocatalysis, and further investigations into these effects are in progress. However, for the potential analytical applications of the electrocatalysis that form the focus of this work, it is sufficient to note that the largest oxidation currents for glucose were seen near pH 13. Therefore, this value was employed for the CME electrocatalysis in all our subsequent CV and LCEC work. Numerous other carbohydrates examined in the course of this work showed cyclic voltammetric behavior nearly identical with that described above for glucose. The compounds studied included fructose, galactose, ribose, sucrose, maltose, and lactose. None exhibited significant redox activity a t unmodified carbon paste electrodes, but large irreversible anodic waves with E,’s of +0.42 V (&lo mV) were observed at the CoPC CME. Thus, the electrode showed more or less equivalently enhanced response toward mono- and disaccharides, pyranose and furanose rings, and reducing and nonreducing sugars. In all cases, stable CME response required that the electrode was cycled to potentials of -0.3 V vs. Ag/AgCl before each positive-going scan. Interestingly, 2-deoxyribose exhibited no response a t the CoPC CME. LCEC. On the basis of the CV results described above, it appeared likely that amperometric detection of carbohydrates via LCEC might be carried out at the CoPC CME at potentials on the order of +0.4 V vs. Ag/AgCl. This potential is easily low enough to provide the high sensitivity and selectivity for which LCEC is noted. Furthermore, the requirement of the electrocatalysis for very basic solution conditions presented no chromatographic difficulty as precisely these mobile phase conditions have recently been shown to be effective for the ion exchange separation of carbohydrates (4, 5 , 9). The CoPC CME response for simple amperometric monitoring of glucose a t +0.39 V vs. Ag/AgCl is illustrated in Figure 2, curve A. Qualitatively similar results were obtained for all the other sugars examined except once again for 2deoxyribose. The results shown represent the currents obtained under flow injection conditions (i.e., no chromatographic column used) for repeated injections of a 5 X M glucose solution. This corresponded to the injection of 2.5 nmol of analyte. As anticipated, large anodic signals were obtained. Unfortunately, the signals were not stable but
1st
B
2nd
3rd
4th
I
Figure 2. Chromatograms obtained for four injections of 2.5 nmol of glucose using constant potential detection at (A) 2% CoPC CME and (B) unmodified carbon paste electrode: E = 4-0.39 V vs. Ag/AgCI; mobile phase, 0.15 M NaOH; flow rate, 1.0 mL/min.
I 1st
2nd
A
3rd
4th
TIME in ms
B
Figure 3. (A) Chromatograms obtained for four injections of 2.5 nmol of glucose using pulsed detection. Mobile phase and flow rate are as in Figure 2. (B) Potential-time sequence used in dual-pulse detection. decreased so rapidly that, after four injections, the peak height was only 30% of its initial value. No signal a t all, however, was observed when plain carbon paste electrodes were employed under identical detection conditions. The most obvious reason for the loss of CME response under the constant potential amperometric detection scheme was that, as shown in the earlier CV experiments, maintenance of CME activity required periodic application of negative potentials on the order of -0.3 V. This hypothesis was confirmed by switching from constant potential LCEC detection to a pulsed approach. The specific potential sequence decided on consisted of holding the electrode at +0.39 V for 50 ms for detection and then at -0.30 V for an equal length of time for electrode regeneration; the actual detection was performed during the last 16.7 ms of the +0.39 V step. (Note that the potential sequence described, a symmetrical square wave, is exactly what is used in normal pulse voltammetry; the waveform can be generated and the resulting current sampled, with many commercially available general purpose electrochemical analyzers. Also, the pulse rate employed enabled current sampling at a 10-Hz rate that would be rapid enough for nearly all HPLC applications.) The results so obtained are shown in Figure 3, curve A. As can be readily seen, the pulsed approach succeeded extremely well in stabilizing the activity of the CME for repeated glucose exposures. The results shown are for only four successive injections, but subsequent stability tests over longer periods of heavy electrode usage showed that indistinguishable response could be
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
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100 nA
/
0.25
0.30 0.35 POTENTIAL in volts vs. AglAgCl
0
0.40
Figure 4. Hydrodynamic voltammograms of 5.0 X M (A) glucose and (B) fructose at 2% CoPC carbon paste electrode. Mobile phase and flow rate are as in Figure 2.
maintained for at least 40 injections taking place over more than 5 h. Peak heights obtained in this manner yielded a relative standard deviation of 2%, with the 30th glucose injection producing more than 98% of the current exhibited for the first injection. Hydrodynamic voltammograms (HDVs) obtained under HPLC conditions for glucose and the other carbohydrates were identical in shape, each exhibiting maximum electrode response in the vicinity of +0.4 v. For the pulsed potential waveform utilized here, this refers to the more positive "detection potential" as opposed to the -0.3 V "reactivation potential". (See Figure 4 for the HDVs obtained for glucose and fructose.) Thus, the potential dependences of the electrocatalysis for flow injection and LCEC were virtually the same as for cyclic voltammetry. Unlike HDVs observed for uncomplicated diffusion-limited electrode processes, those obtained here for the carbohydrates did not have a simple plateau shape at more positive potentials. Rather, decreases in chromatographic peak currents occurred at higher potentials and resulted in distinctly peak-shaped HDVs. In fact, operation of the CME at potentials greater than +0.45 V even for just a few minutes produced a permanent loss of response toward the carbohydrates that could be restored only by installation of a new carbon paste surface. Behavior similar to this was also seen for CoPC electrocatalyses involving the oxidation of both sulfhydryls and a-keto acids (19,20). Several different redox processes are possible for CoPC, some occurring a t the metal center and some on the phthalocyanine ring (21). The former are usually reversible and account for CoPC's catalytic activity while the latter often lead to irreversible degradation of the compound and loss of activity. Most likely, potentials more positive than +0.4 V are sufficient to cause such a deactivation process to take place under the conditions employed here. However, as long as the applied potential was never permitted to exceed +0.45 V (and, of course, was also returned to -0.3 V periodically), no appreciable loss of CME response was noted. Maximum sensitivity was therefore obtained by setting the positive limit of the pulsed potential waveform to +0.39 V vs. Ag/AgCl. The negative half of the pulse was always -0.30 V. Under these conditions, the chromatogram shown in Figure 5 and illustrating the separation and CME detection of glucose, fructose, sucrose, and maltose was obtained. Note that the column employed was an anion exchange column and that the rising base line during the chromatogram was due to a slow
I
,
2
/
I
4
I
,
,
,
6
s
RETENTION TIME in min.
Flgure 5. Chromatogram of (1) 5.0 X M glucose, (2) 1.0 X M fructose, (3) 5.0 X lo-' M sucrose, and (4) 5.0 X M maltose at 2 % CoPC CME. Mobile phase and flow rate are as in Figure 2; E = +0.39 V vs. Ag/AgCI. Table I. LCEC of Carbohydrates at CoPC Chemically Modified Electrode" compound
glucose galactose maltose lactose ribose
fructose sucrose
detection limit, ng 18 18 34
34 75 90 171
linear range (M, for 50-1L injection)*
2x 2x 2x 2x 1x 1x 1x
10-6 to 1 x 10-6to 1 x 10-6 to 1 x 10-6to 1 x 10-5to 1 x 10-5 to 1 x 10-5 to I x
10-4 10-4 lo4 10-4
10-3 10-3 10-3
"Using dual-phase sequence with detection at +0.39 V vs. Ag/ AgCl. *From least-squares analysis of at least four concentrations; correlation coefficients were always greater than 0.99. drift in base potential that could be avoided by use of a more stable potentiostat designed with the particular requirements of the pulsed LCEC experiment in mind. The detection of glucose and maltose was somewhat more sensitive than was that of the others. Detection limits (signal/noise = 3) were in the 100-pmol range in the more favorable cases and 500 pmol in the others. Linear response was generally obtained over a range approximately 2 orders of magnitude above the detection limits. Results obtained for the individual carbohydrates are summarized in Table I.
CONCLUSIONS Numerous aspects of carbohydrate oxidation at CoPC CMEs still remain to be fully characterized. However, even a t this stage, the analytical performance of these electrodes in LCEC determination of carbohydrates appears comparable to that reported for metallic electrodes. Glucose detection limits in the 100 pmol (or 18 ng) range with the CME compares favorably with reported limits of 460 ng for Pt (2),200 ng for Au ( 6 ) ,and 1 ng for Ni (9). We anticipate that appreciable improvements in detection could be achieved for the CMEs simply by use of a pulse instrument designed or modified with LCEC usage in mind; in particular, larger background current offset and more flexible pulse width capabilities than those available with the general-purpose instrument employed in this study would be of direct benefit. The 2 order of magnitude range of linear response for direct calibration plots obtained with the CoPC electrode is somewhat larger than that seen with Pt and Au electrodes where
Anal. Chem. 1907, 59, 1770-1774
1770
adsorption effects serve to alter the peak current-concentration at higher concentrations. Last?but perhaps most significantly, the operating principles and therefore the selectivities of the various approaches are different. SpecifiCallY, the metallic electrdes Provide considerably more general detection responding not only to carbohydrates but to and as well. Interestingly, the CoPC CMEs, operating by an entirely have shown response Only a-keto acids different and mercaptans in addition to carbohydrates.
LITERATURE CITED Hughes, S.; Johnson, D. C. Anal. Chim. Acta 1981, 132, 11-22. Hughes, S.; Johnson, D. C. J . Agric. Food Chem. 1982, 3 0 , 7 12-714. Hughes, S.; Johnson, D. C. Anal. Chim. Acta 1983, 149, 1-10, Rocklin, R. D.; Pohl, C. A. J . Liq. Chromatogr. 1983, 6 , 1577-1590. Edwards, P.; Haak, K. K. Am. Lab. (Fairfield, Conn.) 1983 (April), 78-87, Neuberger, G. G.; Johnson, D. C. Anal. Chern. 1987, 5 9 , 150-154. Schick, K. G.; Magearu, V. G.; Huber, C. 0. Ciin. Chem. ( Winston-Sa/em, N C ) 1978, 2 4 , 448-450. Buchberger, W.; Winsauer, K.; Breitwieser. Ch. Fresenius' 2 . Anal. Chem. 1983, 315, 518-520. Reim, R. E.; Van Effen, R. M. Anal. Chern. 1986, 5 8 , 3203-3207.
(10) Wieck, H. J.; Heider, G. H., Jr.; Yacynych, A. M. Anal. Chim. Acta 1984, 158, 137-141. "keds, H,; Osajima, y, Ana/, Chem, (11) Matsumoto, K,; Hamada, 0,; 1986, 5 8 , 2732-2734. (13) (12) Takata, Brunt, K.y,; Analyst Muto, (London) G, Anal, Chem, 1982, 107, 1973,1261-1271. 4 5 , 1864-1868, ( I 4) Hangos-Mahr, M.; Pungor, E. Modern Trends in Analytical Chemistry;
Analytical Chemistry Symposia Series; Elsevier: Amsterdam, 1982; Vol. 18, pt. A, pp 307-315.
(15) Watanabe, N,;Inoue, M, Ana/, Chem, 1983, 5 5 , 1016-1019. (16) Green, J. W. The Carbohydrates, 2nd ed.; Pigman, W., Horton, D., Ed.; Academic: New York, 1980; Vol. IB, Chapter 24. (17) Moser, F. H.; Thomas, A. L. The Phthalocyanines; CRC Press: Boca Raton, FL, 1983; Voi. 1. (18) Korfhage. K. M.; Ravichandran, K.; Baldwin, R. P. Anal. Chern. 1984, 56, 1514-1517. (19) Halbert, M. K.; Baidwin, R. P. Anal. Chem. 1985, 5 7 , 591-595. (20) Santos, L. M.; Baldwin, R. P. Anal. Chem. 1986, 58, 848-852. (21) Zecevic, S.; Simic-Glavaski, B.; Yeager, E.; Lever, A. 5.P.; Minor, P. C. J . Nectroanal. Chem. 1985, 196, 339-358. (22) Binkley, W. W. The Carbohydrates, 2nd ed.; Pigman, W., Horton, D., Eds.; Academic: New York, 1970; Vol. IIB, pp 760-764. (23) Pigman, W.; Anet, E. F. L. J. The Carbohydrates, 2nd ed.; Pigman, W., Horton, D., Eds.; Academic: New York, 1972; Vol. IA. Chapter 4.
RECERXDfor review January 28,1987. Accepted April 9,1987. This work was supported by the National Science Foundation through EPSCoR Grant 86-10671-01.
Interaction between Halide Anions and Lithium and Substituted-Ammonium Cations or Acids in Acetonitrile by Direct Current Polarography Masashi Hojo,* Hironori Nagai, Minoru Hagiwara, and Yoshihiko Imai Department of Chemistry, Faculty of Science, Kochi University, Kochi 780, Japan
A polarographic method to study complex formation has been applied to the anodic (mercury dissolution) waves of halide ions In acetonitrile. The formation constants of complexes, such as (Li+),CI-, (HR),Ci- (HR = p-bromophenol), and (HA),Ci- (HA = acetic or benrok acid) were obtained by the positive shift in the half-wave potential (€,,*) of one of the anodlc waves of Ci- upon the addition of a large excess of (Lewis) acid. Cation acids (alkyl-substituted ammonium, pyridlnlum, and anillnium) gave similar species, (R, NH,-,+),Ci- ( n = 1-3), etc. The coexistence of Li' and HR in a Ci- solution produced the (Li+)(HR)Ci- species. On the other hand, the coexistence of Li+ and HA caused the formation of (LI+),( HA)CI-, in which, one iithlum ion seemed to be bound to Ci- and the other lithium ion to the oxygen atom of the carboxylic acid. As for the bromide ion, the (overall) formation constants of (HR)Br- and (HR),Br- and (R,NH,+)Brand (R,NH,,+),Brwere obtained. However, apparently only a strong cation acid (N,N-dhnethylanilinium) could make the (R,NH+),I--type species for the iodide ion.
Many inconsistent phenomena have been reported for alkali halides, especially for LiCl, in nonaqueous solvents ( I d ) . For instance, Brookes et ai. ( 1 ) reported that the value of the ion-pair contact distance (a = 2.15 A) for LiCl in acetone deduced from conductivity measurements was practically impossible because the sum of the crystallographic radii is 2.41 A. These problems have not been solved yet, although some
suggestions have been made on the possibility of triple-ion formation or of strong solvation. In previous studies (&IO), we have developed a polarographic method to obtain complex formation constants. The method makes use of the positive potential shift in the anodic (mercury dissolution) wave of a base in the presence of a large excess of (Lewis) acid. A number of new species in nonaqueous solvents have been discovered (6-10) by this method. The identification of the [Li,A]+ and [LiAJ (A = the acetate or benzoate ion) complexes (6) made it possible to understand the inconsistency between experimental and calculated values of conductivity of lithium trifluoroacetate in propylene carbonate by Jansen and Yeager (11). Jansen has discussed the formation of the triple ions (12).
The formation of the [CH3COOLi2]+species in acetonitrile was first proposed by Itabashi (13). We have indicated (6) some limitation of the solubility method of measuring the formation constants of the homoor heteroconjugation reaction (formation of A-(HA), or A(HX),) for simple carboxylates, such as the acetate or benzoate because such a simple carboxylate readily forms the [Li,A]+ type species in acetonitrile. Incidentally, the mechanism of the "leveling effect" in the polarographic reduction of some acids in pyridine containing Li+ or Na+ as the supporting electrolyte was also confirmed by the formation of [Li,A]+ or [Na,A]+ ( 1 4 ) . Tsuji and Elving
0003-2700/87/0359-1770$01.50/0 D 1987 American Chemical Society
