Solubility of Organophosphorus Metal Extractantsin Supercritical Carbon Dioxide

Yoshihiro Meguro,* Shuichi Iso, Takayuki Sasaki, and Zenko Yoshida

Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan

The solubility (S) of several liquid organophosphoruscompounds, tributyl phosphate (TBP), diisodecylphos-phoric acid (DIDPA), di-(2-ethylhexyl)phosphoric acid(DEHPA), dihexyl-(N,N-diethylcarbamoyl)methylphos-phonate (CMP), and octyl(phenyl)(N,N-diisobutylcarbam-oyl)methylphosphine oxide (CMPO), in supercritical CO2

was determined over temperature and pressure rangesof 30-90 °C and 7.5-25 MPa. The solubility of thecompound increases with increasing density (G) of CO2

fluid, showing a linear relationship: ln S ) p ln G + q (pand q are constants). The slope p measured at 60 °C is21.8, 11.4, 13.1, 10.8, or 7.5, and the constant q, givensolubility at G ) 1 g/mL, is 26.3, 0.4, 2.7, 5.1, or -0.1respectively for TBP, DIDPA, DEHPA, CMP, or CMPO.A homogeneous mixture of TBP or CMP with CO2 isreadily obtained even at relatively low pressure, where thedensity of CO2 is relatively low. It is found that all thecompounds examined are soluble enough in CO2 toprepare an organophosphorus compound-CO2 mixturewhich can be used in supercritical CO2 fluid extractionof metal ions.

Supercritical fluid extraction (SFE) using an extractant-supercritical CO2 mixture instead of an extractant-organic solventmixture has recently been recognized to be promising as anadvanced method for separation of metals from liquid samples oreven from solid samples for the purpose of analytical pretreatmentor hydrometallurgy. Hence, an increasing number of studies onthe development of SFE of metals1-9 is available. One of severalimportant advantages of SFE is that extraction efficiency andextraction selectivity can be enhanced by tuning the pressure and/or temperature. Also, SFE can minimize the amount of solventwaste. Examples of SFE applications include, e.g., an SFE methodfor the separation of metal ions such as uranium(VI) and fissionproduct elements from nitric acid solution into supercritical CO2

containing an organophosphorus extractant such as tributylphosphate (TBP).8,9

There have been studies on the solubility and/or phasebehavior of organic compounds such as aromatic hydrocarbons10,11

and pesticides12 in supercritical CO2 in order to establish a suitablecondition for the extraction of these substances from a sample.Several empirical or theoretical equations to clarify the solubilitybehavior have been proposed.10-16 Recent work is directed to thesolubility of extractants such as diethyl dithiocarbamates,3,17 crownethers,18 and organophosphine oxides19,20 in supercritical CO2 forevaluating the applicability of extractant in SFE and for designinga new extractant feasible to SFE of a metal ion. Solubilities ofextractants and metal-containing compounds were reviewedrecently.21

Determination of extractant solubility into supercritical CO2

media is indispensable from both fundamental and practical view-points: (i) For elucidating and formulating an extraction reaction,in which the distribution equilibrium of an extractant can beexpressed by the ratio of extractant solubilities into both phases,the distribution equilibrium of the extractant itself betweenaqueous and supercritical CO2 phases should be taken intoaccount. (ii) Solubility of an extractant in supercritical CO2 is, ingeneral, fairly lower than that in a conventional organic solvent,which may restrict the preparation of supercritical CO2 mediacontaining the extractant of sufficiently high concentration.Understanding the solubility of the extractant, therefore, is a topicof importance for further development of SFE.

In the present study, the solubilities of five organophosphoruscompounds which are liquid at an ambient temperature, tributylphosphate (TBP),22 diisodecylphosphoric acid (DIDPA),23 di(2-ethylhexyl)phosphoric acid (DEHPA),24 dihexyl(N,N-diethylcar-

(1) Wang, J.; Marshall, W. D. Anal. Chem. 1994, 66, 3900.(2) Furton, K. G.; Chen, L.; Jaffe, R. Anal. Chim. Acta 1995, 304, 203.(3) Lin, Y.; Smart, N. G.; Wai, C. M. Trends Anal. Chem. 1995, 14, 123.(4) Zolotov, Yu. A.; Glazkov, I. N.; Efimov, I. P.; Revel’skii, I. A.; Zirko, B. I.;

Yashin, Yu. S.; Shakhpenderyan, E. A. Vestn. Mosk. Univ., Ser. 2: Khim.1995, 36, 41.

(5) Wang, S.; Wai, C. M. Environ. Sci. Technol. 1996, 30, 3111.(6) Toews, K. L.; Smart, N. G.; Wai, C. M. Radiochim. Acta 1996, 75, 179.(7) Laintz, K. E.; Tachikawa, E. Anal. Chem. 1994, 66, 2190.(8) Iso, S.; Meguro, Y.; Yoshida, Z. Chem. Lett. 1995, 365.(9) Meguro, Y.; Iso, S.; Takeishi, H.; Yoshida, Z. Radiochim. Acta 1996, 75,

185.

(10) Bartle, K. D.; Clifford, A. A.; Jafar, S. A.; Shilstone, G. F. J. Phys. Chem. Ref.Data 1991, 20, 713.

(11) Chen, P.-C.; Tang, M.; Chen, Y.-P. Ind. Eng. Chem. Res. 1995, 34, 332.(12) Macnaughton, S. J.; Kikic, I.; Rovedo, G.; Foster, N. R.; Alessi, P. J. Chem.

Eng. Data 1995, 40, 593.(13) Liu, G.-T.; Nagahama, K. J. Supercrit. Fluids 1996, 9, 152.(14) Chen, J.-W.; Tsai, F.-N. Fluid Phase Equilibria 1995, 107, 189.(15) Chrastil, J. J. Phys. Chem. 1982, 86, 3016.(16) Yakoumis, I. V.; Vlachos, K.; Kontogeorgis, G. M.; Coutsikos, P.; Kalospiros,

N. S.; Tassios, D.; Kolisis, F. N. J. Supercrit. Fluids 1996, 9, 88.(17) Wang, J.; Marshall, W. D. Anal. Chem. 1994, 66, 1658.(18) Wang, S.; Elshani, S.; Wai, C. M. Anal. Chem. 1995, 67, 919.(19) Schmitt, W. J.; Reid, R. C. Chem. Eng. Commun. 1988, 64, 155.(20) Lin, Y.; Smart, N. G.; Wai, C. M. Environ. Sci. Technol. 1995, 29, 2706.(21) Smart, N. G.; Carleson, T.; Kast, T.; Clifford, A. A.; Burford, M. D.; Wai, C.

M. Talanta 1997, 44, 137.(22) Schulz, W. W., Burger, L. L., Navratil, J. D., Eds. Science and Technology of

Tributylphosphate, Vol. III; CRC Press: Boca Raton, FL, 1990.

Anal. Chem. 1998, 70, 774-779

774 Analytical Chemistry, Vol. 70, No. 4, February 15, 1998 S0003-2700(97)00739-7 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 02/15/1998

bamoyl)methylphosphonate (CMP),25 and octyl(phenyl)(N,N-diisobutylcarbamoyl)methylphosphine oxide (CMPO),26 weremeasured over temperature and pressure ranges of 30-90 °C and7.5-25 MPa. These liquid compounds have been widely em-ployed in solvent extraction of metal ions, particularly in the fieldof nuclear technology, because of their high extractability towardmetal ions from highly acid aqueous solution and high radio-chemical stability. The relation between the solubility and densityof CO2 was investigated, and the feasibility of these extractantsfor SFE was evaluated from a viewpoint of the preparation of ahomogeneous mixture of the extractant and the supercritical CO2.

EXPERIMENTAL SECTIONApparatus. The apparatus used for solubility measurements

is shown in Figure 1. The main part of the apparatus consistedof twin equilibrium cells (1 and 2) of stainless steel and a collectingglass vessel containing 1.5 g of quartz wool (1-5 µm), both ofwhich were kept in a thermostated oven at a defined temperature.The syringe pump (Isco Co. Ltd., 260D), plunger-type pump (JEOLCo. Ltd., CAP-L02), prewarming coil, filter (2 µm in pore size),restrictor (Isco Co. Ltd., stainless steel capillary of 57 µm i.d. ×27 cm in length, 71 µm × 48 cm, or 71 µm × 110 cm), andthermostat oven were identical with those employed in previouswork.8,9 Extra heating of the restrictor part to avoid a pluggingproblem was not necessary in the present work to measure thesolubility of such liquid organophosphorus compounds.

Gas chromatographic analysis was conducted with a Hitachitype 260-30 gas chromatograph, equipped with a temperature-programming device and a flame ionization detector. The columnemployed was made from stainless steel tubing of 3 mm i.d. and1000 mm in length, packed with 1.5% OV-101 on ChromosorbG-HP substrate. Helium of 0.6 kgf/cm2 (at 100 °C) was used ascarrier gas.

Procedure. An appropriate volume of organophosphorusliquid compound was taken in the equilibrium cells 1 and 2. Thesyringe pump was filled with liquid CO2 at 25 °C. Carbon dioxidefluid was allowed to flow through the system at constant pressure.The supercritical CO2 flow, after being warmed in the prewarmingcoil, was introduced into the thermostated equilibrium cells. Theeffluent from the equilibrium cells was introduced into thecollecting vessel, where the pressure was reduced to atmosphericby the aid of a restrictor. Effluents were discarded in the first 2min of an experiment, and then a solute in the effluent wascollected in the vessel over a definite time. The amount of thesolute collected was determined by gravimetry and/or gaschromatography.

Gravimetric weighing of the collection vessel before and afterthe solubility measurement procedure is sufficiently precise fordetermination of the amount of solute recovered, provided thesample compound is pure enough. When the sample reagentcontains appreciable quantities of an impurity, in particular, if theimpurity has preferential solubility into the CO2 phase, gaschromatographic analysis of the solute is necessary. For gaschromatography, the solute recovered in the collecting vessel wasquantitatively dissolved with ethanol (for TBP, CMP, or CMPO)or hexane (for DIDPA), and an aliquot of the solution was takenand subjected to analysis. Initial temperature and rate of tem-perature increase in temperature-programmed gas chromatogra-phy were 80 °C + 15 °C/min, 80 °C + 20 °C/min, 100 °C + 20°C/min, or 150 °C + 10 °C/min for the determination of TBP,CMP, CMPO or DIDPA, respectively.

The concentration (C; in moles per liter) of a compounddissolved in CO2 was calculated using eq 1,

where w is the amount in grams of compound collected, M is themolecular weight of the compound, fCO2(25,P) is the flow rate inmilliliters per minute of CO2 at the syringe pump (at 25 °C andpressure P), t gives the collection time in minutes, FCO2(T,P) is thedensity of CO2 at temperature T and pressure P, and Fp is thedensity of a compound measured at 25 °C. Here, Fp can beassumed to be independent of temperature and pressure in therange investigated in the present work. The flow rate of CO2 fluidwas controlled using restrictors of various inner diameter andlength.

Materials. TBP (Koso Chemicals, g98% purity guaranteedby the company), DIDPA (Daihachi Chemical, 92.9% as analyzedby the company), CMP (Occidental Chemical, g95% guaranteed),DEHPA (Aldrich, g97% guaranteed), and CMPO (M&T Chemi-cals, 98.6% analyzed) were used without further purification.

A liquid CO2 cylinder of ∼6 MPa and 99.99% pure, suppliedby Shin Tokyo Teisan Co. Ltd., was used.

RESULTSA solubility equilibrium of CO2 flow with solute should be

achieved during the residence time of the flow in the twinequilibrium cells (cf. Figure 1). With 8 and 5 mL of compoundbeing placed in cells 1 and 2, respectively, the concentration ofcompound determined was independent within the error, (5%,

(23) Kubota, M.; Dojiri, S.; Yamaguchi, I.; Morita, Y.; Yamagishi, I.; Kobayashi,T.; Tani, S. In High-Level Radioactive Waste and Spent Fuel Management;Slate, S. C., Kohout, R., Duzuki, A., Eds.; The American Society ofMechanical Engineers: Fairfield, NJ, 1989; p 537.

(24) Ceccaroli, B.; Alstad, J. J. Inorg. Nucl. Chem. 1981, 43, 1881.(25) Horwitz, E. P.; Muscatello, A. C.; Kalina, D. G.; Kaplan, L. Sep. Sci. Technol.

1981, 16, 417.(26) Reddy, M. L. P.; Damodaran, A. D.; Mathur, J. N.; Murali, M. S.; Krishna,

M. V. B.; Iyer, R. H. Solv. Extr. Ion Exch. 1996, 14, 793.

Figure 1. Apparatus for measuring solubility of the liquid organiccompound. 1,2, equilibrium cells; 3, collection vessel; 4, quartz wool;5, syringe pump; 6, prewarming coil; 7, filter; 8, restrictor; 9, thermostatoven; 10, CO2 cylinder; 11, plunger-type pump; 12, liquid compound.

C ) w/M × 1000fCO2(25,P)t (FCO2(25,P)/FCO2(T,P)) + w/Fp

(1)

Analytical Chemistry, Vol. 70, No. 4, February 15, 1998 775

on the flow rate of the CO2 in the range from 0.02 to 1.8 mL/min.Thus, we hereafter define the concentration of compound beingmeasured in this flow rate range as an equilibrium solubility. Itwas observed that the dissolution reaction did not attain equilib-rium when a single cell was used with CO2 of relatively high flowrate. For example, the solubility of TBP measured using a singlecell containing 5 mL of TBP remarkably decreased with increasingthe flow rate in the range between 0.02 and 1.0 mL/min.

The recovery efficiency of solute in the collection vessel wasexamined by the following procedure. After allowing the CO2 fluidto flow through the apparatus with empty cells 1 and 2, a knownamount of sample compound was added to the CO2 stream at aknown flow rate using a plunger-type pump (11 in Figure 1). Byanalyzing the solute in CO2 after collection, a recovery of g98%was confirmed.

Three repeated experiments at given pressure and tempera-ture, e.g., 15 MPa and 60 °C, showed the reproducibility of themeasurements to be (5%, irrespective of the amount of the solutecollected in the range 0.15-5 g.

It has been well-known28 that the presence of modifierenhances the solubility of a solute when an appreciable amountof modifier coexists and the amount of the modifier is in excessof the amount of an objective solute in CO2. The modifier effectdue to impurities contained in organophosphorus compoundsemployed in the present work is estimated to be negligible within

the experimental error of (5%, since total concentration ofimpurities is low enough, except for DIDPA, and the concentrationof impurities dissolved in CO2 is lower than that of the organo-phosphorus compound. The modifier effect on the DIDPAsolubility due to impurity will be discussed below.

Solubility of Tributyl Phosphate. In the gas chromatogramof reagent grade TBP, whose purity was given as g98%, no peakindicating the impurity could be observed. If there were animpurity with preferential solubility relative to TBP, this impurityshould be concentrated in the solute collected after the solubilitymeasurement procedure. No peak due to the impurity wasdetected in the gas chromatogram, even for the solute collectedby the solubility measurements at 80 °C and 10 MPa (amount ofthe solute collected, 70 mg) or at 90 °C and 10 MPa (solute, 100mg). These results suggest that gravimetry is precise enough todetermine the solubility of TBP.

The results of concentration (C) of TBP dissolved in CO2 at30-90 °C and 7.5-25 MPa are summarized in Table 1. The TBPconcentration remarkably increases with an increase of pressureat temperature higher than 50 °C and in a relatively lower pressureregion. The TBP concentration in this range, where a strongpressure dependence of C is observed, corresponds to a pressure-dependent solubility of TBP in supercritical CO2.

The concentration of TBP in CO2 measured at g50 °C becomesless dependent on the pressure when the pressure is considerablyhigher and the TBP concentration ranges from 1 to 1.5 mol/L.TBP solubilities measured at 30-40 °C were found in the range1-1.5 mol/L and showed relatively weak pressure dependence.Under these dissolution conditions, the mixture of TBP and CO2

(27) Angus, S., Armstrong, B., de Reuck, K. M., Eds. IUPAC InternationalThermodynamic Tables of The Fluid State. Vol. 3, Carbon Dioxide; PergamonPress: New York, 1976.

(28) Taylor, L. T. Supercritical Fluid Extraction; John Wiley & Sons: New York,1996; Chapter 3.

Table 1. Solubility of Tributyl Phosphate (TBP) in CO2

temp(°C)

pressure(MPa)

density ofCO2 (g/mL)a

concn ofTBP (mol/L)

temp(°C)

pressure(MPa)

density ofCO2 (g/mL)a

concn ofTBP (mol/L)

30 8.5 0.726 1.3 65 9.0 0.220 0.0008410.0 0.772 1.4 10.0 0.266 0.02015.0 0.847 1.4 11.0 0.320 0.2420.0 0.891 1.5 12.0 0.381 0.3325.0 0.923 1.4 15.0 0.553 1.0

20.0 0.692 1.340 8.5 0.335 0.76 25.0 0.762 1.4

10.0 0.625 1.215.0 0.781 1.3 70 10.0 0.248 0.0009220.0 0.840 1.4 10.5 0.270 0.007025.0 0.880 1.4 11.0 0.293 0.14

11.5 0.319 0.3150 7.5 0.194 0.0094 15.0 0.505 1.0

8.5 0.249 0.07 20.0 0.660 1.29.0 0.284 0.17 25.0 0.737 1.3

10.0 0.371 0.7315.0 0.701 1.3 80 8.5 0.175 0.0006620.0 0.785 1.4 10.0 0.222 0.001725.0 0.835 1.5 11.0 0.257 0.0035

12.0 0.297 0.04960 8.5 0.212 0.00064 15.0 0.427 0.65

9.0 0.235 0.0045 20.0 0.595 1.29.3 0.250 0.011 25.0 0.687 1.39.6 0.266 0.13

10.0 0.289 0.42 90 10.0 0.203 0.001612.0 0.429 0.71 12.0 0.265 0.008915.0 0.605 1.1 15.0 0.372 0.4320.0 0.724 1.2 20.0 0.534 1.225.0 0.787 1.3 25.0 0.637 1.2

a Taken from ref 27 (given for neat CO2).

776 Analytical Chemistry, Vol. 70, No. 4, February 15, 1998

in the equilibrium cell may form a single TBP-CO2 phase, inwhich the amount of TBP dissolved in the CO2 phase increasesand the amount of CO2 dissolved in the TBP phase also increases,and finally both phases become of identical composition, resultingin formation of the single phase. The single phase formation ofTBP and CO2, e.g., at 60 °C and g15 MPa, was confirmed visuallyin a separate experiment using a sapphire window cell. Theminimum pressure at which the single phase formation occursincreases with increasing temperature.

Solubility of Diisodecylphosphoric Acid. In the gas chro-matogram of DIDPA reagent, a peak corresponding to an impuritywas observed with retention time, tR, of 2.9 ( 0.3 min, while tR ofthe main peak due to DIDPA was 10.4 ( 0.4 min. The gaschromatogram recorded with the solute collected in the solubilitymeasurement showed peaks for both DIDPA and impurity. A ratioof peak areas of both peaks, which is defined as R ) [area ofimpurity peak(s)]/[area of pure reagent peak], in the gas chro-matogram for the solute collected was larger than R for thereagent as received; the results of R measured under variousconditions are summarized in Table 2. The larger R for the solutecollected implies the preferential dissolution of the impurity intoCO2 and, thus, the enrichment of the impurity by the solubilitymeasurement procedure. The enrichment effect is more clearlyobserved when the amount of the solute collected is smaller, asshown in Table 2. Hence, gravimetry could not be applied to thedetermination of the solubility of DIDPA, because the gravimetricresult includes a fairly large positive error due to the impurity.The solubility data for DIDPA determined by gas chromatographyare listed in Table 3. The solubility increases with an increase ofthe pressure in the range 12.5-25 MPa and with an increase ofthe temperature in the range 40-70 °C.

If the impurity which is enriched in CO2 fluid enhances thesolubility of DIDPA through the so-called modifier effect, thenthe solubility determined by collecting a smaller amount of thesolute must be higher than that determined by collecting a larger

amount of the solute. The experimental results showed that thesolubility of DIDPA was independent of the amount of the solutecollected in the range of 0.15-1.5 g. This implies that the impuritydoes not enhance the DIDPA solubility appreciably.

Solubility of Di-2-ethylhexylphosphoric Acid. The purityof DEHPA used in this study is rather high. To confirm theaccuracy of solubility data from gravimetric measurement, 0.20-2.5 g of solute (at 60 °C and 15 MPa) was collected. Solubilitiesdetermined by both experiments were 0.022 ( 0.001 and did notdepend on the amount of the solute collected. These resultsindicate that the enrichment effect of an impurity is insignificantfor DEHPA, indicating that interferences by impurity in thegravimetric determination were negligible. The solubility ofDEHPA at 60 °C was determined by gravimetry, and the resultsare summarized in Table 4. Significant pressure dependence ofthe solubility is observed.

Solubility of Dihexyl-(N,N-diethylcarbamoyl)methylphos-phonate. In the gas chromatogram of reagent CMP, at least threepeaks due to impurities were detected at tR ) 2.9 ( 0.2, 5.8 (0.1, and 8.2 ( 0.1 min. The tR of the CMP peak was 6.8 ( 0.3min. Peak area ratio R was calculated to be 0.02 ( 0.01. Identicalpeaks of impurities were observed for all solute samples collected.The R values, as shown in Table 2, were determined for the soluteat 60 °C and 10-15 MPa and for wide ranges of the amountcollected. It is found that R determined for the solute is identicalwith R for the reagent CMP and is not influenced by the amountof the solute collected ranging from 40 mg to 2 g. This indicatesthat there is no enrichment effect of the impurity in supercritical

Table 2. Ratio of Peak Areas, R, of Impurities andPure Compound in the Gas Chromatogram ofOrganophosphorus Compounds

condition of solubility measurement

compdtemp(°C)

pressure(MPa)

amount of solutecollected (g) Ra

DIDPA 40 12.5 1.80 0.0750 12.5 0.74 0.1960 15 0.93 0.1770 17.5 1.44 0.11

DIDPA reagent as received 0.02

CMP 60 10 0.039 0.0211 0.039 0.0212 1.48 0.0215 2.22 0.02

CMP reagent as received 0.02 ( 0.01

CMPO 60 10 0.016 ∼012 0.30 0.1615 0.48 0.0120 0.89 ∼025 1.36 ∼0

CMPO reagent as received ∼0

a R ) (∑ peak area for impurities)/(peak area for pure compound).

Table 3. Solubility of Diisodecylphosphoric Acid(DIDPA) in CO2

temp (°C) pressure (MPa) density (g/mL) concn (mol/L)

40 12.5 0.732 0.01515 0.781 0.03225 0.880 0.12

50 12.5 0.612 0.003215 0.701 0.01725 0.835 0.12

60 15 0.605 0.004620 0.724 0.04125 0.787 0.090

70 17.5 0.599 0.005920 0.660 0.01925 0.737 0.057

Table 4. Solubility of DEHPA, CMP, and CMPO in CO2 at60 °C

organophosphoruscompound

pressure(MPa)

density(g/mL)

concn(mol/L)

DEHPA 15 0.605 0.02220 0.724 0.1625 0.787 0.81

CMP 10 0.289 0.0002311 0.355 0.001012 0.429 0.07815 0.605 0.41

CMPO 10 0.289 0.00008212 0.429 0.001515 0.605 0.01720 0.724 0.089

Analytical Chemistry, Vol. 70, No. 4, February 15, 1998 777

CO2 during solubility measurement procedure. Both gravimetryand gas chromatography give enough precise solubility data,which was ascertained by comparison of results from these twomethods. Solubilities given in Table 4 were measured at 60 °Cand 10-15 MPa using gas chromatography. The solubility ofCMP was found to increase with pressure.

Solubility of Octyl(phenyl)(N,N-diisobutylcarbamoyl)-methylphosphine Oxide. There was no noticeable peak corre-sponding to an impurity in the gas chromatogram for reagentCMPO. The tR for CMPO was 8.7 ( 0.2 min. The solute samplecollected in the solubility measurement at 60 °C and 10-25 MPawas analyzed by gas chromatography, and the results are shownin Table 2. Four peaks with tR ) 0.7, 1.9, 3.1, and 7.4 min weredetected in the gas chromatograms for the solute collected at 12and 15 MPa, showing R ) 0.16 and 0.01, respectively (Table 2).The solubility data for CMPO obtained by gas chromatographyare summarized in Table 4.

DISCUSSIONThe solubilities of the five organophosphorus compounds

investigated in the present work increase significantly with anincrease of the pressure, except in the case of a formation of asingle phase in the equilibrium cells, typically observed in thesolubility of TBP at a relatively high pressure region. Such apressure effect has been explained in terms of an effect ofsupercritical fluid density on the solubility of a solute. A simplerelation, as given by eq 2, between the solubility S and the densityF of CO2 was proposed15 for systematizing the solubilities ofcarboxylic acids, carboxylic esters, and water. A recent review21

suggests the validity of this correlation for the solubility of a varietyof organic compounds,

where F is the density of neat CO2 in grams per milliliter, p is aconstant relating to the solvation of the solute in the supercriticalfluid, and q is the solubility at F ) 1 g/mL.

The solubility data listed in Tables 1, 3, and 4 were plottedagainst the density of supercritical CO2, some examples of whichare illustrated in Figure 2. As shown by plot 1 for the solubilityof TBP at 60 °C, the ln S-ln F plot has a clear linear portion.Identical linear relations were observed in ln S-ln F plots

measured at 50-70 °C. The p and q values determined on thebasis of eq 2 are listed in Table 5. These values are calculatedby linear least-squares regression, and the uncertainty denotesits standard deviation. The slope p at 50 °C is smaller than thoseat 60-70 °C, and the average p at 60-70 °C is 20.5 ( 4.7. Theuncertainty is large but results mostly from the very high slope.The ln S-ln F plots at 80 and 90 °C do not show a linear portionin the wide range of the density of CO2. Density dependences ofsolubility below 0.2 g/mL are smaller than those at higher density.It is assumed that the contribution of the volatility of TBP to theoverall solubility becomes larger at higher temperature and at thelower CO2 density region. A similar behavior was reported forthe solubility of menthol in supercritical CO2, indicating a decreasein the slope of the ln S vs ln F plot in the lower density region.29

The linear relationship between ln S and ln F is clearlyobserved for the solubility of DIDPA at 40-70 °C, and the resultat 60 °C is shown as plot 5 in Figure 2. As given in Table 5, slopep of DIDPA is not influenced by the temperature in the rangeexamined, and q increases with an increase of temperature. TheDIDPA solubility at a given density increases with an increase oftemperature, though such a systematic temperature effect cannotbe observed for TBP.

Plots 2-4 for solubilities of CMP, CMPO, and DEHPA at 60°C indicate linear relationships between ln S and ln F, explainedby eq 2. The constants p and q are summarized in Table 5.

Slope p in eq 2 has been considered to be an indication of thesolvation of the solute in the supercritical fluid, and there was anattempt to correlate p directly to the number of CO2 moleculescoordinated onto the solute.15 Slopes p of five extractants (at 60°C) are 21.8 (TBP, molecular weight, 266), 13.1 (DEHPA, 322),10.8 (CMP, 363), 11.4 (DIDPA, 379), and 7.5 (CMPO, 408). It isnoteworthy that slope p decreases with an increase in themolecular weight of the extractants, even though a reason cannoteasily be given.

It is found that all organophosphorus compounds examinedhave sufficiently high solubility which makes it possible to preparea homogeneous extractant-CO2 mixture for SFE of metal ions.For example, if the pressure is sufficiently high to obtain CO2

density above 0.3 g/mL for TBP, 0.45 g/mL for CMP, 0.7 g/mL

(29) Sovova, H.; Jez, J. J. Chem. Eng. Data 1994, 39, 840.

Figure 2. Ln S-ln F plots for organophosphorus compounds in CO2

at 60 °C. Compounds: 1, TBP; 2, CMP; 3, CMPO; 4, DEHPA; 5,DIDPA.

ln S ) p ln F + q (2)

Table 5. Solubility Constants of OrganophosphorusCompounds in the Equation ln S ) p ln G + q

compdpressure

range (MPa) temp (°C) p q

TBP 7.5-12 50 6.5 ( 0.6 6.2 ( 0.860 21.8 ( 2.1 26.3 ( 2.965 15.2 ( 1.0 16 ( 1.370 24.4 ( 3.4 27.1 ( 4.3

DIDPA 10-25 40 11.4 ( 0.1 -0.6 ( 0.150 11.6 ( 0.3 -0.0 ( 0.160 11.4 ( 0.6 0.4 ( 0.270 10.8 ( 0.7 0.5 ( 0.3

DEHPA 15-25 60 13.1 ( 2.1 2.7 ( 0.8

CMP 10-15 60 10.8 ( 2.3 5.1 ( 2.2

CMPO 10-20 60 7.5 ( 0.2 -0.1 ( 0.2

778 Analytical Chemistry, Vol. 70, No. 4, February 15, 1998

for DEHPA, 0.75 g/mL for CMPO, or 0.8 g/mL for DIDPA, a 0.1mol/L extractant-CO2 homogeneous mixture can be prepared.The regions shown by dotted lines for TBP and CMP in Figure 2correspond to the formation of the single phase of extractant-CO2 mixture in the equilibrium cells. This phase behavior reflectsthe fact that TBP and CMP are more soluble in CO2 than CMPO,DEHPA, and DIDPA. Other organophosphorus compounds, suchas trioctylphosphine oxide and triphenyl phosphate,19 were alsoreported to be less soluble in comparison with TBP and CMPstudied in the present work.

CONCLUSIONSolubilities were determined for five organophosphorus liquid

extractants using a twin equilibrium cells method. Results suggestthat all extractants examined are sufficiently soluble in CO2 mediaand can be employed as a metal extractant in SFE. In SFE ofmetal ions from aqueous solution into the supercritical CO2 phase,the distribution of extractant between aqueous and supercriticalphases is defined as the ratio of solubilities of extractant betweenboth phases. It is obvious from the results discussed in this paperthat the solubility of the extractant in supercritical CO2 can becontrolled by changing the CO2 density, namely by tuningpressure and temperature. It is thus expected that the extraction

equilibrium of a metal can be controlled by tuning the density ofCO2 fluid, because the extraction equilibrium of the metal stronglydepends on the distribution of the extractant itself.9 Studies ofthe SFE equilibrium of U(VI), Pu(IV), and lanthanide(III) ionsbetween nitric acid solution and supercritical CO2 using TBP orDIDPA are now in progress.

ACKNOWLEDGMENTThe authors thank Professor C. M. Wai of University of Idaho,

Professor A. A. Clifford of University of Leeds, and Dr. N. G. Smartof British Nuclear Fuels plc. for their helpful discussion andencouragement throughout this work.

SUPPORTING INFORMATION AVAILABLEFigures showing ln S vs pressure plots for TBP and ln S vs ln

F plots for TBP or DIDPA (3 pages). Ordering and accessinformation is given on any current journal masthead page.

Received for review July 9, 1997. Accepted December 2,1997.

AC9707390

Analytical Chemistry, Vol. 70, No. 4, February 15, 1998 779