Anal. Chem. 1982, 5 4 , 329-331
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AIDS FOR ANALYTICAL CHEMISTS Vacuum Thin-Layer Electrochemical Cell for Nonaqueous Spectroelectrochemistry Elmo A. Blubaugh" and Lawrence M. Doane Organic Analytical Research Division, Natlonal Bureau of Standards, Washington, D.C. 20234
The electrochemical and spectroscopic behavior of organic and inorganic molecules have been investigated by many different spectroelectrochemical techniques ( I ) . Of these techniques, thin-layer spectroelectrochemistry at an optically transparent thin-layer electrode (OTTLE) has been shown to be an effective means of following spectral responses to electrochemical perturbations (2). The formal redox potential, E O ' , and the number of electrons transferred, n,for an electrochemical reaction may be determined accurately by thinlayer transmission experiments. Much of the thin-layer spectroelectrochemical work has centered around the redox chemistry of biological components in aqueous media. However, electrochemistry involving radicals or water-sensitive molecules is difficult or impossible to perform without using nonaqueous solvents. There have been recent reports of thin-layer spectroelectrochemical studies involving aprotic solvents (3-5) and a report on the construction of a small-volume, high-performance, semiinfinite diffusion electrochemical cell for nonaqueous spectrochemistry has appeared (6). We report here the construction of a spectroscopic cell that employs an OTTLE and that is not attacked by common nonaqueous solvents. The cell and ancillary apparatus were designed with the following criteria: (1)small cell volume, (2) allowance for optical windows, (3) preparation and maintenance of solution in an oxygen- and water-free environment, (4) convenient and easy interchange between thin-layer and bulk electrochemical experiments, and (5) small overall cell dimensions. The performance of this cell in nonaqueous spectroelectrochemistry was evaluated from measurements of methyl viologen (1,l'dimethyl-4,4'-dipyridiniumdichloride) in propylene carbonate.
EXPERIMENTAL SECTION Instrumentation. All bulk and thin-layer voltammetric experiments were performed with a three-electrode system using a Princeton Applied Research (PAR) Model 173 potentiostat equipped with a PAR Model 179 digital coulometer. Triangular waveforms for cyclic voltammetry were generated with a PAR Model 175 universal programmer. An X-Y recorder was used to record the current-voltage curves. In spectroelectrochemical experiments, the potential applied to the cell was obtained from an Amel Model 551 potentiostat. Cell current was measured with a digital multimeter. The spectrum at each applied potential was recorded with a Cary 14 spectrophotometer, which was modified as follows: The three electrode leads and the purging gas were brought into the Cary 14 sample compartment through a brass guide tube that replaced the carriage handle. Since the cell when fully assembled extended approximately 5 cm above the sample compartment walls, the cover plate was removed and replaced with an aluminum box, open at the bottom. The aluminum box had a port at the top to accommodate the OTTLE suction tube. The cell was held in place with a clamp that was fixed to the carriage plate. Cell Description. The vacuum cell was designed to function equally well in either of two configurations, i.e., as a spectroscopic cell in spectropotentiostatic experiments employing an OTTLE (Figure 1A) and as an electrochemical cell in voltammetric experiments (Figure 1B). The cell (in either configuration) consists of two parts: (i) the main U-shaped cell, which contains the
solution and provides support for the electrodes and optical windows and (ii) the inlet adapter, which in part forms an isolated auxiliary electrode compartment and a port for soldtion introduction. The auxiliary electrode is isolated with two glass frits: one at the end of the auxiliary electrode chamber and the other in the base of the main cell. Approximately 2 mL of solution is required to adequately fill the cell for an experiment. Although the joints are smdard taper, some modification had to be employed to effect a greaseless vacuum seal which would not seize. With joints of adequate wall thickness, a groove is ground in the inner joint to accommodate an O-ring, which provides a vacuum seal. With joints of inadequate wall thickness, two ground joints of different size are employed, e.g., 10/30 inner joint with a 12/30 outer joint. In this case, the O-ring is placed between two tapered Teflon bushings which give the seal mechanical support. The auxiliary cap is machined to accommodate an O-ring and compression fitting that form a vacuum seal when tightened about the platinum wire. The outer wall of the auxiliary cap (as well as the cell cap described below) is grooved and fitted with an O-ring for sealing purposes. Spectroscopic Cell. As shown in Figure lA, the optical window support is made by joining the screw portion of a small bottle to the sides of the cell body. The window seat is formed by sealing a small length of glass tubing to the inside of the screw portion. The bottle cap along with a Teflon cylinder spacer forces the window against an O-ring to form a vacuum seal with the seat. The cell cap has three ports machined in it to accommodate tubing up to 2 mm in diameter. These ports are fitted with O-rings and compression fittings to form vacuum seals. The center hole of the cell cap serves a dual purpose depending on the experiment performed. In spectroelectrochemical and thin-layer experiments, the hole serves as a passage for a hypodermic needle and support for a compression fitting which forms the vacuum seal (see cell cap cross section of Figure 1A for details). A Kel-F tension clamp, which is pressed onto the tip of the hypodermic needle, provides the attachment support for the OTTLE when the OTTLE is forced between the jaws. Electrical connection is made by soldering a fine lead wire from the OTTLE to a heavy gauged lead wire supported in one of the three cell cap ports. By turning the hypodermic needle, one can rotate the OTTLE in or out of the optical beam. One of the remaining two cell cap ports accommodates another hypodermic needle which is used to equalize pressure within the cell to that of the atmosphere (details in procedure section). Rubber tubing is forced into the luer end of the hypodermic needle and clamped at the loose end to give a vacuum seal when the cell is being evacuated. The remaining cell cap port is used to support the Ag/AgC10, reference electrode (7). The OTTLE is of standard construction (5). Electrochemical Cell. In bulk electrochemical experiments, the center hole of the cell cap accommodates a binding post to which a working electrode is attached. The binding post when screwed in place presses against an O-ring forming a seal (see cell cap cross section of Figure 1B). A spring, with a brass contact pin inserted in one end, is attached to the end of the binding post and provides electrical contact with the electrode material. The working electrodes are fabricated from 1/4 in. Kel-F rod. The interior of the electrode body is threaded so that the electrode can be attached and removed easily. Four different electrode materials (gold,,platinum, silver, and pyrolytic carbon) are employed. Rods of each material are turned oversize by 50 to 125 wm and then press fitted into holes bored in the base of the electrode body and polished to a mirror finish.
This article not subject to US. Copyright. Publlshed 1982 by the Amerlcan Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
r
n
I
Inlet Adapter
Inlet Adapter
#. -!L
n
/'
-
Electrode
3 19138
Joint Gasket Assembly
3 12/30'
Auxlllary Electrode Chamber
I 19/38
cell Body
I Figure 1. Vacuum electrochemical cells: (A) SDectroelectrochemlcalapparatus and (B) bulk electrochemical apparatus. The asterisk indicates an O-ring. Solvents and Electrolytes. Tetraethylammonium perchlorate (TEAF') Fluka A. G., purum, at a concentration of 0.5 mol/L, was used as the supporting electrolytein all experiments. TEAP was recrystallized from water and dried for 24 h in vacuo at a temperature of 120 "C. Methyl viologen (MV), supplied as the chloride salt by Pfaltz and Bauer, was used as received, The propylene carbonate (PC)supplied by Aldrich was distilled in vacuo (2 mmHg) from a 90-cm packed column at a temperature of 72 "C under an atmosphere of argon. All joints of the distillation apparatus were mated with Teflon sleeves. The receiver flask, which was kept under an argon atmosphere, was fitted with Teflon high-vacuum stopcocks to allow direct transfer of the solvent to the solution ampule without possible contamination from greased stopcocks. Procedure. The appropriate weights of analyte and supporting electrolyte are transferred into the ampule which contaim a "flea" stirring bar. With only the transfer head and inlet adapter joined to the ampule, the partially assembled apparatus is connected to the vacuum line and solvent reservoir. The ball joint to the vacuum line and the upper portion of the ground joint of the solvent reservoir are greased to effect a vacuum seal, since the solvent never contacta these joints. The partial assembly is then removed from the vacuum line and solvent reservoir, tared, and reattached to the vacuum line and solvent reservoir. After the ampule is evacuated, the appropriate amount of solvent is transferred into the ampule and the solution stirred to mix all components. The partial assembly is then reweighed to determine the amount of solvent added. At this point, the cell body, the cell cap, and the inlet adapter are connected and the whole assembly is attached to the vacuum line (see Figure 2). Four freeze-pump-thaw cycles under argon are performed to remove all traces of oxygen. The solution is transferred into the cell under an argon atmosphere. In the spectroelectrochemical and thin-layer electrochemical experiments the atmosphere relief needle is opened to allow the argon pressure within the cell to equalize with atmosphere pressure. This is necessary since solution has to be drawn into the OTTLE and an imbalance in pressures would muse ambient atmosphere to be drawn into the cell. As depicted in Figure 2, the assembly is slowly rotated counterclockwise about the axis perpendicular to the plane of the figure and the solution allowed to pass into the cell to the desired level. With solution in the cell and valve (F) of the inlet adapter closed, the entire cell is removed from the ampule at the inlet adapter/ampule connection. Once the cell is connected to the appropriate in-
C
-_ - ARGON _
A
3
7i U
Figure 2. The vacuum electrochemical cell (A) Is shown attached to the solution ampule (B) and solvent header (C), which in turn Is attached to the vacuum llne (D). Teflon vacuum stococks (E 4- F) are used to Isolate the contents of the apparatus. strument, the two sections of tubing attached to the hypodermic needles are connected to their respective lines (argon or suction line). In bulk experiments the solution can be delivered without argon/atmosphere equalization.
RESULTS AND DISCUSSION The utility of the cell described here and its effectiveness in providing an oxygen-free atmosphere were evaluated from electrochemical measurements of MV in PC. Two separate
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
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Table I. Determination of the Number of Electrons ( n ) for Charge ( Q )Passed in Thin-Layer Controlled Potential Coulometry of 1.04 mmol/L Methyl Viologen in Propylene Carbonate runa
Qfonvard
x
400
500
600
700
%orward
3
0.87 0.95 0.93 0.92 k0.04
4 5 6
4.008 3.953 3.898
1.96 1.94 1.91 1.94 k0.03
av std dev
av std dev
c
1.780 1.935 1.896
1 2
a For runs 1, 2, and 3, the potential steps were -0.970: -1.438:-0.970V vs. AgIAgC10,. For runs 4, 5, and 6, the potential steps were -0.970:-1.945:-0.970 V vs. Background Ag/AgClO,. Cell volume = 20.3 pL. charge was found to be negligibly small.
Wavelength, nm
Figure 3. Spectra of 1.04 mmoi/L methyl viologen in an optically : (1) open circuit, transparent thin-layer electrode for potentials E, (2) -1.120, (3) -1.157, (4) -1.182, (5) -1.207, ( e 1 . 2 2 2 , (7) -1.232, (8) -1.242, (9) -1.257, (10) -1.282, (11) -1.307, (12) -1.342 V VS. Ag/AgCIO,.
charge transfer process are seen in the cyclic voltammogram and have been assigned (8) to E,O‘
MV2+ + e- eMV+.
(1)
E p
MV+. + e- eMVO
(2)
Formation of the methyl viologen radical cation produces a dark violet-blue solution, the ultraviolet and visible absorption spectrum of which has been reported (9). The spectra of the methyl viologen radical cation can be seen to grow as the applied potential is made progressively more negative (Figure 3). By use of the absorbance changes at 610 nm, the log ([O]/[R]) vs. Eapplied plot is linear, with a slope of 0.059 indicating the first reduction process is in fact a le- reduction of MV2+ to MV+. (see eq 1). When the OTTLE functions as a thin-layer cell only, it is a rapid means of obtaining controlled potential coulometric data on relatively small amounts of solution. In a typical charge-time curve, total electrolysis is completed in approximately 200 s. Renewal of the solution within the OTTLE is useful for multiple coulometric experiments on the same solution where statistical information is required. Solution renewal is easily and rapidly accomplished by drawing the solution up through the OTTLE while inert gas is allowed to replace the displaced solution. The large iR drop associated with nonaqueous solvents, although still present, is minimized by utilizing a thin-layer cell which has a small electrode area. Occurrence of edge effects with the smaller electrode area employed with the OTTLEs in this work is not evident in either the charge-time or spectropotentiostatic experiments. It has been demonstrated that oxygen can oxidize the methyl viologen radical cation to the dication (10,11). If oxygen were entering the cell, the n values of MV would vary significantly from integer values. Table I gives the results of thin-layer controlled potential coulometry performed with the OTTLE for several experiments on the same solution over the span of 7 h. No evidence of any contamination by oxygen was observed.
A great deal of mobility has been obtained by enclosing the OTTLE in a vacuum cell and minimizing its size. Since the system is closed and under an oxygen-free atmosphere, the cell can be transported easily without the need of continuous purging. This is especially helpful in laboratories where the solution preparation facilities are remote from the spectrophotometer. In addition, the lack of continuous purging removes the possibility of trace impurities in the purging gas accumulating in the solution. The small size of the cell simplifies the amount of modification needed to adapt the cell to the sample compartment of conventional spectrophotometers. Although the apparatus was designed principally for use as a spectroelectrochemicalcell, it has the flexibility to be used in bulk electrochemical experiments. The cell body, which provides a vacuumtight envelope for the OTTLE in spectroeletrochemical experiments, serves as a bulk electrochemical cell by merely changing the components of the cell cap. The conversion is accomplished by removing the OTTLE apparatus and inserting the electrode assembly (Figure 1). Bulk controlled potential electrolysis can be implemented as well by mounting a foil working electrode in one of the unused cell cap ports. The centrally located disk electrode may then be employed as an indicator electrode to follow the process of the electrolysis.
LITERATURE CITED (1) Bard, A. J.; Faulkner, L. R. “Electrochemical Methods: Fundamentals and Applications”; Wiiey: New York, 1980; Chapter 14. (2) Heineman, W. R. Anal. Chem. 1978, 50, 390 A-402 A. (3) Rohrbach, D. F.; Heineman, W. R.; Deutsch, E. Inorg. Chem. 1979, 78, 2536-2542. (4) Hurst, R. W.; Heineman, W. R. Deutsch, E. Inorg. Chem. 1981, 20, 3298. (5) Rhodes, R. K.; Kadish, K. M. Anal. Chem. 1981, 53, 1539-1541. (6) Hawkridge, F. M.; Pernberton, J. E.; Mount, H. N. Anal. Chem. 1977, 49, 1646-1647. (7) Klrowa-Eisner, E.; Gilead, E. J . flectroanal. Chem. 1970, 25, 481-487. (8) Kuwana, T.; Wlnograd, N. I n “Electroanaiytlcal Chemistry”; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, p 64. (9) Kosower, E. M.; Cotter, J. L. J . Am. Chem. SOC. 1964, 86, 5524-5527. (IO) Hawkridge, F. M.; Kuwana. T. Anal. Chem. 1973, 45, 1021-1027. (11) Sullivan, B. P.; Salmon, D. J.; Meyer, 1.J. Inorg. Chem. 1979, 78, 3369-3374.
RECEIVED for review July 28,1981. Accepted September 23, 1981. Identification of commercial products does not imply endorsement by the National Bureau of Standards.
