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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
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Figure 1. PMF-OTE form-fitting cell wkh separated counterelectrode and reference electrode compartments with asbestos flber junctions: (A) PMF working electrode; (El) platinum wire counterelectrode; (C) SCE reference electrode; (D) optical window.
ameter Teflon tube was sealed into the top of the cell for filling the cell by suction. An optical mask of opaque tape was positioned on the front of the OTE to confine the optical beam to the central portion of the PMF. The Au PMF-OTE was made by electrciplating gold on nickel PMF substrate (Oak Ridge National Laboratory), using the Sel-Rex Original Bright Gold patented process (Sel-Rex Corp., Nutley, NJ). The Au PMF-OTE was completed by sealing the electrode between quartz microscope slides as described above. The mercury-coatedPJi PMF-OTE was made from a previously assembled Ni PMF-OTIE by electroplating:mercury in a manner similar to that described by Heineman et al. (7). The lack of a substantiallynegative hydrogen overpotential and a mottled visual appearance exhibited by the mercury-coatednickel PMF electrode were evidence of the difficulty in obtaining an even coating of mercury on PMF which had been previouisly mounted between quartz plates. Because of this difficulty, modified cell design was utilized. An unmounted section of Ni PMF-OTE was rinsed first with ethanol to remove any surface organic impurities and then with distilled water. The PMF was placed in 1 M KC1, reference electrode and counterelectrode were placed in the solution, and the surface of the PMF was cathodized for about 5 min at a potential where hydrogen evolution was clearly visible. The electrode was again rinsed with water, and placed in a nitrogen-saturated Hg-plating solution of M HgClz, 0.1 M KCl, and 0.01 M HC1. A potential of -1.15 V/SCE was applied. After about 10 min, the PMF was removed from the solution and was rinsed with water and then with ethanol and left to air-dry. The PMF section was then dipped into a pool of mercury. After a minute or two the PMF section was removed from the pool and excess Hg was dislodged by vigorous shaking. The section of Hg-plated PMF was then inserted into a form-fitting glass cell equipped with side a ~ m and s asbestos fiber junctions, as illustrated in Figure 1. The above-mentioned OTE cells have volumes from 120 pL to 3 mL and optical path lengths from 1.6 to 6.0 mm. The cell volumes were estimatedl by measuring the electrode dimensions and assuming a 90% void volume. The cell path lengths were determined by comparing the absorbance of a standard NiSOl solution in the OTE to that of the same solution in a 1.0-cm cuvette. Apparatus. Spectral were recorded with a Cary Model 14-H spectrophotometer. Electrochemical measurements were performed with an EG&G PARC Model 173D/179D/175 potentiostat/coulometer/unk,rsal programmer. Voltammograms were recorded with an Esterline Angus Model XY530 recorder. All potential measurements were made vs. a SCE reference electrode. The pH measurements1 were made with a combination glass electrode and a Cornin,g Model 130 pH meter. Reagents. Hydrated lanthanide saltti were obtained from Research Chemicals, Phoenix, AZ. All other chemicals were (EL
50
55, NO. 9, AUGUST 1983 * 1535
100 TIME
t 50
2 00
(5)
Flgure 2. Chronocoulogram for the reduction of 6.5 X M K,Fe(CN), in 1 M KCI; Ni PMF-OTE (Series 240-10): potential stepped from no potential applied to -0.50 V/SCE; cell volume, 120 pL; cell path length, 1.6 mm.
-
ACS-certified reagent grade and were used without further purification. All solutions were prepared with deionized water.
RESULTS AND DISCUSSION The PMF-OTE is a “sintered” metal matrix electrode with a unique three-dimensional construction similar to that of RVC. P M F has a porous structure with a free void volume of from 85 to 90% (5) and a surface area certainly comparable to that of RVC (65 cm2/cm3(4)). The light transmittance of PMF is a function of the pore size and the electrode thickness. For a 0.6-cm slice of Series 280 PMF, the transmittance is about 13%. For a 0.4-cm section of Series 260, the transmittance is about 25%. For a given electrode the transmittance is essentially constant throughout the entire UV-VIS spectral region. The PMF-OTE, therefore, provides a substantial electrode surface area with a reasonably long light path, while maintaining sufficient light transmittance and requiring only a small sample volume. The three-dimensional structure of PMF allows smaller pore size PMF-OTE’s to behave electrochemically similar to the RVC-OTE (4) and the optically transparent thin-layer electrodes (OTTLE’s) (6) in that electrochemical equilibrium is attained rapidly. Since the electroactive material is always within a relatively short diffusion distance to some portion of the electrode surface, complete electrolysis of the electroactive species is achieved in a short time. This behavior is illustrated in Figure 2 by the chronocoulogram of a standard K,Fe(CN), solution. The ferricyanide solution was reduced in a Series 240-10 Ni PMF-OTE with a 1.6-mm path length by stepping the potential from no potential applied to -0.5 V/SCE. The charge increases rapidly and levels off at Qt when electrolysis is complete. The background charge, Qb, was measured by electrolysis of the supporting electrolyte solution without the electroactive species of interest. The short electrolysis time (-3 min) is evidence of the OTTLE-like behavior exhibited by the PMF-OTE. In an effort to optimize the optical path length parameter of the PMF-OTE, larger pore-sized PMF was utilized. It was found that the times for electrolysis of comparable solutions in these cells were reasonable (-20 min), but were not as short as those reported for the 100 pores-per-in. RVC-OTE (4) or the OTTLE (6) (-3 min). Electrolysis times in the PMFOTE are a function of the pore size; the larger the pore size, the longer the electrolysis time, since electroactive material in the bulk solution must diffuse a longer distance to reach the electrode surface. The chronocoulogram of a standard K3Fe(CN), solution in a Series 260-10 Ni(Hg) PMF-OTE with a 0.6-cm path length is shown in Figure 3. The potential was
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ANALYTICAL CHEMISTRY, VOL. 55, NO.
9,AUGUST 1983 I
t 10
O
l 20
30
1
I
I
C e 4 + + e-
I
I
I
1
Ce3+
i
1
TIME (rnin)
Flgure 3. Chronocoulogram for the reduction of 4.0 X M K,Fe(CN)Bin 1 M KCI; Ni(Hg) PMF-OTE (Series 260-10): potential stepped from no potential applied to -0.60 V/SCE; cell volume, 1.0 mL; cell path length, 6.0 mm (the background current was subtracted prior to plotting of the data).
r
0
I
10
I
20 TIME ( m i n )
30
40
Flgure 4. Absorbance-time curve for the reduction of 1.0 X M Eu(II1) in 1 M KCI at pH 4; Ni(Hg) PMF-OTE (Series 260-10): potential stepped from no potential applied to -0.8 V/SCE; cell volume, 1 mL; cell path length, 6.0 mm; wavelength, 320 nm.
-
stepped from no potential applied to -0.6 V/SCE. The charge-time curve increased rapidly to a constant equilibrium value in about 22 min. The reasonably rapid electrolysis time for the 0.6 cm thick Ni(Hg) PMF-OTE is further demonstrated by the spectroelectrochemistry of europium. The absorbance-time curve for the reduction of Eu(II1) to Eu(I1) in 1M KC1 at pH 4 is shown in Figure 4. An identical PMF-OTE, loaded with solvent only, was placed in the reference beam of the spectrophotometer to compensate for P M F absorbance. The equilibrium time for the reduction of a lC3M Eu(II1) solution was about 30 min. Although Eu(I1) has a high molar absorptivity (>2000 M-l cm-’), it should be noted that 6 times the above-mentioned concentration is required in a 1-mm RVC-OTE and about 30 times this concentration in a 0.2-mm OTTLE cell in order to obtain absorbance of comparable magnitude to that obtained by using the PMF-OTE. The usefulness of the PMF-OTE for spectropotentiometric determinations of redox potentials has also been demonstrated. A cyclic voltammogram for the Ce(IV)/Ce(III) couple in concentrated K&03 is shown in Figure 5. A slow potential scan rate (1mV/s) was required to minimize charging-current contributions and internal cell resistance because the P M F electrode has a substantial surface area. The anodic and
I
I 0
I -0.2 -0.4 POTENTIAL ( v o I t s / S C E 1
i
I -0.6
Flgure 5. Cyclic voltammogram for the Ce(IV)/Ce(III) couple in 5 M K&O, at pH 13.6;Au PMF-OTE (Series 280-10):cell volume, -3 mL; 1.0 X lo-*M Ce(II1); scan rate, 1 mV/s.
cathodic peak currents are equal, indicating retention of the electroactive product in the immediate vicinity of the electrode surface. The formal reduction potential of the Ce(IV)/Ce(III) couple in K2C03solution was “estimated” from the cyclic voltammogram recorded in the PMF-OTE to be -0.26 V/SCE by simply calculating the midpoint between the anodic and cathodic peak potentials. The spectropotentiometric determination of the formal potential of the cerium couple in 2 M K2C03a t pH 12.9 was performed by using a Au PMF-OTE (Series 280-10) with a 0.6 cm path length and a cell volume of -3 mL. The absorbance of Ce(1V) (4 x M Ce) was monitored at 304 nm. The rather long time (2-3 h) to reach equilibrium was most likely due to the viscosity of 2 M K2C03 and the large pore size of Series 280-10 PMF. This particular pore size was used in order to obtain a long cell path length. The Nernstian plot of the spectropotentiometric data for the cerium couple conformed quite well to a straight line with a slope of 0.068 f 0.001 mV (n = 1.16 electrons). They intercept yielded a value of -0.234 f 0.002 V/SCE for the formal potential of the Ce(IV)/Ce(III) couple. This value is in excellent agreement with that (-0.233 f 0.002 V/SCE) determined by using a platinum screen thin-layer OTE (8, 9) and is in reasonable agreement with the value estimated from the cyclic voltammogram for this system and with the value (-0.20 V/SCE) obtained a t a RVC-OTE (10).
CONCLUSIONS Nickel PMF was used as received or was modified by plating with gold or mercury in the present work; however, various metal PMF materials such as copper, molybdenum, tungsten, tantalum, silver, platinum, etc. are also available directly from the manufacturer. Other electrode materials could be used as surface coatings to modify the electrochemical characteristics of base-metal PMF. A doped tin oxide coating (2) could be used to extend the anodic range of PMF. Platinum metal could be electroplated onto P M F followed by a “platinumblack” procedure to provide a substantial increase in the electrode surface area. A method less expensive than electroplating P M F with a precious metal would be to dip the PMF repeatedly into “liquid gold” or “liquid silver” (Engehard Corp., East Newark, NJ). P M F may provide a versatile
Anal. Chem. 1983, 55,
substrate for chemical1,y modified electrode surfaces (11). Inexpensive base-metal F’MF materials could certainly provide a highly conductive substrate for a large number of electrode surfaces. Bulk P M F is available in a number of metals with various porosities and can be easily formed or cut into any required shape. As is the case with RVC, the choice of several porosities permits user control over the optical and electrolytic characteristics of the electrode. PMF has at least two advantages over RVC of being less fragile and exhibiting lower electrical resistance. The PMF-CITE combines the desirable characteristics of a metal or metal oxide thin-layer electrode with the favorable structure of the RVC-OTE; such that electrochemical equilibrium can be attained in a short time while a substantial optical path length can be maintained by using only a small volume of solution. In conclusion, the PMF-OTE has been shown to be a new, simple, and highly versatile OTE for spectroelectrochemical studies of redox systems. In particular, the PMF-OTE is quite useful for studies of weakly absorbing splecies and/or dilute solutions, where optimization of the cell path length is required. Many applications will be found1 for using P M F as optically transparent electrodes as well as for use as conventional working electrodes where versatility, porosity, and large electroactive surface area are advantageous, such as for bulk electrolysis in flow-through process chemistry and for electrochemical detectors in liquid chromatographic systems. ACKNOWLEDGMENT The authors wish to thank G. Mamantov of The University of Tennessee (Knoxville), L. N. Klatt of Oak Ridge National
1637
1637-1639
Laboratory, and F. Gorman and J. M. Stamper of Astro Met Associates, Inc., for their helpful discussions. The form-fitting cell described herein was fabricated expertly by R. W. Poole, B. R. Vermillion, and E. R. Mellon of Oak Ridge National Laboratory. Registry No. Eu, 7440-53-1; Ce, 7440-45-1; K,C03, 584-08-7; nickel, 7440-02-0;gold, 7440-57-5;mercury, 7439-97-6;ferricyanide, 13408-62-3;ferrocyanide, 13408-63-4. LITERATURE C I T E D (1) Helneman, W. R. Anal. Chem. 1980, 5 0 , 390A-402A. (2) Kuwana, T.; Helneman, W. R. Acc. Chem. Res. 1976. 9 , 241-248. (3) Brewster, J. D.: Anderson, J. L. Anal. Chem. 1982, 5 4 , 2560-2566, and references therein. (4) N o ~ e l l V. , E.; Mamantov, G. Anal. Chem. 1977, 49, 1470-1472. (5) Bulletin No. 66-1-A Rev. 2, Astro Met Assoclates, Inc., Cinclnnatl, OH 45215. (6) DeAngelis, T. P.; Heineman, W. R. J . Chem. Educ. 1978, 5 3 , 594-597. (7) Heineman, W. R.; DeAngells, T. P.; Goelz, J. F. Anal. Chem. 1975, 4 7 , 1364-1369. ( 8 ) Hellwege, H. E.; Hobart, D. E.; Peterson, J. R., 1982, data to be submitted for publication. (9) Peterson, J. R. et al. U S . Department of Energy Document No. DOE/ ER/04447-152; pp 18-26. (10) Hobart, D. E.; Samhoun, K.; Young, J. P.; Norveil, V. E.; Mamantov, G.; Peterson, J. R. Inorg. Nucl. Chem. Leff. 1980, 16, 321-328. (11) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141.
RECEIVED for review February 28,1983. Accepted April 15, 1983. This research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U S . Department of Energy, under Contracts DE-AS05-76ER04447 with The University of Tennessee (Knoxville) and W-7405-eng-26with the Union Carbide Corporation.
Determination of Thorium in Geological Materials by X-ray Fluorescence Spectrometry after Anion Exchange Extraction I wan Roelandts Department of Geology, P~~trology and Geoche?mistry,University of Licige, B 4000 Sari Tilman, LlSge 1, Belgium
Precise and accurate determinations of thorium in rocks and minerals of diverse nature are of importance not only for understanding its geochemical behavior during magmatic differentiation but also for the development of nuclear technology. A survey of the literature (1)shows, that radiochemical or nondestructive neutron activation analysis (using thermal, epithermal, or delayed neutrons) and maw spectrometry are the most widespread and particularly suited methods for the estimation of thorium a t the microgram and submicrogram levek of concentration in geological specimens. Unfortunately, these elaborate techniques require specialized instrumentation not available in most geochemical and industrial laboratories. Over the last few years, X-ray fluorescence spectrometry (XRF) has proved to be a useful analytical technique in geochemistry, mainly for bulk analysis of common silicate rocks. For certain trace elements present a t low levels of concentration, conventional separation and preconcentration procedures have been proposed in order to enhance both sensitivity and accuracy. Ion exchange i13a convenient and extremely efficient method for this purpose. Van Niekerk et al. ( 2 )have previously reported on a cation exchange X-ray fluorescence method for thorium, but an EDTA titration seemed to offer more accurate and flexible results (3). Anion exchange preconcentratiion is probably a more selective and favorable technique ( 4 ) . The aim of the present investigation was to verify the ap-
plicability of a combined batch anion-exchange-XRF method for the determination of thorium in geological samples. The so-called “matrix effects” which are severe drawbacks in these complex geological matrices will thus be overcome to a large extent after the removal of the main constituents. The calibration will be achieved by using pure chemical standards processed in the same way as the samples and therefore be independent upon internationally available geochemical reference materials. Two Canadian syenite reference rocks (SY-2, SY-3) as well as some South African thorium ores and concentrates were analyzed to control if the outlined procedure is suitable in practice. EXPERIMENTAL SECTION Apparatus. Batch equilibrations were performed by means of a Turbula system Schatz WAB shaking machine. A stainless steel die and a hydraulic press (Weber, Stuttgart, Uhlbach, Germany) were used for pelletizing the cellulose supports (5). All X-ray measurements were performed by using a CGR-alpha 2020 semiautomatic spectrometer (Compagnie GBnBrale de Radiologie, France) p equipped with a six-position sample changer. The spectrometer was interfaced to a Hewlett-Packard 9815A calculator. Reagents. The resin Dowex 1x8 (lOC-200 mesh, chloride form) was supplied by Fluka, Buchs, Switzerland. The nitrate form of the anion-exchange resin was prepared from the chloride form
0003-2701~/83/0355-1637$01.50/0 0 1983 American Chemical Society
