Anal. Chem. 1994,66,503-509
Voltammetry of Ferrocene in Subcritical and Supercritical Chlorodifluoromethane Scott A. Olsen and Dennls E. Tallman' Department of Chemistty, North Dakota State University, Fargo, North Dakota 58 105-5516
The voltammetry of ferrocene in sub- and supercritical chlorodifluoromethane was investigated at platinum microelectrodes in the presence of millimolar concentrationsof tetraRbutylammoniumtetrafluoroborate. The diffusion coefficient cm2/s at 25 O C and of ferrocene increases from 2.36 X 5.20 MPa to 1.30 X lod cm2/s at 115 O C and 9.00 MPa. The analytical sensitivity increases from 12.8 to 62.2 pA/pM for the same change in fluid conditions. The ferrocene couple is shownto behave reversibly in the liquid and at liquidlike densities in the homogeneous supercritical fluid. The earliest report of supercritical fluid chromatography demonstrated the elution of nickel etioporphyrin in halocarbon solvents, one of which was chlorodifluoromethane (CDFM).' In recent years there has been a greater focus on the use of carbon dioxide in supercritical fluid chromatography because carbon dioxide has a relatively mild critical temperature (3 1 "C) and an accessible critical pressure (7.39 MPa or 1071 psi) and is nontoxic and inexpensive. However, the low dielectric constant of carbon dioxide (1.18 at the critical point) limits its usefulness with more polar analytes. Recent work has demonstrated that CDFM is superior to carbon dioxide as a solvent for supercritical fluid e x t r a ~ t i o nand ~ ! ~chromat o g r a p h ~of~ polar molecules. The physical properties of CDFM include a critical temperature (TJof 369.3 K (96.15 "C), a critical pressure (Pc)of 4.97 MPa (721 psi), a critical density of 0.522 g/cm3, a dipole moment of 1.44 D, and a dielectricconstant ranging from 7.51 at 5.0 MPa, 0 OC (liquid) to 2.31 at the critical point. Electrochemical detection is one of the more sensitive detection techniques for liquid chromatography. Anticipated increases in mass-transfer rates by diffusion in the supercritical fluid state should lead to even higher analytical sensitivities for supercritical fluid chromatography with electrochemical detection (SFC/ECD). However, the highly resistive nature of most fluids having moderate critical temperature and critical pressure precludes the use of conventional electrochemical detector designs employing working electrodes of millimeter dimension (macroelectrodes). Earlier work by Bard and coworkers with macro electrode^'-^ and with microelectrodes8 Klesper, E.; Corwin, A. H.; Turner, D. A. J. Org. Chem. 1962, 27, 700. Ong, C. P.; Lee, H. K.; Li, S. F. Y . Anal. Chem. 1990, 62, 1389. Li, S. F. Y.;Ong, C. P.; Lee, M. L.; Lee, H. K. J. Chromatogr. 1990. 515, r l I
JlJ.
(4) Hawthorne, S.B.; Langenfcld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992, 64, 1614. ( 5 ) Crooks, R. M.; Fan, F.-R. F.; Bard, A. J. J. Am. Chem. SOC.1984,106,6851. (6) McDonald, A. C.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1986, 90, 196. (7) Crooks, R. M.; Bard, A. J. J. Electroanal. Chem.-Interfacial Electrochem. 1988, 243, 117. (8) Cabrera, C. R.; Garcia, E.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochem. 1989, 260, 457.
0003-2700/94/0366-0503$04.50/0
0 1994 American Chemical Society
in polar supercritical fluids such as ammonia, water, and acetonitrile demonstrates the feasibility of supercritical fluid electrochemistry. These fluids are impractical for SFC/ECD because of their high critical temperatures and pressures. In addition, the corrosive nature of supercritical ammonia and water shortens equipment lifetime and adversely affects background currents. The use of a microelectrode (an electrode of micrometer dimension) permits electrochemical measurements in rather resistive media, including solvents with very low electrolyte levels, organic solvents, and The very small current flow at a microelectrode leads to tolerable potential control errors (or ohmic distortion, i&), even for rather large uncompensated resistance (R,,). Furthermore, microelectrodes exhibit enhanced mass transfer compared to macroelectrodes, a result of convergent diffusion, and this in turn results in increased current density, which generally translates into improved signal-to-background and signal-to-noise ratio~.'~ Wightman and co-workers examined the usefulness of microelectrodes in supercritical carbon dioxide and concluded that it is not possible to perform electrochemistry in neat supercritical carbon dioxide or even in supercritical carbon dioxide containing a small amount of supporting electrolyte.18 Rather, they found it necessary to add a polar modifier to enhance ionic conduction and demonstrated the use of water, methanol, a salt melt, and an ion exchange polymer to create ionic films at electrode surfaces.lg-22 A more polar solvent such as CDFM solvates and separates polar analytes more efficiently than carbon dioxide, without the addition of modifiers that make mobile-phase characteristics difficult to predict. Furthermore, the more polar solvent permits the dissolution of a small quantity of electrolyte, ~
~~~~
(9) Bond, A. M.; Flcischmann, M.; Robinson, J. J . Electroanal. Chem.Interfacial Electrochem. 1984, 168, 299. (10) Cooper, J. B.; Bond, A. M. J. Electroanal. Chem. Interfacial Elecrrochem. 1991, 315, 143. (1 1) Cooper,J. B.; Bond, A. M.; Oldham, K.B. J. Electroanal. Chem. Interfacial Electrochem. 1992, 331, 877. (12) Drew, S. M.; Wightman, R. M.; Amatore, C. A. J. Electroanal. Chem. Interfacial Electrochem. 1991, 317, 117. (13) Pendley, B. D.; Abrufia, H. D.; Norton, J. D.; Benson, W. E.; White, H. S. Anal. Chem. 1991,63, 2766. (14) Norton, J. D.; Anderson, S.A.; White, H. S. J. Phys. Chem. 1992, 96, 3 . (15) Bento, M. F.; Medeiros, M. J.; Montenegro, M. I.; Beriot, C . ;Pletchcr, D. J . Electroanal. Chem. Interfacial Electrochem. 1993, 345, 273. (16) Bond, A. M.; Pfund, V. B. J. Electroanal. Chem. Interfacial Electrochem. 1992, 335, 28 1. (17) Bond, A. M.; Bixler, J. W. Anal. Chem. 1986, 58, 2859. (18) Philip, M. E.; Deakin, M. R.; Novotny, M. V.;Wightman, R. M. J. Phys. Chem. 1981, 91, 3934. (19) Niehaus, D.; Philips, M.; Michael, A,; Wightman, R. M. J.Phys. Chem. 1989, 93, 6232. (20) Michael, A. C.; Wightman, R. M. Anal. Chem. 1989, 61, 270. (21) Michael, A. C.; Wightman, R. M. Anal. Chem. 1989, 61, 2193. (22) Niehaus,D. E.; Wightman,R. M.; Flowers,P. A. Anal. Chem. 1991,63,1728.
Analytical Chemisfry, Vol. 66, No. 4, February 15, 1994 503
i mm
Figure 1. Scanningelectronmicrograph of the surface of an electrode assembly. Visible as two small dots are separate 25-pmdiameter Pt disk electrodes. Also visible are the glass capillary, the Torr-Seal epoxy, and the surrounding stainless steel tube which serves as the quasi-reference/auxiliary electrode.
making possible voltammetric measurement at a microelectrode with only a modicum of ohmic distortion. In this paper we demonstrate the feasibility of performing electrochemistry in homogeneous supercritical CDFM and describe our initial results for the test compound, ferrocene, in sub- and supercritical chlorodifluoromethane. EXPERIMENTAL SECTION The CDFM manufactured by du Pont has a listed minimum purity of 99.8% and was used without further purification. Ferrocene (Aldrich) was sublimed prior to use. The supporting electrolyte was tetra-n-butylammonium tetrafluoroborate (TBATFB) from Fluka (puriss) and was recrystallized twice from 95% ethanol, dried in a vacuum oven, and stored in a vacuum desiccator. Cobaltocenium hexafluorophosphate (Aldrich) with a listed purity of 98% was used as received and was stored in a vacuum desiccator. The oxygen trap employed in some experiments was from J&W Scientific. Disk microelectrodeswere constructed by heat sealingeither a 10- (Goodfellow) or a 25-pm- (Aesar) diameter platinum wire into one side of a 8 capillary (World Precision Instruments), a capillary which is divided into two hemicyclinders by a glass partition. The other side of the capillary accommodated either a second working electrode (as in the electrode assembly shown in Figure 1) or a large metal wire of up to 250-pm diameter as a quasi-reference electrode. Among the metals tested as quasi-reference electrodes, platinum gave the best mechanical seal but the poorest stability as a reference, while silver wires could not be satisfactorilysealed in the glass. Experience has shown that tungsten is a reasonable compromise, although repeated experimental cycles (particularlylarge changes in temperature)eventually caused stress fractures in the glass around the tungsten. The majority of electrode assemblies constructed for this work had two working electrodes sealed in the 8 capillary. The finished capillary was epoxied into a stainless steel tube (0.125-in. o.d., 0.085in. i.d., Alltech) with Torr-Seal (Varian), an epoxy that does not swell or expand under the experimental conditions employed in this study. The stainlesssteel tube and cell acted 504
AnalyticalChemistry, Vol. 66, No. 4, February 15, 1994
as the auxiliary electrode in three-electrode experiments and as the quasi-referenceand auxiliary electrodein two-electrode experiments. Provided there were no trapped air bubbles in the capillary, these electrode assemblies withstood pressures greater than 20 MPa and temperatures up to 140 OC. An end-on view of a completed electrode assembly having two working electrodes is shown in Figure 1. The supercritical fluid cell was machined locally from a block of type 304 stainless steel and had an internal volume of 5.8 mL. The cell was sealed with a Teflon O-ring and a threaded stainless steel plug, through which a '/*-in. hole was bored for insertion of the electrode assembly. The electrode assembly was held in place using finger-tight fittings (Upchurch). The cell body was also machined to accept two cartridge heaters and a resistance temperature detector (Omega,Inc.). An SFC 500 Microflow pump (Ism) controlled the pressure, while an Omega CN 922 1 controller regulated the temperature. In addition, the cell was electricallyisolated from the pump by using poly(ether ether ketone) (PEEK) tubing, so that either two- or three-electrode electrochemistry could be performed. Since the cartridge heaters and the resistance thermal detector were in close contact with the cell, placement of the cell in a Faraday cage did little to reduce environmental noise. However, placing the cell in a Faraday cage greatly reduced temperature fluctuations due to drafts and thermal convection, and all experiments were performed in such a cage. An optical supercritical fluid cell, designed for fluorescence and absorbance measurements, was used for visual observations of phase behavior. This cell was locally machined from a block of type 3 16 stainless steel. The fluid chamber consists of two intersecting, perpendicular cylinders drilled through the center of the block. The ends of each cylinder were machined to accept a Teflon O-ring, a sappire window (Esco, Inc.), a Delrin O-ring, and a threaded hollow plug. To perform electrochemistry concurrent with visual observations, one of the sapphire windows was replaced with a locally machined plug that accepted the same electrode assembly described above. Prior to each experiment the electrode assembly was resurfaced on 600-grit sandpaper and polished to a mirror finish with 1- and 0.3-pm alumina (Buehler). The electrode was rinsed with deionized water (Millipore Milli-Q), wiped dry, and stored in a vacuum oven until used. To initiate an experiment, a small volume of a standard ferrocene solution in hexane was dispensed into the cell and the hexane solvent was allowed to evaporate. A weighed amount of TBATFB electrolyte was placed directly in the cell. The cell was sealed, the electrode was inserted, and gaseous CDFM was allowed to flow through the system for approximately 1 min at a reduced pressure to remove atmospheric oxygen, following which, valve 3 (Figure 2) was closed. The pump cylinder was chilled with ice until filled with liquid CDFM, valve 1 was closed, and the cell was warmed. After the cell reached operating temperature, the pressure was increased to the desired level. Sufficient time was allowed for solution equilibrium to be achieved, typically 30-60 min each time pressure or temperature was changed. The cyclicvoltammetric waveform was generated by a PAR Model 175 Universal programmer and applied to the cell via
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______---Figure 2. Block diagram of the supercritical fiuld electrochemistry system.
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a potentiostat built in-house. The current flow was measured with a Keithley 487 picoammeter and transferred over an IEEE-488 interface to a personal computer for storage and data analysis. A block diagram of the experimental configuration is shown in Figure 2. The results presented below were obtained in the twoelectrode mode. No significant advantage could be discerned from using the three-electrode approach. Indeed, as mentioned above, Pt, W, and Ag quasi-reference electrodeseach presented particular difficulties. Additionally, the availability of two working electrodes in each assembly proved advantageous, for if one became fouled, the other was available for use. Some electrode assemblies contained both 10- and 25-pmdiameter electrodes, permitting fluid resistance and kinetic effects to be probed. Overall, the two-electrode mode performed well and the stainless steel tube quasi-reference/ auxiliary electrode exhibited a remarkably stable potential ( f 2 mV) once thermal and pressure equilibrium was attained. Safety is always a concern when working at elevated pressures. The pressures employed in this work were modest, seldom exceeding 15 MPa (2200 psi). All components under pressure were rated to at least 69 MPa, the exceptions being the PEEK tubing (35 MPa) and the electrode assemblies (pressure rating unknown, but tested to 20 MPa). The Faraday cage described above was sufficiently robust to serve as a safety shield for the cell/electrode assembly. At the conclusion of an experiment, the nontoxic CDFM was collected by pumping the fluid to a collection tank attached to the exhaust side of valve 3 (Figure 2), where it was stored for commercial recycling.
RESULTS AND DISCUSSION Neat CDFM in the supercritical state is highly resistive. Initial two-electrode voltammetry in neat supercritical CDFM produced an ohmic response (Le., a linear i-V curve), the slope of which yielded a resistance of 2.3 GQ at 105 OC and 5.2 MPa. This resistancewas traced to theelectrodeassembly itself. An electrode assembly was positioned in an empty supercritical fluid cell so that the resistance as a function of temperature could be examined under noise conditions duplicating those of a typical experiment. The resistance between the platinum wire (forming the disk working electrode) and the stainless steel tube (counter/quasi-reference electrode) was greater than 600 GQat 25 OC. The resistance
25 OC, 5.2 MPa ........ 85OC,5.2Mpa
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Potential (V) Figure 9. Voltammograms of neat chlorodifluoromethane at 25 O C and 5.20 MPa and at 85 O C and 5.20 MPa; 50 mV/s at 25-pmdiameter Pt disk electrodes. Separate worklng electrodes were used for the positive and negatlve sweep dlrections, each sweep originating from 0 V. The inset is an expanded vlew of the voltammograms obtained at 85 O C and 5.20 MPa.
decreased as the temperature was increased, apparently the result of a decrease in the bulk resistivity of the epoxy and the borosilicate glass (0 capillary) with increasing temperature. The resistance fell to 1.2 GQ at 105 OC and 0.8 GQ at 115 OC where the slopes of the current-voltage curves were identical to those obtained with the cell filled with neat supercritical CDFM. This current leakage between electrodes was negligible for the majority of experiments performed in the presence of added TBATFB electrolyte, especially when the supercritical fluid density was great enough to solvate millimolar quantities of the electrolyte. However, the leakage current did contribute to a sloping baseline for voltammograms recorded on the most sensitive current ranges. Teflon-lined stainless steel tubing is currently being explored as one approach to increase the high-temperature electrical resistance between electrodes of the assembly. The use of an aluminosilicate capillary might further increase interelectrode resistance. Two-electrode cyclic voltammograms in neat CDFM in the liquid state did not exhibit a well-defined solvent oxidation limit at potentials up to +7.0 V. However, a reduction limit occurs at -1 -75V. The solid curve of Figure 3 shows a typical background voltammogram of the neat liquid at a 25-pm Pt disk, with the fluid at 25 OC and 5.20 MPa, the current density being less than 1 mA/cm* over the potential range +7 to -2 V. As the critical point is approached isobarically, poorly defined oxidation and reduction limits are observed (dashed curves of Figure 3), probably a consequence of the decreasing dielectric constant and increasing resistance of the fluid. The voltammogram of neat CDFM at 105 OC and 5.20 MPa (not shown) displays the ohmic behavior of the electrode assembly described above, implying that the fluid resistance has become so great that the dominant method of current flow is through the body of the electrode assembly. Interestingly, increasing the density (i.e., the pressure and, thus, the dielectric constant) of the supercritical fluid has little further effect on the ohmic response. Ana&ticalChemlstty, Vol. 86, No. 4, February 15, 1994
505
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poTENllAL(v) Flgurr 5. Voltammograms of 96.7 pM ferrocene In sub- and supercritical CDFM: electrolyte, 12.49 mM TBATFB scan rate, 20 mV/s; working electrode, 10-pm Pt disk. Voltammograms were recorded at 26 "C and 5.20 MPa, at 105 "C and 5.20 MPa, and at 105 "C and 15.00 MPa.
The resistive and electrochemical behavior of the fluid changes dramatically when a small amount of TBATFB (approximately 10 mM) is added as a supporting electrolyte (Figure 4). The small wave under liquid conditions at ca. -1.75 V (solid curve, Figure 4) appears to be due to an adventitious impurity and corresponds closely to the shoulder wave in the voltammogram of the neat liquid revealed by careful inspection of the solid curve of Figure 3. The impurity is very likely molecular oxygen since the wave dissappears when an oxygen trap is placed in-line between the CDFM tank and valve 1 (Figure 2). In any event, the potential range in the presence of TBATFB within which the current density is less than 1 mA/cm2 is from +2.8 to-1.5 V (Figure 4), and to -2.0 V with the impurity wave removed. This potential window is comparable to that reported for methylene chloride.23 The potential window remains virtually unchanged (within 100 mV) as temperatures and pressures approach and surpass the critical point (Figure 4). This is in contrast to other supercritical fluids such as ammonia, water, acetonitrile, and sulfur di0xide,5~for which the potential window becomes considerably smaller as the temperature is ihcreased. Ferrocene was selected as the test analyte in these studies because it has been studied in a wide variety of solvents, permitting comparisons with the results obtained from this work in CDFM. Furthermore, ferrocene is a frequently used internal reference couple in nonaqueous solvents and may prove useful as such in future work in CDFM. Figure 5 shows a comparison of the ferrocene wave under three different temperature and pressure conditions. At 5.20 MPa and 26 "C CDFM is a liquid, so the wave under these conditions can be compared with the ferrocene wave in other liquid solvents. A plot of E versus ln((id - i)/i) for the wave shown has a value of 0.0264 V, only slightly greater than the Nernstian value of 0.0258 V, demonstrating that under normal liquid conditions ferrocene is reversible in CDFM. Bond and c0-workers2~ (23) Coutagne, D. M. Bull. SOC.Chim. Fr. 1971.5, 1940. (24) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.:Mann, T. F.; Thormann, W.; Zoski, C.G.Anal. Chem. 1988,60, 1878.
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examined the behavior of ferrocene in methylene chloride, a solvent with a dipole moment and dielectric constant similar to CDFM. They found that ferrocene is reversible over a wide temperature range in methylene chloride when a suitably small microelectrode is used. It was not possible to obtain reproducible electrochemical behavior when the fluid conditions were near the critical point, presumably because the critical temperature and pressure are very sensitive to the fluid composition (amount of electrolyte added), making the precise control of density and dielectric constant extremely difficult. Indeed, a binary phase is observed at 105 "C and 5.20 MPa, even when only small amounts (
