Art ides Anal. Chem. 1994,66, 3553-3557
Instrument for Evaluating Phase Behavior of Mixtures for Supercritical Fluid Experiments Steven H. Page,' Janet F. Morrison, Richard G. Christensen, and Steven J. Choquette Organic Analytical Research Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Fluid phase behavior has a profound impact on supercritical fluid chromatography performance. The effect of fluid phase behavior on supercritical fluid extraction performance has not been studied. Generally, methods available to generate phase diagrams of fluid mixtures are time-consuming. This paper describes relatively easily constructed instrumentationfor the rapid screening of fluid mixtures to determine whether or not they are single phase. Either scattered or transmitted light can be used to probe the phase behavior of the fluids. Examples using COz/methanol and COJwater are presented. The phase behavior of multicomponent mixtures of C02 or Freon (R22, chlorodifluoromethane) with methanol/triethylamine/water was determined. If thep-T-x conditions employed during supercritical fluid chromatography (SFC) produce phase separation, then solute retention and mobile phase flow become erratic, the baseline becomes noisy, and solute peak shapes become irregular and widened. l4 Although the solubility of analytes and cosclvents in supercritical fluids has been in~estigated,~-~ the effect of the phase behavior of the extraction fluid on supercritical fluid extraction (SFE) performance has not been studied. Phase separation occurs when the saturation limits of a modifier (cosolvent) are exceeded in a mixed mobile phase. Modifiers are added to a primary fluid (often C02) to improve the solvation power (SFC and SFE) and the extraction efficiency (SFE) of polar and high molecular weight analytes. Berger, T. A.; Deye, J. F. Chromatographia 1991, 31, 529-534. Page, S. H.; Goates, S. R.; Lee, M. L. J. Supercrit. Fluids 1991.4, 109-1 17. Page, S . H.; Raynie, D. E.; Goates, S. R.; Lee, M. L.; Dixon, D. J.; Johnston, K. P. J . Microcolumn Sep. 1991, 3, 355-369. (4) Page,S. H.;Sumpter,S.R.;Lee, M.L. J. MicrocolumnSep. 1992,4,91-122. (5) Foster, N. R.; Yun, S . L. J.; Ting, S . S . T. J. Supercrit. Fluids 1991, 4, 127-1 30. (6) Mackay, M. E.; Paulaitis, M. E. Ind. Eng. Chem.Fundam. 1979,18,149-153. (7) Czubryt, J. J.; Myers, M. N.; Giddings, J. C. J. Phys. Chem. 1970, 74,426C4266. (8) Lemert, R. M.; Johnston, K. P. Fluid Phase Equilib. 1989, 45, 265-286. (9) Mitra, S.; Chen, J. W.; Viswanath, D. S . J . Chem. Eng. Data 1988,33,35-37. This article not subject to US. Copyright. Published 1994 by the American Chemical Society
Unless thepT-x conditions at which phase separation occurs are known, saturation limits can be exceeded within the pressure (8-60 MPa), temperature (40-200 "C), and concentration (0-20 mol % modifier) regions where SFC and SFE are routinely perf~rmed.~ Although there is an enormous amount of fluid phase equilibria data in the literature, the phase behavior of only 8 of 38 binary CO2-based mixtures used to date in SFC has been sufficiently evaluated in the p-T-x space described ab~ve.~JOThe systems that have sufficient p T - x data include binary mixtures of C02 with acetone,11- 13 benzene:, 14-2 1 ethanol,4,13,16,22-25 hexane:, 14,18,26,27 m e t h a n ~ l , * , ~ J ~t .o*l~~- ~e ~n e ,tri-n-butyl ~ , ~ ~ ~phosphate,' and (10) Page, S.H. Ph.D. Dissertation, Brigham Young University, Provo, UT, 1992. (1 1) Page, S . H.; Sumpter, S . R.; Goates, S. R.; Lec, M. L. J. Supercrit. FIuids 1993, 6, 95-101. (1 2) Katayama, T.; Ohgaki, K.; Maekawa, G.; Goto M.; Nagano, T. J. Chem. Eng. Jpn. 1975,8, 89-92. (13) Panagiotopoulos, A. Z.; Reid, R. C. In Supercritical Fluids, Chemical and Engineering Principles and Applications; Squires, T. G., Paulaitis, M. E., Eds.; ACS Symposium Series 329; American Chemical Society: Wahsington, D.C., 1987, Chap. 10. (14) Ohgaki, K.; Katayama, T. J. Chem. Eng. Data 1976, 21, 53-55. (15) Hicks, C. P.; Young, C. L. Chem. Rev. 1975, 75, 119-175. (16) Nagahama, K.; Suzuki, J.; Suzuki, T. Proceedings of the International Symposium onSupercritical Fluids; Nice, France, 1988; Vol. 1, pp 143-156. (17) Takenouchi, S.; Kennedy, G. C. Am. J. Sci. 1964, 262, 1055-1074. (18) Kaminishi, G.-I.; Yokoyama, C.; Takahashi, S. Fluid Phase Equilib. 1984, 34, 83-99. (19) Kim, C.-H.; Vimalchand, P.; Donohue, M. D. Fluid Phase Equilib. 1986,31, 299-3 1 1. (20) Inomata. H.: Arai. K.: Saito. S . Fluid Phase Eauilib. 1987. 36. 107-1 19. (21) Gupta, M. K.; Li,Y.-H.; Hulsey, B. J., Robinson, R. L., Jr: J. Chem. Eng. 1982, 27, 55-57. (22) Suzuki, K.; Sue, H.; Itou, M.; Smith, R. L.; Inomata, H.; Arai, K.; Saito, S. J. Chem. Eng. Data 1990, 35, 63-66. (23) Suzuki, K.; Tsuge, N.; Nagahama, K. Fluid Phase Equilib. 1991.67, 213226. (24) Takishima, S.; Saito, K.; Arai, K.; Saito, S . J. Chem. Eng. Jpn. 1986, 19, 48-56. (25) Feng,Y.S.;Du,X.Y.;Li,C.F.;Hou,Y.J.InProceedingsoftheInternationaI Symposium on Supercritical Fluids; Nice, France, 1988; pp 75-84. (26) Li, Y.-H.; Dillard, K. H.; Robinson, Jr., R. L. J. Chem. Eng. Data 1981,26, 53-56. (27) Wagner, Z . ; Wichterle, I. Fluid Phase Equilib. 1987, 33, 109-123. (28) Ambrose, D.; Sprake, C. H. S.; Townsend, R. J. J . Chem. Thermodyn. 1975, 7, 185-190.
Analytical Chemistry, Vol. 66,No. 21, November 1, 1994 3553
~ a t e r . ~ J 5 J ~Estimation $ ~ ~ 6 methods used to predict either the critical properties or gas-liquid equilibrium of mixtures in the near-critical region are often ~nreliable.~ Finally, the concentration of the mobile phases or extraction fluids employed in SFC and SFE are often expressed in volume percents. It is often impossible to convert these values into mole percents due to the highly nonlinear mixing of fluids in the near-critical region and a lack of density data for the mixtures. For example, under the experimental conditions in this paper, a 9.7 vol % mixture of COz + methanol was equivalent to 13.2 mol % methanol at 39.48 MPa and 68.5 mol % at 5.92 MPa. A semiautomated system for evaluating the phase behavior of fluid mixtures has recently been described.47 Although this system is well-suited for efficient phase behavior mapping of binary mixtures, an inordinate amount of time would be required to sufficiently evaluate the phase behavior of multicomponent mixtures (mixtures containing a modifier with one or more additives or derivatizing reagents) prior to method development for analytical SFC or SFE. Other disadvantages of the previously reported system are the custom machining required for some of the components, the requirement that a single phase be established at the beginning of a run, and problems with seal integrity. Teflon will flow under pressure, resulting in an explosion hazard due to extrusion of the O-rings from the view cell when high temperatures and high pressures are employed. The large volume (approximately 30 mL) of highly compressible fluid in the view cell exacerbates the problem. A complete phase diagram of a fluid mixture is valuable for routine method development in SFC or SFE. Phase separation could be avoided by programming the p-7'-x conditions of the SFC or SFE experiment to avoid regions delineated in the phase diagram as inhomogeneous. However, the phase behavior of multicomponent mixtures can be extremely complicated and thorough mapping may be very time-consuming. Because a homogeneous, single-phase fluid is desirable for operation in SFC, the purpose of this paper is to describe an inexpensive instrument that can rapidly and accurately determine whether or not a multicomponent fluid exists in this condition. Briefly, modifier is delivered into a (29) Bezdel, L. S.; Teodorovich, V. P. Gazov. Promst. 1958, 8, 3 8 4 3 . (30) Brunner, E. J. Chem. Thermodyn. 1985, 17, 671-679. (31) Chang, T.; Rousseau, R. W. Fluid Phase Equilib. 1985, 23, 243-258. (32) Hong, J. H.; Kobayashi, R. Fluid Phase Equilib. 1988, 41, 269-276. (33) Krichevskii, I. R.; Lebedeva, E. S. Zh. Fiz. Khim. 1947, 27, 715-718. (34) Ohgaki, K.; Katayama, T. J . Chem. Eng. Data. 1976, 21, 53-55. (35) Robinson, D. B.; Peng, D.-Y.; Chung, S. Y.-K. Fluid Phase Equilib. 1985, 24, 25-41. (36) Shenderei, E. R.; Zelvenskii, Y. D.; Ivanovskii, F. P. Khim. Promst. 1959,4, 328-331. (37) Takenuchi, K.; Matsumura, K.; Yaginuma, K. Fluid Phase Equilib. 1983,14, 255-263. (38) Weber, W.; Zeck, S.; Knapp, H. Fluid Phase Equilib. 1984, 18, 253-278. (39) Semenova, A. I.; Emel'yanova, E. A.; Tsimmerman,S. S.;Tsiklis, D. S. Rum. J . Phys. Chem. (Engl. Transl.) 1979, 53, 1428-1430. (40) Morris, W. D.; Donohue, M. D. J . Chem. Eng. Data 1985, 30, 259-263. (41) Sebastian;H. M.; Simnick, J. J.; Lin, H.-M.; Chao, K.-C. J. Chem. Eng. Data 1980, 25, 246-248. (42) Kim, C. H.; Vimalchand, P.; Donohue, M. D. Fluid Phase Equilib. 1986,31, 299-3 1 1 . (43) Ng, H.-J.; Robinson, D. B. J. Chem. Eng. Data 1978, 23, 325-327. (44) Wiebe, R. Chem. Reu. 1941, 29, 4 7 5 4 8 1 . (45) Brunner, G. Thesis (for professorship), University or Erlangen, Nurnberg, Germany, 1978. (46) Briones, J. A.; Mullins, J. C.; Thies, M. C. Kim, B.-U. Fluid Phase Equilib. 1976, 36,235-246. (47) Page, S . H.; Yun, H.; Lee, M. L.; Goates, S. R. Anal. Chem. 1993,65, 14931495.
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Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
Ferrule
Figure 1. Schematic diagram of the modified stainless steel cross.
view cell, pressurized with the primary fluid, and monitored to determine the miscibility of the modifier in the fluid. EXPERIMENTAL SECTION View Cell. A view cell was constructed by simple modifications made to a 6.4 mm (1/4 in.) stainless steel cross (Swagelok Co., Solon, OH), as illustrated in Figure 1. Briefly, two of the connections were milled flat to enable the seating of 6.4-mm-diameter X 1.5-mm (l/16 in.)-thick sapphire windows (Meller Optics, Providence, RI). Because a cross configuration is used, the milled connections can be at right angles to each other for 90' sampling of scattered light or at 180' angles to each other for transmitted light (on-axis) or scattered light (off-axis) analyses. In this work the 180' configuration was used. Delrin disks [6.4-mm 0.d. X 4.5mm-(3/16in.) i.d X 1-mm thick] were used as seals. A brass tube (6.4-mm 0.d. X 4.5-mm i.d.) with an integral flange was used to seat the seals against the windows. To minimize the volume in the cross, 1.5-mm-0.d. X 0.15-mm4.d. stainless steel tubing was silver-soldered into a 6.4-mm-0.d. X 1.5mm-i.d. stainless steel cylinder. The cylinder and associated stainless steel tubing were positioned on the outlet, just outside theoptical path of theview cell, resulting in an internal volume of 1.03 mL. In order to precisely position the ferrules and prevent slipping, the back ferrule was silver-soldered to the stainless steel. Swagelok reducing fittings were used at the fluid introduction port of the view cell. Safety Considerations:Teflon O-rings are not satisfactory for high-pressure and high-temperature applications as explained above. Delrin disks proved to be much more stable but tended to deform with increasing temperature changes. Whenever the oven temperature was reduced (even moderately), the nuts holding the windows in place were tightened to prevent leakage. The pressure rating for the sapphire windows and stainless steel cross are well above the operating conditions described in this paper. The oven door was always secured prior to pressurization of the cell. Despite the fact that the system described here operated without incident for more than 1 month, all visual observations were performed from behind appropriate shielding. Finally, chemical compatibility between the fluid mixtures and the polymeric seals must be ensured before pressurizing the system. Instrumentation. The instrumentation used in these experiments is shown schematically in Figure 2 and consisted
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Figure 2. Schematic diagram of the instrumentation used for phase behavior analysis.
of a source cylinder of COz, an SFC syringe pump (Model 50 1, Dionex Corp., Sunnyvale, CA), a high-pressure external loop injector (Model 7125, Rheodyne Inc., Cotati, CA) for introduction of 100 pL of modifier into the view cell, a GC oven (Model 3700, Varian, Sunnyvale, CA), the view cell described above, a two-way needle valve (SSI, High Pressure Equipment Co., Erie, PA) for stop flow control to allow for static equilibration of the cell contents, and a vent line. The associated optics and electronics include a He/Ne laser (Hughes Aircraft Co., Torrance, CA), a mirror for directing the beam through the cell windows, a glass rod (3mm diameter) for collection of scattered or transmitted light, a silicon photodiode operated in the photovoltaic mode, a transimpedance amplifier (Model 1OlC, UDT Sensors, Hawthorne, CA) for amplification of the photodiode signal, an A/D converter (Model 960, PE Nelson, Cupertino, CA) for signal digitization at 10 Hz, and data acquisition hardware and software (Turbochrom, PE Nelson) for signal processing. Evaluation of Fluid Phase Behavior. The view cell was first flushed with at least 10 cell volumes of COz or Freon (R22, chlorodifluoromethane) to remove atmospheric gases. Upon decompressing the view cell to ambient laboratory pressure, a baseline signal of transmitted or scattered laser light was monitored. The postcell needle valve was subsequently closed to stop flow, and 100 pL of modifier was introduced into the sample loop, swept into the view cell with C02 or Freon, and rapidly pressurized at +4.93 MPa min-l (50.0atm min-l) to the final pressure of 5.92 (60 atm) or 39.48 MPa (400.0 atm). Temperature was maintained at 110 OC. Because phaseseparation will occur on a much faster time scale than the digitization of the ~ i g n a l , ~ abrupt ,~J~ changes in the signal would be expected during phase separation. The gain on the amplifier was adjusted to lo5lo6. All measurements were performed in duplicate. Transmitted light was sampled by positioning the glass rod on-axis with the optical path of the laser light exiting from the view cell. By moving the glass rod 3-4 mm off-axis and parallel to the optical path, scattered light could be efficiently sampled. Fluid Mixtures. ECD-grade methanol (J. T. Baker, Phillipsburg, NJ), water distilled on-site, SFC-grade C02 (Scott Specialty Gases, Plumsteadville, PA), and 99.9% chlorodifluoromethane (Freon, Scott Specialty Gases) were used. The modifiers were degassed by sonication prior to use.
0
5
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25
30
Concentration of Water (mol%)
Figure 3. Phase diagram of COp/water at 110 O C . (0)shows the conditions of the phase behavior experiment, 39.48 MPa and 28.0 mol % water; (A) Is the single-phase region, and (B) is the two-phase gas-liquid region.
The Freon and C02 were filtered with a 0.2-pm filter during filling of the syringe pump. Estimation of Mole Percent. Weighed amounts of modifier were added to theview cell, pressurized with COz, and allowed to equilibrate. Valves were closed to trap the contents in the view cell, and thecell with thevalves was weighed todetermine the amount of CO2 equilibrated with the methanol. The weighing was performed after the optical studies in order not todisturb thealignment of theview cell with the laser. Because the sample loop was calibrated, equivalent amounts of modifier were delivered into the flow cell for both procedures. Corrections for the residual C02 inside the view cell and valves prior to the introduction of modifier were performed with the Ely equation of state.48
RESULTS AND DISCUSSION Initial Results. Visual observation of the phase behavior with a flashlight proved possible. When a phase boundary was crossed, the formation of gas bubbles (bubble point crossed), liquid droplets (dew point crossed), or critical opalescence (critical point crossed) efficiently scattered the light passing through the cell. Upon sweeping the modifier plug into the view cell with pressurized COz, turbulence and transient phase immiscibilityduring the mixing were observed. The transmitted light intensity fluctuated during the mixing. If the mixture was miscible after equilibration (C02 + methanol), then the light intensity was observed to return to approximately its initial intensity before pressurization. If the mixture was immiscible (C02 + water), then the light intensity remained diminished due to the efficient scattering of the light at gas-liquid interfaces. Because the visual observation method precluded the storage and sharing of data, and was tedious, optics were added to monitor the light intensity passing through the cell. Also, a laser was used instead of the flashlight to minimize scattered light. Transmitted Light. The glass optical rod was carefully aligned with the path of the laser light to sample the transmitted light. The first mixture evaluated with the present system was C02 + water at 110 OC and 39.48 MPa. A portion of the phase diagram for COz/water in the general p - x region of the experimental conditions is shown in Figure 3.2,4J3-17 F. Proceedings of the 65th Annual Gas Processors Association, San Antonio, TX, 1986; pp 185-192.
(48) Ely, J.
Analytical Chemistry, Vot. 66, No. 21, November 1, 1994
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Flgure 4. Transmitted light signal during compression of C02/water at 110 OC: (A) Decompressed view cell, (B) 28.0 mol % water introducedinto view cell rapidly compressedto 39.48 MPa, and (C) fully compressed fluid. Signal to noise ratio (SIN) is 7.9.
The dot (0)in Figure 3 denotes the experimental conditions employed here (1 10 OC, 39.48 MPa, and 28.0 mol 9%water), which lie in the two-phase, gas-liquid region. The region to the left of the vertical line is a single-phase liquid. The signal obtained during the introduction of water (9.7 vol %and 28.0 mol 7%) into the cell and pressurization with C02 to 39.48 MPa is shown in Figure 4. An order of magnitude decrease in signal intensity is apparent upon introduction of the water. Because the concentration of water exceeds the saturation limit of C02 at 110 OC, gas-liquid separation occurs. The liquid layer settles to the bottom of the view cell while the gas phase rises, creating a gas-liquid interface. Light is efficiently scattered by liquid droplets, by gas bubbles, and at the interface between the liquid and gas phases, resulting in multiple reflections and theobserved decrease in signal intensity. Visual observation confirmed that the laser light passing through the view cell was no longer collimated. A screen was positioned in front of the window to determine whether the beam passed through the cell. A partially silvered mirror was then positioned in front of the window to view the contents of the cell. At no time was the laser light directly viewed in order to minimize the potential of eye damage. In contrast to the results obtained with water/C02 are those obtained upon the introduction of methanol into the view cell under the same experimental conditions (1 10 "C and 39.48 MPa). The p - x phase diagram of methanol/COz is shown in Figure 5 . 2 3 4 3 1 9 - 3 1 The upper dot ( 0 )in Figure 5 denotes the experimental conditions employed here (1 10 OC, 39.48 MPa, and 13.2 mol % methanol), which lie in the singlephase region; therefore, the saturation limits of C02 were not exceeded. Equilibrium must be established before the contents are single phase. Because the modifier is swept directly into the view cell, the kinetics of mixing of modifier with the primary fluid can be studied. Modifier is often injected into the extraction cell in SFE, so similar mixing kinetics are expected in SFE as in the view cell. The signal obtained during the introduction of 100 pL of methanol (9.7 vol % and 13.2 mol %) into the view cell is shown in Figure 6. The signal is erratic during the equilibration of modifier with the primary fluid. Equilibration appeared to occur after approximately 1 min despite the fact that the 3550
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Analytical Chemistty, Vol. 66, No. 21, November 1, 1994
Flgure 5. Phase diagram of COp/methanolat 110 OC. (0)shows the conditions of the phase behavior experiments with the upper dot at 39.48 MPa and 13.7 mol % and the lower dot at 5.92 MPa and 68.5 mol %. (B) is the two-phase gas-liquid region.
0
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e
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nime (mm)
Flgwe 6. Transmttted lightsignal duringcompression of COp/methanoi to 39.48 MPa at 110 OC. (A-C) are the same as in Figure 4, except 13.7 mol % methanol was introduced into the cell. S/N is 5.7.
pressure was equilibrated in approximately 26 s. Visual observation (with a flashlight) confirmed that the transient liquid droplets persisted in the primary fluid for approximately 1 min before complete miscibility was established. After miscibility was established, the signal rose above baseline by a small increment. The rise in signal is expected after an organic solvent-enriched solvent is mixed with the C02. The index of refraction for the mixture is larger than for pure C02 at 0.1 MPa; therefore, less light will be lost due to scaterring within the view cell. A small screen positioned over the view cell confirmed that the beam remained collimated, verifying the existence of a single phase. ScatteredLight. By moving the optical rod off-axis slightly (3-4 mm), scattered light can be sampled. The lower dot (0) in Figure 5 denotes the experimental conditions employed here (110 OC, 5.92 MPa, and 68.5 mol % methanol), which lie in the two-phase gas-liquid region. These experimental conditions resulted in phase separation within the cell. Figure 7A shows the signal obtained during the introduction of methanol (9.7 vol %and 68.5 mol %) into the view cell at 110 "C and 5.92 MPa. Because the saturation limits are exceeded, transmitted light is severely attenuated, while scattered light is increased due to scattering of the laser light at the interface between the two phases, especially 180' to the source. When the laser light is scattered at the interface between the phases, the scattered light intensity abruptly increases. In the case
Table 1. Phase Behavior of Mlxtureo of Methanol, Water, or TEA in C02 or Freon at 110 O C and 39.5 MPa
phases present modifiers neat methanol" water' TEAa methanol + TEAb methanol + watep water + TEAb methanol + TEA + wate#
, 0
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io
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Freon single single coexisting single single single coexisting single
coz single single coexisting single single single coexisting single
" 9.7%modifier (vol % in COz or Freon). 8.3%methanol or water and 1.4%TEA. 8.7%methanol and 1.0%water. 7.4%methanol, 1 .O% water, and 1.3% TEA.
Time (min)
Flgure 7. Scattered light signal during compression of COp/methanoi at 110 OC. Final pressures are (A) 5.92 and (B) 39.48 MPa. S/N is 50 for (A) and 17.3 for (B).
of COz/methanol at 5.92 MPa, 68.5 mol % methanol, and 110 OC, the signal increased by over 300%. Visual observation confirmed the phase separation. Subsequent compression of the fluid to 39.48MPa resulted in complete miscibility of methanol in C02 and in a large reduction in the intensity of the scattered light (see Figure 7B). The results show that a single phase was established after equilibration at 39.48MPa, which is expected since these experimental conditions lie in the single-phase region outside the gas-liquid envelope shown in Figure 5. Transient signal spikes were observed until miscibility was established. Although the scattered light signal baseline was noisier than the transmitted light mode, the large change in signal intensity between single-phase and multiple-phase conditions enabled accurate assessment of phase behavior. Multicomponent Fluid Screening. Because the instrumentation was shown to be reliable with the binary mixtures tested, ternary and quaternary mixtures of Freon or C02 with water/ triethylamine (TEA)/methanol were screened to determine whether or not they were single phase. These fluid mixtures were being evaluated as extractants for cocaine, so it was desirable to know their phase behavior under the experimental conditions employed. Table 1 lists the results of the screening at 110 OC and 39.48 MPa. The water and water/TEA modifiers produced immiscibility in both C02 and Freon. Approximately 40 min was required to screen each fluid. The majority of this time was consumed in equilibrating the mixture. If a dual-pump system was employed to dynamically feed the modifier and primary fluid together, then the screening time would be much faster. Nevertheless, the method of introducing modifier directly into the extraction cell is common in SFE.
CONCLUSIONS The optical cell described here is inexpensive, relatively easy to construct, and simple to use. Analysis times can be less than 4 min for each run. Therefore, only a fraction of the analyst's time needs to be spent on determining whether or not the fluid mixture is homogenous, which will greatly facilitate method development. Single-phase conditions are readily differentiated from coexisting phases; however,pT-x data along phase envelopes cannot be determined. Unlike decompression studies, where a single phase must be achieved before a transition point can be determined, this method does not require a single phase. Either scattered or transmitted light may be monitored, depending upon the placement of the glass optical rod. Both the scattered and transmitted light modes for obtaining a signal produced accurate results for determining the physical state of the fluid mixture. The equipment can be as simple as the following: modified view cell, pump, oven, flashlight, and visual observation. ACKNOWLEDGMENT The authors thank Dr. Edgar S . Etz for the sapphire windows. S.H.P. and J.F.M. thank the National Research Council (NRC) for the NRC/NIST postdoctoral fellowship award. Suppliers of the commercial equipment, instruments, and materials listed in this report were identified to specify adequately the experimental procedure. Such identification should not be construed as endorsement or recommendation by the National Institute of Standards and Technology, nor does it imply that the best available equipment or materials were used. Received for review March 30, 1994. Accepted July 25, 1994." e Abstract
published in Adwnce ACS Abstracts, September 15, 1994.
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