Anal. Chem. 1983, 5 5 , 1177-1179
1177
Thin-Layer Spectroelectrochemistry Cell with Demountable Gold or Mercury-Gold Minigrid Electrodes Harry 0. Flnkka” Department of Chemistty, Virginia Polytechnic: Institute and State University, Blacksburg, Wrglnia 2406 7
Robert K. Boggess” and Johnny W. Trogdon Department of Chomlstty, Radford Unlverslfy, Radford, Virglnia 2 4 742
Franklln A. Schultz
*‘
Department of Chemistty, Florida Atlantic Uniwersify, Boca Raton, Florida 3343 7
The technique of spectroelectrochemistry in thin layers of solution (1, 2) has been used with increasing frequency t o characterize the redox and spectral properties of inorganic (3-6), organic (7-Z0),and biological (21-24) species. Accompanying this increased interest, numerous designs have been reported (15-26) for optically transparent thin-layer electrode (OTTLE) cells which provide one or more features needed for a particular spectroelectrochemical experiment, viz., exclusion of oxygen, compatibility with nonaqueous solvents, ultraviolet transparency, and utilizatioln of small sample volumes. Comprehensive spectroelectrochemical characterizations, however, may require a wide range of solvents, electrode materials, and accessible spectral energies and electrode potentials. We wish to report a design for a versatile OTTLE cell which incorporates many of the fieatures needed in spectroelectrochemical experimentation, yet is simple t o construct and use and is relatively inexpensive. Advantages of the cell include: (1) compatibility with aqueous or nonaqueous solvents; (2) a demountable gold or amalgamated-gold minigrid working electrode and the extended negative potential limit provided b y the latter in aqueous solution; (3) transparency from the ultraviolet to near-infrared spectral regions; (4) small Elample volume requirement; (5) flow through capability; and (6)the ability to handle air-sensitive materials. A novel feature of the cell is the containment of and electrical contact to the minigrid working electrode entirely within the thin-layer sample compartment. This property enables easy dismantling of the cell for exchange of minigrid materials or replacement of the amalgamated grid upon its eventual deterioration. A number of optically transparent mercury-onsubstrate electrodes have been reported (27-30). From the standpoint of increased ]-I2 overvoltage the most satisfactory of these is the Hg-Au minigrid (29). Yet wider application of this electrode has been forestalled by the need to construct a new OTTLE cell each time the underlying gold mesh deteriorates. The present cell circumvents this problem and should enable increased’ application of mercury electrode surfaces in spectroelectrochemical investigations.
EXPERIMENTAL SECTION Cell Construction and Use. The cell, shown in Figure 1,is adapted from the design of Heineman et al. (22)with modifications made to enable use in nonaqueous solvents and rapid reassembly with a new minigrrd electrode. A 4.5 X 4.0 X 1.25 cm Teflon block serves as the cell body. An 8-mm hole drilled through its center defines the optical path. Concentric to this hole are machined a cylindrical cavity for the smaller quartz window ( I O mm diameter, 2 mm thick, Esco Products, Oak Ridge, NJ)and a 0.19 cm deep circular groove for a large silicone O-ring (1.71 an id., 2.22 cm o.d., Catalog No. 2-115, Presently on leave at: Charles F. Kettering Research Laboratory, 150 E. South College St., Yellow Springs, OH 45387.
Robert Hoose Co., Miami, FL). This O-ring defines the thin-layer volume of the cell. A solution inlet and a reference electrode port are drilled into the cell block, from which they access the thin-layer volume through 1.27 mm diameter holes. These holes are enlarged to ca. 5 mm diameter on the cell block periphery, where they are threaded to accept Cheminert liquid chromatographic fittings. A 1.27 mm diameter hole is drilled at a 45O angle into the reference electrode port for emplacement of the auxiliary electrode. A more recently constructed cell contains separate ports and solution paths from the thin-layer section to the reference and auxiliary electrodes. To provide a path for electrical contact to the working electrode, a 1.42 mm diameter hole is drilled through the cell block to the thin-layer compartment. This hole is enlarged to a diameter of 0.42 cm and threaded on the cell block exterior. As the final step in cell construction the 10-mm quartz window is cemented into the central cavity with silicone adhesive (General Electric 2561-71DP). To ensure that this window dries flush with the Teflon surface, a large quartz disk (25 mm diameter X 1.6 mm thick) is placed over it and clamped for 24 h with a Lucite pressure plate. If necessary, the small window may be removed by pressing from the rear. The fiist step in assemblingthe cell is installation of the contact to the working electrode. A slightly undersized gold rod (1.40 mm diameter, 2.5 cm long, Alfa Products) is passed through an externally threaded aluminum nut of our own construction, a small O-ring (1.19 mm i.d., 3.57 mm o.d., Catalog No. 2-02, Robert Hoose Co.), and the Teflon block until it projects very slightly above the cell surface. Tightening the nut simultaneously seals the O-ring against the gold rod and the bottom of the cavity in which it rests. In practice, very little solution appears to leak along the rod between this seal and the main compartment of the thin-layer cell. A similar means of internally contacting a plate-type SnOz working electrode in an OTTLE cell has been described recently by Hawkridge (16). The large O-ring is then placed in its groove on the cell block, and a minigrid electrode (31,32)is constructed within its circumference from a 1.0 X 1.5 cm piece of gold mesh (100,500, or 2000 lines per in. (lpi),Buckbee Mears Co., St. Paul, MN) and one or two thicknesses of Fluorofilm tape (DF-1200, 5-mil thick, Dilectrix Carp., Farmingdale, NY) (see Figure 1).The large quartz window is positioned over the working electrode and the entire cell is fastened with a Lucite pressure plate (drilled for optical transparency) and four nuts and bolts (not shown). The cell is mounted on a 4 X 4 X 1 cm Lucite block to increase its stability when handled. The auxiliary electrode is a 1.27 mm diameter P t needle (Hamilton No. KF 718PT 51162) which is press fitted into the diagonal hole. A Hamilton lMM1 valve is attached to this needle and to tubing leading t o an empty flask. A Cheminert adapter is threaded into the reference electrode port and a Hamilton 3MM2 valve is attached to it. A flask containing the reference electrode is connected to this valve via Teflon tubing. A Cheminert adapter, Hamilton valve, tubing, and connector are attached to the solution inlet, permitting samples to be introduced by syringe. Prior to use the cell is cleaned in a radio frequency argon plasma. To fill the cell, the syringe is filled with deaerated electrolyte and attached to the inlet port. Electrolyte is forced through the cell with all valves open until all bubbles have been
0003-2700/83/0355-1177$01.50/00 1983 American Chemical Soclety
1178
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7,JUNE 1983 Hg-Au OTTLE a
d
l
p5qij ,d, ,~ F?? ',, U L Au MINIGRID
0 RING
T
Pt AUX rR$j
/ , , , , ~CA ' F ; 5 ; O~~ET
THREADED NUT
\ 0 RING W A R T Z WINDOWS TEFLON BLOCK
/
PRESSURE PLATE
Figure 1. Cell design. Not shown are four screw holes and accompanying screws and wing nuts for clamping the cell assembly. These are located near the four corners of the Teflon block.
removed. The auxiliary electrode and inlet valves are shut to prevent siphoning. If it is necessary to refii the syringe, all valves are shut before removing the syringe. After the syringe is refilled, the auxiliary electrode and inlet valves are opened and electrolyte allowed to back-siphon before reattaching the syringe. The object is to remove air which can form a bubble when the syringe is inserted into its adapter. Amalgamation of the Au Minigrid. Heineman et al. (29) developed a two-step procedure for amalgamating Au-minigrid electrodes wherein calomel (HgZCl2)is first deposited by POtentiostatic reduction of an Hg2+/C1-solution at -0.7 V vs. SCE and then converted to Hg"by electrolysis at -1.2 V in mercury-free electrolyte. This procedure led to difficulty with the present cell. The large currents resulting from potentiostatic reduction of concentrated Hg2+solutions formed bubbles at the Pt needle auxiliary electrode, and these eventually terminated electrolysis by causing an open-circuit condition. We determined that the cell could sustain currents of only ca. 100 pA without breaking electrical contact during the amalgamation step. Consequently, the following procedure was used to amalgamate the Au grid electrode. The cell was filled with saturated Hg (NO,), in 0.5 M KC1/0.1 M HC1, and Hg,C12 was deposited galvanostatically with a current of 100 FA. The cell then was filled with 0.5 M KC1/0.1 HC1, and the HgzClzreduced to Hgo by either continued galvanostatic reduction at 100 pA or potentiostatic reduction at -0.4 V vs. SCE. With a 2000 lpi grid, approximately 1 C of charge is required for the initial plating of a 1.5-cm2electrode.
RESULTS AND DISCUSSION The present cell has been used successfully with Au and Hg-Au minigrid electrodes in aqueous solution and with an Au electrode in the following nonaqueous solvents: acetonitrile, dichloromethane, and dimethyl sulfoxide. Results obtained with the cell have been described in recent publications (33,34). These results demonstrate the utility of the cell in obtaining spectral information on potentially reactive species under a wide range of conditions and in determining formal potentials and electron stoichiometries by the spectropotentiostatic technique of Heineman (4, 5, 31). The following paragraphs describe the operating characteristics of the cell and provide further experimental examples of its versatility. Figure 2 shows the extended negative potential limit attainable in aqueous solution with the Hg-Au minigrid electrode. A current density of 10 pA/cm2 (geometric electrode area) is reached a t -0.88 f 0.06 V in 0.5 M KC1/0.1 M HC1 (Figure 2a) and a t -1.5 f 0.1 V in pH 9.4 catechol buffer (Figure 2b). These results are in good agreement with the Hg-Au minigrid of Heineman et al. (29). Although the Hz evolution potential is still several hundred millivolts less negative than that of pure mercury, spectroelectrochemical characterization of many difficult-to-reduce aqueous samples can be achieved with the Hg-Au OTTLE cell. Figure 2c shows the reversible one-electron reduction of methyl viologen dication in pH 7 phosphate buffer. The
U d L d -02 -0.6 -10 -14 E V v s SCE
Figure 2. Aqueous solution cyclic voltammograms in the Hg-Au OlTLE cell recorded at 4 mV s-I: (a) 0.5 M KCI, 0.1 M HCI, S = 100 pA; (b) 1 M KCI, 0.15M catechol, pH 9.4, S = 100 /*A; (c) 0.5 M KCI, pH 7 phosphate buffer, 0.98 mM methyl viologen, S = 100 pA; (d) 1 M KCI, 1 M NH,CI/NH,, 0.15 M catechol, pH 9, 2.0 mM Mo(cat),*-,S = 200 MA. iR compensation of 600, 600, 1000, and 250 il was applied in (a), (b), (c), and (d), respectively.
well-defined voltammetric wave a t E"' = -0.700 V vs. SCE is completely resolved from Hz evolution. Figure 2d contains a thin-layer cyclic voltammogram for the reversible Mo(IV)/Mo(III) couple generated by controlled-potential reduction of the Mo(V1)-catechol complex, MoOz(cat)22-,at E < -1.0 V in pH 9 catechol buffer. Determination of oneelectron stoichiometry and a formal potential of -1.09 V vs. SCE by thin-layer spectroelectrochemistry aided in assigning the half-reaction Mo(cat),2e- = M ~ ( c a t ) ~ ~ (1)
+
to this couple (33). Examination of this electron transfer reaction could not have been accomplished in aqueous solution at an electrode surface other than mercury. As noted by Heineman (29) the Hz overpotential of the Hg-Au minigrid diminishes by a few hundred millivolts over a period of 1-2 days. Deposition of a fresh layer of Hg restores the original characteristics. In practice, the Hg surface can be renewed 3-5 times before the underlying mesh deteriorates; thus, the lifetime of a single minigrid is 1-2 weeks with frequent use. The demountable feature of the cell facilitates replacement of a spent Hg-Au electrode. Construction of a new minigrid electrode and reassembly of the cell requires about 45 min. Deposition of the initial layer of mercury with constant current electrolysis at 100 pA requires 2-3 h. Subsequent coatings of Hg take ca. 30 min. The chromatographic fittings on the cell facilitate sample introduction and also could enable flow-through studies (20). When constructed with two layers (ca. 250 pm thickness) of 5-mil Teflon tape, the volume enclosed in the thin-layer region is ca. 60 pL. However, because of the undersized minigrid electrode, the electrochemicallyaccessible volume (determined by controlled potential coulometry) is ca. 35 pL. The solution inlet and reference and auxiliary electrode fittings add considerable "dead-volume'' to the cell. Typically, 1-2 mL of solution is required to flush and fill the cell. Samples are deoxygenated by purging with argon in a septum-sealed flask. Solution is withdrawn from the flask and transferred to the cell by use of a gastight syringe. This procedure appears to be satisfactory in handling relatively air-sensitive samples such as alkaline catechol (Figure 2) in aqueous solution, where entrained O2is reduced innocuously to water (14). However, the cell cannot be considered strictly anaerobic when used in this way, and 0, intrusion is noted with nonaqueous solvents. If more stringent oxygen exclusion is required, it is recommended that the cell be filled in an inert atmosphere chamber. To avoid the oxygen permeability of Teflon, the cell body
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE
1983
1179
broadened relative to the normal oxide stripping peak. This artifact apparently results from the creation of extremely thin and nearly inaccessible regions under or along the gold rod where large resistance distorts the oxide reduction wave. Inordinately long thin-layer electrolysis times (215 min) also may be noticed. The problem is reduced by placing one Teflon spacer on each side of the gold grid and passing the contact rod through a hole cut in the nearer strip. Registry No. Gold, 7440-57-5; mercury, 7439-97-6.
LITERATURE CITED
1
I
300
nm
LOO
Figure 3. Spectra of Mn(SALDPT)N, In 0.5 M (C2H,),NCI0,/Me,S0 as a function of applied potential: (a) 0.000, (b) -0.205, (c) -0.220, (d) -0.235, (e) -0.250, and (f) -0.400 V vs. SSCE.
could be constructed from Kel F or Macor (24). Nonaqueous solvents also can be used in the OTTLE cell. Figure 3 shows spectra ag a function of potential for the Schiff base complex, [a,a’-(iminobis(trimethylenenitri1o))di-o-cresolato]manganese (111) azide, Mn (SALDPT)N3, in dimethyl sulfoxide with 0.5 M tetraethylammonium perchlorate as supporting electrolyte. A 100 lpi Au minigrid served as the working electrode. A plot of E vs. log (A - AR)/(Ao - A ) , in which A is the absorbance of the solution a t the potential E, AR is the absorbance of the fully reduced complex (curve f), and A0 is the absorbance of the fully oxidized complex (curve a), monitored a t 370 nm, provides a straight line from which n = 0.92 and Eo’= -0.212 V vs. SSCE are determined for the Mn(III)/Mn(II) redox couple. A cyclic voltammogram of the same compound in the OTTLE cell gives Eo’= -0.206 V, in good agreement with the result from spectral data. Several limitations are encountered in operating the cell; some of these can be alleviated by modifications in design. As is typical of thin-layer electrochemistry (35),uncompensated solution resistances of 300-1000 fl (aqueous) and >loo0 Q (nonaqueous) are observed between the working and reference electrodes Figure 2 shows that t h k resistance can be partially compensated by positive potentiostatic feedback, but measurable separation between anodic and cathodic peak potentials still exists. Lower uncompensated resistance is achieved by providing separate entrances for the reference and auxiliary electrodes to the thin-layer volume and by increasing the diameter of these paths. After these modifications are made, a cyclic voltammogram of 1mN[ Mn(SALDPT)N3 in 0.5 M (C2H6),NClO4/Me2SOrecorded at a sweep rate of 2 mV s-l without positive feedback exhibitca a peak separation of 114 mV, a relatively small value for nonaqueous thin-layer electrolysis. These changes also should alleviate the problems arising from bubble formation at the auxiliary electrode during amalgamation. A problem sometimes arises when Teflon spacers are placed directly opposite the gold contact rod during cell assembly (see Figure 1). In this cafie, a second Au oxide reductive wave is observed in 0.5 M H2804 which is shifted -150 mV and
Heineman, W. R. Anal. Chem. 1978, 50, 390A. Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976, 9 ,241. Kadish, K. M.; Rhodes, R. K. Inorg. Chem. 1981,20, 2961. Rohrbach, D. F.; Heineman, W. R.; Deutsch, E. Inorg. Chem. 1979, 18, 2536. Rohrbach, D. F.; Deutsch, E.; Heineman, W. R. I n “Characterization of Solutes In Non-Aqueous Solvents”; Mamantov, G., Ed., Plenum: New York, 1978;pp 177-195. Rieder, K.; Hauser, U.; Siegenthaler, H.; Schmidt, E.; Ludi, A. Inorg. Chem. 1975, 14, 1902. Jarbawi, T. B.; Heineman, W. R. J . Necfroanal. Chem. 1982, 132,
323. Stutts, K. J.; Powell, L. A.; Wightman, R. M. J . Electrochem. SOC. 1981, 128, 1248. Langhus, D. L.; Wilson, G. S. Anal. Chem. 1979,51,1139. Owens, J. L.; Marsh, H. A., Jr.; Dryhurst, G. J . Elecfroanal. Chem.
1978,91,231. Heineman, W. R.; Meckstroth, M. L.; Norris, B. J.; Su, C.-H. Bioelectrochem. Bioenerg. 1979, 6 , 577. Sailasuta, N.; Anson, F. C.; Gray, H. 8. J . Am. Chem. SOC. 1979, 101, 455. Mark, H. B.. Jr.; Kenvhercz, T. M.: Kissinaer. P. T. ACS Svmo. Ser. ~,
1977, No. 38,1.
Heineman, W. R.; Norris, B. J.; Goelz, J. F. Anal. Chem. 1975, 4 7 , 79 Rubinson, K. A.; Mark, H. B., Jr. Anal. Chem. 1982, 5 4 , 1204. Bowden, E. F.; Cohen, D. J.; Hawkridge, F. M. Anal. Chem. 1982,54,
1005. Muth, E. P.; Fuller, J. E.; Doane, L. M.; Blubaugh, E. A. Anal. Chem.
1982, 54,604. Blubaugh, E. A.; Doane, L. M. Anal. Chem. 1982, 5 4 , 329. Rhodes, R. K.; Kadish, K. M. Anal. Chem. 1981, 53, 1539. Pinkerton, T. C.; Hajizadeh, K.; Deutsch, E.; Heineman, W. R. Anal. Chem. 1980, 52, 1542. Mamantov, G.; NoNell, V. E.; KIatt, L. J . Nectrochem. SOC. 1980,
127. 1768. Anderson, C. W.; Halsall, H. B.; Heineman, W. R. Anal. Slochem.
1979, 93.368. Anderson, J. I.. Anal. Chem. 1979,51,2312. Hawkridge, F. M.; Pemberton, J. E.; Biount, H. N. Anal. Chem. 1977,
49, 1646. NONell, V. E.; Mamantov, G. Anal. Chem. 1977, 4 9 , 1470. Norris, B. J.; Meckstroth, M. L.; Helneman, W. R. Anal. Chem. 1978, 4 8 , 630. Helneman, W. R.; Kuwana, T. Anal. Chem. 1971, 43, 1075. Helneman, W. R.; DeAngelis, T. P.; Goelz, J. F. Anal. Chem. 1975, 4 7 , 1364.
Meyer, M. L.; DeAngeiis, T. P.; Heineman, W. R. Anal. Chem. 1977,
49,602. DeAngelis, T. P.; Hurst, R. W.; Yacynych, A. M.; Mark, H. B., Jr.; Helneman, W. R.; Mattson, J. S. Anal. Chem. 1977, 49, 1395. DeAngeiis, T. P.; Heineman, W. R. J . Chem. Educ. 1978, 53,594. Murray, R. W.; Helneman, W. R.; O’Dom, G. W. Anal. Chem. 1987, 39, 1868. Charney, L. M.; Flnklea, H. 0.; Schultz, F. A. Inorg. Chem. 1982,21,
549. Smith, D. A.; McDonald, J. W.; Flnkiea, H. 0.; Ott,V. R.; Schultz, F. A. Inorg. Chem. 198PV21,3825. Hubbard, A. T.; Anson, F. C. I n “Electroanalytical Chemistry”; Bard, A. J., Ed.; Marcel Dekker: New York, 1970;Vol. 4, pp 129-214.
RECEIVED for review December 9,1982. Accepted March 14, 1983. Acknowledgment is made to the National Science Foundation (Grant No. CHE-8020442, F.A.S., and TFI-802595, R.K.B.) and to the donors of the Petroleum Research Fund, administered by the American Chemical Society (R.K.B.), for support of this research.
