Anal. Chem. 1998, 70, 2421-2425
UV-Visible Spectroscopic Measurement of Solubilities in Supercritical CO2 Using High-Pressure Fiber-Optic Cells M. J. Carrott and C. M. Wai*
Department of Chemistry, University of Idaho, Moscow, Idaho 83843
The design and construction of a microscale, fiber-opticsbased system for the measurement of solubilities in supercritical CO2 by UV-visible spectroscopy is described. This system consists of three high-pressure fiberoptic cells, with path lengths ranging from 38 µm to 1 cm, constructed from standard 1/16-in. stainless steel fittings and silica fibers. It is capable of withstanding pressures in excess of 300 atm, and spectra over the entire UV-visible range (200-900 nm) can be obtained. Use of three cells with different path lengths enables compounds of high or low solubility to be measured over a concentration range of several orders of magnitude. The solubility of a uranium complex, UO2(tta)2‚TBP, in supercritical CO2 at 40 °C and over the pressure range 100-325 atm was determined, and it was found to be possible to attain solubilities in excess of 10-2 M for metal species in unmodified supercritical CO2. Also, the small volume of this system allows solubilities to be measured with relatively small amounts of compounds. Supercritical fluid extraction (SFE) has been applied to a wide range of samples and analytical problems1 and is now an acceptable alternative to traditional solvent extraction methods. More recently, SFE has been applied to the extraction of metal species2-7 from solid and liquid matrixes. The key to solvating metal ions in supercritical CO2 lies in neutralization of the charge on the metal by the use of an appropriate ligand and the formation of a complex with an appreciable solubility in the supercritical fluid. Ligands such as lithium bis(trifluoroethyl)dithiocarbamate2,3 and ionizable crown ethers4 have been used for the extraction of Cu2+ and Hg2+. The most widely used ligands are the β-diketones, such as thenoyltrifluoroacetylacetone (tta), and organophosphorus reagents, such as tributyl phosphate (TBP), which have been successfully applied to the extraction of lanthanides and actinides (1) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1996, 68, 487R514R. (2) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Anal. Chem. 1992, 64, 2875-2878. (3) Wai, C. M.; Lin, Y.; Brauer, R.; Wang, S.; Beckert, W. F. Talanta 1993, 40, 1325-1330. (4) Wang, S.; Elshani, S.; Wai, C. M. Anal. Chem. 1995, 67, 919-923. (5) Lin, Y.; Wai, C. M. Anal. Chem. 1994, 66, 1971-1975. (6) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Anal. Chem. 1993, 65, 2549-2551. (7) Lin, Y., Smart, N. G.; Wai, C. M. Environ. Sci. Technol. 1995, 29, 27062708. S0003-2700(97)01077-9 CCC: $15.00 Published on Web 05/01/1998
© 1998 American Chemical Society
from liquid and solid matrixes.5-7 This technology has obvious benefits in the area of nuclear waste remediation and cleanup of toxic metals, as it has the potential to provide a solvent-free extraction process, thus minimizing or eliminating the production of secondary wastes. To assess the feasibility of using this technique for metal extraction, develop theoretical models for the extraction process, and optimize extraction conditions, several fundamental parameters need to be measured, the most important of which is the solubility of the metal chelates. A recent article reviewed the solubility data currently available for 49 metal-containing compounds and 15 ligands in supercritical CO2.8 The solubility of metal chelates in supercritical fluids is dependent on several factors, including pressure and temperature of the fluid, use of modifiers, ligand, identity of the metal, and oxidation state of the metal ion. It has been shown that metal complexes containing fluorinated ligands are more soluble than their nonfluorinated analogues, often by several orders of magnitude,9,10 whereas complexes with phenyl-substituted ligands show reduced solubilities.10 Solubility has also been shown to be dependent on oxidation state (for β-diketone complexes of Mn2+/Mn3+ and Co2+/Co3+), with the higher oxidation states showing the greater solubility.11 This can be attributed to the improved shielding of the central metal ion by the additional ligands and the increased solute-solvent interactions. The solubility data for these metal chelates span a concentration range over 8 orders of magnitude, and the most soluble compound reported to date is chromium(III) dipivaloylmethane (Cr(thd)3), which has a solubility of 0.126 M at 310 atm and 40 °C.10 A range of techniques has been used to measure the solubility of metal chelates in supercritical fluids including gravimetric,12 spectroscopic,9,10,13 and chromatographic14 methods. Gravimetric and chromatographic methods are usually time-consuming; the (8) Smart, N. G.; Carleson, T.; Kast, T.; Clifford, A. A.; Burford, M. D.; Wai, C. M. Talanta 1997, 44, 137-150. (9) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. J. Supercrit. Fluids 1991, 4, 194-198. (10) Laglante, A. F.; Hansen, B. N.; Bruno, T. J.; Sievers, R. E. Inorg. Chem. 1995, 34, 5781-5785. (11) Saito, N.; Ikushima, Y.; Goto, T. Bull. Chem. Soc. Jpn. 1990, 63, 15321534. (12) Wai, C. M.; Wang, S.; Yu, J. Anal. Chem. 1996, 68, 3516-3519. (13) Hansen, B. N.; Laglante, A. F.; Sievers, R. E.; Bruno, T. J. Rev. Sci. Instrum. 1994, 65, 2112-2114. (14) Cowey, C. M.; Bartle, K. D.; Burford, M. D.; Clifford, A. A.; Zhu, S.; Smart, N. G.; Tinker, N. D. J. Chem. Eng. Data 1995, 40, 1217-1221.
Analytical Chemistry, Vol. 70, No. 11, June 1, 1998 2421
former also lacks sensitivity for low-solubility compounds in addition to requiring large amounts of solute. In situ spectroscopic measurements allow rapid determination of solubilities and require relatively small amounts of the compound. However, it requires the construction of a precision-engineered view cell, containing one or more optically transparent windows to enable the fluid to be probed using an appropriate technique. These cells are generally difficult and time-consuming to clean; the windows are expensive and difficult to replace if damaged or dirty and are prone to window seal-related leaks. Since the metal complexes exhibit solubilities spanning over 8 orders of magnitude, the use of fixed path length optical cells limits both the concentration range and compounds that can be measured and imposes restrictions on the temperature and pressure range that can be investigated. To overcome the problems associated with conventional highpressure cells, several groups have constructed cells based on fiber optics15-23 to probe supercritical solutions with a range of spectroscopic techniques. Applications have included fluorescence measurements of organic compounds,15,20 use of Raman spectroscopy to monitor reactions in supercritical water21 and dissolved gases (H2, N2) in supercritical CO2,22 and, more recently, real-time monitoring of SFE by FT-IR16 and fluorescence17 spectroscopy. These cells are considerably smaller, with volumes from 100 nL to 100 µL, compared to several milliliters for conventional high-pressure cells. They require very little in terms of precision engineering and are frequently constructed from commercially available chromatographic fittings. Additionally, the use of fiber optics alleviates many of the problems associated with cleaning or replacing spectroscopic windows, and the path length of the cell can be easily adjusted. In this work, we report the development of a fiber-optic-based system for the determination of solubilities in supercritical CO2 using UV-visible spectroscopy. Three fiber-optic cells with different path lengths were constructed and connected in series, thus facilitating measurement of compounds with high or low solubility over a wide concentration range. EXPERIMENTAL SECTION The components of the apparatus are illustrated in Figure 1. An Isco syringe pump, model 260D (Isco, Lincoln, NB), was used to supply CO2 at the desired pressure to a saturation cell containing the solute. Supercritical CO2 was introduced via a 1.5-m (1/16-in.-o.d. × 0.03-in.-i.d.) stainless steel equilibration coil, to ensure the CO2 was at the correct temperature prior to entering (15) Zagrobelny, J.; Li, M.; Wang, R.; Betts, T. A.; Bright, F. V. Appl. Spectrosc. 1992, 46, 1895-1897. (16) Heglund, D. L.; Tilotta, D. C.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1994, 66, 3543-3551. (17) Tena, M. T.; Luque de Castro, M. D.; Valcarcel, M. Anal. Chem. 1996, 68, 2386-2391. (18) Tena, M. T.; Valcarcel, M. J. Chromatogr., A 1996, 753, 299-305. (19) Fjeldsted, J. C.; Richter, B. E.; Jackson, W. P.; Lee, M. L. J. Chromatrogr. 1983, 279, 423-430. (20) Rice, J. K.; Dunbar, R. A.; Bright, F. V. Appl. Spectrosc. 1994, 48, 10301032. (21) Myrick, M. L.; Kolis, J.; Parsons, E.; Chike, K.; Lovelace, M.; Scrivens, W.; Holliday, R.; Williams, M. J. Raman Spectrosc. 1994, 25, 59-65. (22) Howdle, S. M.; Stanley, K.; Popov, V. K.; Bagratashvili, V. N. Appl. Spectrosc. 1994, 48, 214-218. (23) Rhodes, T. A.; Fox, M. A. Appl. Spectrosc. 1997, 51, 358.
2422 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
Figure 1. Schematic diagram of fiber-optic system for solubility measurements in supercritical fluids: (P) pump; (S) switching valve; (V) 3.5-mL saturation vessel; (E) equilibration coil; (F) input and output optical fibers connecting high-pressure cell to the UV spectrometer; (C1) 38-µm path length cell; (C2) 1-cm path length cell; (C3) 733-µm path length cell; (R) restrictor; (RV) restrictor valve.
the cell. A 3.5-mL saturation vessel (Keystone Scientific, Bellafonte, PA) was connected to a Rheodyne six-port valve, which enabled the cell to be switched in or out of the flow path without the need for depressurizing the entire system and also facilitates the cleaning and flushing of the fiber-optic cells. In situ solubility measurements were performed by three high-pressure UV-visible cells, with different path lengths, connected in series to the switching valve. Flow of the saturated supercritical solution through the optical cells was controlled via a high-pressure valve (HIP, Erie, PA) connected to the outlet of the optical cell manifold. Pressure was maintained in the system using a stainless steel restrictor manufactured from 1/16-in. × 0.01-in.-i.d. tubing and crimped to give a flow of 100 µL/min at 300 atm and room temperature. The tip of the restrictor was housed in a heated aluminum block to minimize plugging during depressurization of the saturated supercritical solution. All components of the apparatus were housed in an Eldex HPLC oven (Bodman Industries, Aston, PA) to allow precise control of the temperature ((0.1 °C). UV-Visible Spectrometer. A Cary 1E UV-visible spectrometer (Varian Instruments, Sugarland, TX) was used for all measurements in this work. The instrument was fitted with the fiber-optics interface for the Cary 1E UV-visible spectrometer, which is simply placed in the sample compartment to focus the beam from the source into the input fiber and from the output fiber to the detector. All absorption spectra were collected under identical conditions; scanning from 200 to 800 nm, at a rate of 900 nm/min, with a data interval of 1 nm. The spectrometer was operated in double-beam mode for all experiments. Instrument control and data collection were performed using the Cary FITF (version 3.04) software installed on a Pentium PC. Fiber-Optic Cells. The high-pressure UV-visible cells were constructed from commercially available 1/16-in. stainless steel Valco unions and fittings (Valco Instruments, Houston, TX). The optical fibers (Polymicro Technologies, Phoenix, AZ) used for this work comprised a 600-µm-high OH- silica core surrounded by silica cladding and a polyimide buffer, with a total outer diameter of 710 µm and a numerical aperture of 0.22. Six fibers were obtained in 1-m lengths, and the ends were polished to be defectfree at 40× magnification. The proximal end of each fiber was fitted with an SMA 906 connector for coupling to the Cary 1E UV-visible fiber-optics interface, and the distal end was sealed into a high-pressure cell. The short path length (
