1632
Anal. Chem. 1963, 5 5 , 1632-1634
Butane7
Acetone
7
11 ,t-Amyl
Alcohol
-
-
2 3 4 5 6 7 8 910 fvlinuies
Figure 1. Gas chromatogram of methanol (0.5 ppm) and other oxygenates isolated from llquld butane.
be determined in the range 0.5-250 ppm. Lower concentrations may be determined by employing larger sample sizes. Recovery of trace methanol from synthetic liquid butane standards which were analyzed by the proposed method is given in Table I. The synthetic liquid butane standards were prepared by injecting a weighed amount of methanol into
unsealed Hoke vessels which contained a weighed amount of chilled liquid butane. The Hoke vessels were sealed and allowed to equilibrate at room temperature prior to sampling. The accuracy and precision obtained are well within those required for the above proposed application. Generally, greater than 98% of the methanol is desorbed from the silica gel cartridge by the aqueous eluent as verified by injecting a known amount of methanol into the cartridge. During sampling, no methanol breakthrough was observed by placing two cartridges in series and analyzing each cartridge separately. To ensure that the silica gel is of the proper activity and to verify that complete trapping of the methanol has occurred, it is recommended that a synthetic blend of methanol in liquid butane be analyzed by placing two silica gel cartridges in series and analyzing each separately as described above. 2-Propanol and butyl mercaptan, if present, will interfere with the methanol analysis but other GC columns may be substituted, e.g., 10 ft X 1/8 in. stainless steel 3% SP1500 on 8O/lZO Carbopack B (Supelco, Inc., Bellefonte, PA) programmed at 4 OC/min from 70 to 190 OC. Other water-soluble oxygenates present in the liquid butane, e.g., acetone and ethanol, may also be determined. Water-insoluble polar solutes in LPG's may also be sampled and quantitated by this technique by employing an appropriate organic solvent for desorption. After the sample is taken, the silica gel cartridges may be capped with Swagelok fittings and stored or shipped to any location for analysis. Registry No. Methanol, 67-56-1; butane, 106-97-8.
LITERATURE CITED (1) "Standard Method of Sampllng Llquified Petroleum (LP) Gases"; American Soclety for Testlng and Materials: Philadelphia, PA, 1978; Part 23, pp 641-644.
RECEIVED for review January 3,1983.
Accepted April 15,1983.
Enhanced Electrochemical Reversibility at Heat-Treated Glassy Carbon Electrodes Kenneth J. Stutts, Paul M. Kovach, Werner G. Kuhr, and R. Mark Wightmen" Department of Chemistry, Indiana Unlversity, Bloomlngton, Indiana 47405
Carbon electrodes are frequently used for voltammetry and amperometry. Although these electrodes are useful over a wide potential range, it is generally true that the apparent rates of electron transfer are slower at carbon electrodes than at metal electrodes. Recently it has been shown that the electrochemical rate can be increased for the oxidation of ascorbate a t carbon fiber or graphite-epoxy composite electrodes by pretreatment with high current density ( ~ A 2cm-*) (1,2). Polishing the surface of glassy carbon with a-alumina has also been shown to accelerate the oxidation of ascorbate (3). The rate of the reduction of ferricyanide at carbon can be increased by the judicious use of polishing procedures (4). During an investigation of surface-attached redox mediators for ascorbate oxidation (5),we have found that the conditions employed prior to surface attachment reactions render a surface that greatly accelerates the apparent electrolysis rate for ascorbate and a number of other compounds. Specifically, it will be shown that heating glassy carbon to 500 OC at reduced pressure provides a surface which has greatly improved electrochemical properties.
EXPERIMENTAL SECTION Reagents. K,Fe(CN),, Ru(",),cl,, 3,4-dihydroxybenzylamine hydrobromide (DHBA), 3,4-dihydroxyphenylaceticacid
(DOPAC),ascorbic acid (AA), and 4-methylcatechol(4-MeCat) were reagent grade and used as received. McIlvaine buffers (0.1 M in buffer capacity and adjusted to 1.0 M ionic strength with KCl) were used in the electrochemical studies of all compounds except ferricyanide, which was in 0.5 M K2S04adjusted to the proper pH with concentrated H2S04or NaOH. All solutions were prepared in water distilled from alkaline permanganate and purged with nitrogen prior to use. Apparatus. Cyclic voltammetry was performed at glassy carbon rectangles of two different grades (0.8 X 2.5 cm, Tokai GC-10 or Tokai GC-20), which had been hand polished to a mirror finish with 5.0,0.3, and 0.05 pm alumina (Beuhler, Lake Forest, IL) successively. Identical results were obtained at both grades of carbon. Before use, the glassy carbon was freshly polished with 0.05 pm alumina and Soxhlet extracted in toluene for approximately 4 h. The extracted carbon served as the control electrode. For the reduction of Fe(cN)63-,but not AA oxidation, an increase in reversibility was seen for the refluxed carbon relative to carbon that was freshly polished with 0.05 pm alumina. Some of the refluxed electrodes were transferred t o a Pyrex glass tube that was heated (520-540 "C) under reduced pressure (-1 torr) for 2-24 h. These electrodes were returned to atmospheric pressure, removed from heat, and allowed to cool at the end of the heating period. The glassy carbon formed the floor of a cell fabricated from Plexiglas and a polyethylene gasket defined the electrode area ( A = 0.061 cm2). The cell was designed to allow semiinfinite
0003-2700/83/0355-1632$01.50/00 1983 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
1633
pH
= 3.0 Fe (CN )-:
-
RU(NH&* 4-MeCat DOPAC DHBA AA
C
n
I
4 - MeCat DOPAC DHBA AA
02
0.0 E
t0.4
0.2
0.0
-0.2
IVvsAg/AgCl)
Flgure 1. Cyclic voltammograms of Fe(CN),3’-(left) and AA (right) at glassy carbon: (a, a’) polished and refluxed surface; (b, b‘) freshly heat-treated glassy carbon; (c’) b’ after 240 mini in solution. Conditions: M, pIH 7.4. scan rate 0.100 V s-I, C = 2.0 X
linear diffusion to the electrode surface. Input and exit ports for the admittance of fresh solution were incorporated as were reference and auxiliary electrodes. Cyclic voltammetry was performed with a PAR 174A polarographic analyzer (Princeton Applied Research Corp., Princeton, NJ) in conjunction with an Omnigraphic ZOOOX-Y recorder (Houston Instruments, Austin, TX).Potentials are reported vs. a Ag/AgC1(4 M KC1) refierence electrode. ?‘he auxiliary electrode was a platinum wire. Surface analysis using ESCA was performed by Surface Science Laboratories, Inc., Mountain View, CA.
RESULTS AND DISCUSSION Voltammograms for the oxidation of ascorbate (pH 7.4) at freshly polished glassy carbon electrodes exhibit features of a system which is both chemically and electrochemically irreversible (Figure 1). The chemical irreversibility arises from a hydration reaction of‘the oxidized form (6). However, the degree of electrochemical irreversibility is unique to carbon electrodes-faster electrochemical rates are obtained at gold (7), mercury (B), or iodide-coated platinum electrodes (9). The voltammetry of Fe(CN)6” (pH 7.4) at freshly polished glassy carbon electrodes also exhibits considerably slower electrochemical rates than is observed at noble metals; however, the apparent rate increases with a decrease in pH (IO). For both ascorbate and Fe(CN)63-the degree of reversibility is enhanced by heat treatment of the electrode as evidenced by sharper voltammetric peaks and a shift in the peak potential (Figure 1). A decrease in the separation of the cathodic and anodic peak potentials from 121 to 64 mV is observed for Fe(CN)63-. We have examined the peak current from these voltammograms as a function of sweep rate, and diffusion control is obtained for these compounds a t both polished and heattreated electrodes. The increased electrochemical reversiblity observed with heat treatment is stable for a significant period in solution. The anodic peak current and peak potential for AA were examined over a span of 4 h a t a heat-treated electrode. The current resiponse was found to decrease slightly (6.7%/h) with a concomitant positive shift in anodic peak potential (f13.6 mV/h, Figure 1). The residual current was also found to decrease >withtime. The degree of reversibility is independent of the dluration of heat tireatment. Although surface waves are not apparent at heat-treated electrodes, ,an increase in residual current (4X for a 2 h heat treatment) is observed. If the heat treatment is performed at atmospheric pressure, very large residual current is obtained (30X), resulting in an unusable electrode. Electrodes that have been
l
+05
I
as I
!
t07
0.4
c
-C
i
c0.6
H
L
l
l
S
0 - i
l
l
*03 +01 E ( V vs AgIAgCI)
l
l
,
0 -01
Flgure 2. Voltammetrlc propertles at polished and refluxed glassy carbon (C), and heat-treated glassy carbon (H). The conventions used in expressing the data are according to ref 17. One end of the rectangle is at E,, the other at E,,,. An open rectangle indicates chemical M except Fe(CN), (2.0 irreversibility. All concentrations are 5.0 X X M) and Ru(NH,), (1.0 X M).
4.0
t L-I -A-A-A-A-
2 .o
1.5
1.o
0.5
-log v
Figure 3. Dependence of normalized peak current at heat-treated glassy carbon electrodes. I = i,/ACv”2: the dashed line represents the theoretical two-electron value; AA (A),4-MeCat (0),DHBA (El), DOPAC (V).Condltlons: 5.0 X M, pH 7.4.
treated retain their favorable electrochemical properties even after a week’s exposure to laboratory environment. Polishing of the carbon surface with 0.05-ym alumina after heat treatment eliminates the enhanced reversibility. The voltammetry of several other chemical systems has been examined at polished and heat-treated electrodes (Figure 2). The apparent electrochemical rate of reduction of Ru(NH3)B3+is identical a t both pHs and appears unchanged by the heat treatment. At p H 3.0, the degree of separation of the cyclic voltammetric peaks for Fe(CN)63-i s considerably less than a t pH 7.4, and a slight decrease in this parameter is observed with the heat treatment. For the catechols examined, the largest increase in reversibility is observed with heat treatment when the side chain is charged. The amount of potential shift for all of the compounds tested is reproducible to *5% at different heat-treated surfaces. The voltammetry of several catechols at heat-treated glassy carbon has been investigated as a function of scan rate (v). Several investigators have noted that other treatments which result in an acceleration of the rate of ascorbate oxidation also result in adsorption-dominated voltammograms for catechols (2,3).Adsorption is evidenced by the slope of the normalized peak current (i,/ACu112) vs. scan rate for DHBA and DOPAC a t pH 7.4 (Figure 3), and for DHBA a t pH 3.0; however, the degree of adsorption is sufficiently small that the adsorption is not evident a t slow scan rates. Heating of glassy carbon has long been used to increase the number of functional groups on the surface prior to surface
1634
Anal. Chem. 1983, 55, 1634-1637
derivatization (11). It has been shown that heating graphite and carbon to 500 “C results in the evolution of CO and C 0 2 (11-13). Subsequent exposure to air at this temperature results in an “acidic” carbon (14). Although the chemical nature of the heat-treated carbon surface is unclear, it is likely to be a more oxidized form than the polished and refluxed surface. This hypothesis is confirmed by ESCA analysis which indicates an increase in oxygen from 8.0 to 9.9%. However, the CIS spectrum of heat-treated carbon does not exhibit the weak shoulder seen at higher energies on the polished and refluxed sample indicating a decrease in concentration of C-O functionalities. Aluminum, presumably from the polishing procedure, was found on both samples (1.2% and 0.38%, respectively). Electron micrographs of both grades of glassy carbon at 300X are identical with those published by other investigators (15), and no change is observed with the heat treatment. These facta do not permit an interpretation of the physical or chemical parameters responsible for the increased electrolysis rates observed at heat-treated electrodes. However, it is interesting to note that this method, the use of large anodic currents, and polishing procedures result in cleavage and re-formation of carbon-oxygen bonds on the surface. It has been conjectured that these functional groups are important for the facilitation of electrode reactions (16).
ACKNOWLEDGMENT The gift of glassy carbon from T. Osa is gratefully acknowledged. Registry No. K$F~(CN)~, 13746-66-2;Ru(NH~)~CI~, 14282-91-8;
C, 7440-44-0; DHBA, 37491-68-2;DOPAC, 102-32-9;AA, 50-81-7; 4-MeCat, 452-86-8.
LITERATURE CITED Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386-1389. Falat, L.; Cheng, H.-Y. Anal. Chem. 1982, 5 4 , 2111-2113. Zak, J.; Kuwana, T. J . Am. Chem. SOC. 1982, 104, 5514-5515. Jordan, J. Plttsburgh Conference Abstracts, 1982,52. Stutts, K. J.; Wlghtman, R. M. Anal. Chem. 1983, 55, 1576-1579. Perone, S. P.; Kretlow, W. J. Anal. Chem. 1988, 38, 1760-1763. Rueda, M.; Aldaz, A.; Sanchez-Burgos, F. Nectrochim. Acta 1978,
23,419-424. Brdicka, R.; Zuman, P. Collect. Czech. Chem. Commun. 1950, 15,
766-799.
Lane, R. F.; Hubbard, A. T.; Fukunaga, K.; Blanchard, R. J . Brain Res. 1976, 114, 346-352. Stutts, K. J.; Dayton, M. A.; Wightman, R. M. Anal. Chem. 1982, 5 4 ,
995-998. Watkins, B. F.; Behling, J. R.; Kariv, E.; Miller, L. L. J . Am. Chem. Soc. 1975, 9 7 , 3549-3550. Barton, S. S.;Boulton, G. C . ; Harrison, B. H., Carbon 1972, 10,
395-400. Barton, S. S.; Harrlson, B. H. Carbon 1975, 13, 283-288. Donnet, J. B. Carbon 1988, 6 , 161-176. Miller, C. W.; Karweik, D. H.; Kuwana, T. Anal. Chem. 1981, 53,
2319-2323. Gunaslngham, H.; Fleet, B. Analysf (London) 1982, 107, 896-902. Miner, D. J.; Rice, J. R.; Riggin, R. M.; Kissinger, P. T. Anal. Chem. 1981, 53, 2258-2263.
RECEIVED for review January 31, 1983. Accepted April 22, 1983. This work was supported by NIH (R01-NS-15841) and NSF (CHE 81-21422). R.M.W. is an Alfred P. Sloan Fellow and the recipient of a Research Career Development Award (K04-NS-356).
Optically Transparent Porous Metal Foam Electrode Davld E. Hobart,”’ Vlncent E. Norvell, Peter G. Varlashkin, Herbert E. Hellwege,2 and Joseph R. Peterson Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996- 1600,and Transuranium Research Laboratory (Chemistry Dlvlsion), Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
The use of optically transparent electrodes (OTE’s) for in situ spectral monitoring of electrochemical reactions has proved to be an effective approach to the study of an everincreasing variety of oxidation-reduction (redox) systems (1). OTE’s have been made from a variety of materials for spectroelectrochemical studies of highly absorbing species (1,2). The development of OTE’s with long optical path lengths required for studies of weakly absorbing species has also been reported (3). Many of the long optical path cells are complicated in design, require special expertise in fabrication, and/or involve expensive instrumentation or components (i.e., lasers, fiber optics, etc.). In most cases the bulk analyte solution in these cells must be stirred, pulsed, pumped, or recirculated to obtain electrochemical equilibrium. Often only the electrode-solution interface is observed and equilibrium in the bulk solution is not attained. A long optical path OTE which does not suffer from the above mentioned disadvantages is the reticulated vitreous carbon (RVC)-OTE (4). The three-dimensional glassy carbon matrix of a RVC electrode permits rapid attainment of equilibrium in a small volume of solution with a nominal optical path length (>0.8 mm). The modification of a RVC electrode surface by mercury plating ‘Author to whom correspondence should be addressed. Current address: Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Current address: Department of Chemistry, Rollins College, Winter Park, FL 32789.
has been reported (4). This suggests that other electrode materials in a porous three-dimensional matrix-type configuration would be desirable for use as optically transparent electrodes or as the support for other plated electrode surfaces. The recent development of porous metal foam (5) has made possible the construction of a porous metal foam optically transparent electrode (PMF-OTE). PMF is quite similar to RVC in its structure and optical properties. The fabrication and evaluation of P M F for use as an OTE are presented in this paper.
EXPERIMENTAL SECTION Materials. Nickel porous metal foam, “AmPorMet” (a proprietary item manufactured by Astro Met Associates, Inc., Cincinnati, OH), was obtained in 1.0 mm (Series 240-lo), 1.4 mm (Series 260-lo), and 2.0 mm (Series 280-10) pore sizes of -90% void volume with a specified thickness of 6.35 mm (1/4 in.). This material was cut into assorted sections between 1.5 cm X 4.0 cm and 3.0 cm X 5.0 cm using a band saw, equipped with a metal cutting blade. Certain thickness were cut to high precision from the stock material by electrical discharge machining. Construction of the PMF-OTEs. The nickel PMF-OTE waa made by cleaning a precut PMF section with soapy water, rinsing it with acetone, and then drying it at about 60 “C. Electrical contact was made by soldering an 18 gauge copper wire to the edge of the PMF section. The OTE was assembled by sealing with epoxy the nickel PMF electrode between two quartz microscope slides using Teflon spacers in a configuration similar to that reported by DeAngelis and Heineman (6). A small di-
0003-2700/83/0355-1634$01.50/00 1983 American Chemical Soclety
