ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
997
Determination of the Solubility Behavior of Some Polycyclic Aromatic Hydrocarbons in Water Willie E. M a y * and Stanley P. Wasik National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234
David H. Freeman Chemistry Department, University of Maryland, College Park, Maryland 20747
A dynamic coupled column liquid chromatographic technique was used to obtain aqueous solubility data on 11 aromatic hydrocarbons. The aqueous solubility at 25 OC was determined for each compound. The precision of replicate solubility measurements was better than *3%. The variation of the solubility of each compound with temperature is expressed In the form of either a quadratic or cubic equation based on a least squares fit of the solubility to temperature. These equations can be used to interpolate the solubility to within f2% of the experimentally measured values between 5 and 30 OC. Enthalpies of solution, AH,, were then calculated from the values so obtained and Setschenow constants were calculated from the effect of salinity on solubility.
T h e aqueous solubility is a fundamental parameter in assessing t h e extent and rate of the dissolution of polycyclic aromatic hydrocarbons (PAHs) and their persistence in the aquatic environment. T h e extent t o which aquatic biota are exposed to a toxicant such as a PAH is largely controlled by the aqueous solubility of the toxicant. These solubilities are also of thermodynamic interest, and give information on the nature of these highly non-ideal solutions. T h e aqueous solubilities of benzene, the alkylbenzenes, naphthalene, and the alkylnaphthalenes have been measured by several investigators (1-14). All of these investigators prepared saturated solutions by adding an excess of the solute to be measured to water and mechanically stirring the mixture for at least 24 h. These solutions were usually allowed to settle prior t o filtration, extraction, and quantitative analysis by ultraviolet spectroscopy or gas chromatography. This method worked well for the fairly soluble compounds t h a t were being studied and interlaboratory agreement was good. This basic methodology, with some minor modifications, has been generally adopted for the determination of the aqueous solubilities of PAHs (15-24) even though the solubilities of some PAHs differ from t h a t of benzene by more than a factor of lo6. For the less soluble PAHs, both the accuracy and precision of this method can be adversely affected by a number of processes including: incomplete equilibration of t h e solid hydrocarbons with the aqueous medium; dispersion rather than true dissolution of the solid hydrocarbons (accommodation); failure to remove suspended microcrystals from the solution; adsorptive losses of the PAHs from solution to the surfaces of containers, filters, and transfer tools; non-analyte signals produced by impurities associated with the analyte, the water, or both. The authors have recently described and critically evaluated a dynamic coupled column liquid chromatographic (DCCLC) method which circumvents the problems stated above (25). In this paper we briefly describe this method and give aqueous solubility data for benzene, naphthalene, and nine PAHs. Enthalpies of solution, W s , calculated from the temperature dependence of the solubility and Setschenow constants, K,, calculated from the 0003-2700/78/0350-0997$01.00/0
effect of salinity on solubility, are reported for each compound.
EXPERIMENTAL The DCCLC method for determining PAH aqueous solubilities is based on generating saturated solutions by pumping water through a column packed with glass beads that have been coated with the compound of interest (generator column). The concentration of the desired compound in the effluent of the generator column is measured by a modification of the coupled column liquid chromatographic process that has been previously described by May et al. (26). Reagents. The water used in this study was distilled from a potassium permanganate-sodium hydroxide solution and passed through a column packed with XAD-2 (Rohm and Haas, Philadelphia, Pa.) in order to remove residual trace organics. The acetonitrile and sodium chloride used were spectro and reagent grade, respectively. All of the PAHs used were obtained commercially and were reported by their respective manufacturers to contain less than 3% impurities. Gas and liquid chromatographic analyses of the materials supported these claims. Preparation and Quantitative Analysis of Saturated Solutions. In the present study, saturated solutions of individual PAHs were prepared by pumping distilled or salt water through “Generator Columns”. These columns were usually packed with 60-80 mesh glass beads coated with 1% by weight of the compound of interest. The column for the generation of saturated solutions of benzene was prepared by first pumping 50 mL of benzene through a column packed with uncoated glass beads, then purging the excess benzene from the column with 50 mL of water. The temperature of the generator column was controlled with a constant temperature bath to within hO.05 “C. Extraction of the generated PAH solutions was accomplished by passing a measured volume of that solution through an “Extractor Column”. This 6 X 0.6 cm column was packed with a superficially porous bonded CIBstationary phase (Bondapak CI8, Water Associates, Milford, Mass.) and was found to provide better than 99% extraction efficiency for Less than 25 mL volumes of aqueous PAH solutions. Since the extractor column did not extract aqueous solutions of benzene quantitatively, the generated solutions of benzene were pumped through a stainless steel 23.2-pL sample loop instead of the extractor column, for injection into the chromatographic system. After extraction, a water-acetonitrile solvent blend was passed through the extractor column (or sample loop) to elute the adsorbed PAH. This eluate was then passed through a microparticulate analytical column (pBondapak CI8) for separation of the PAH from non-analyte interferences. Individual response factors were determined by measuring the UV detector signal produced by injecting a known amount of the PAH of interest dissolved in acetonitrile. RESULTS AND DISCUSSION Aqueous Solubilities at 25 OC. Table I compares the solubilities determined by DCCLC with some values reported by other investigators. Of the 11 values reported, there is only one case of gross disagreement with the consensus literature value. The solubilities reported for anthracene are clustered about two values. A possible reason for this phenomenon lies in the fact that most commercial preparations of anthracene contain a t least 2% phenanthrene. ‘Though the two com-
a 1978 American Chemical Society
998
ANALYTICAL C H E M I S T R Y , VOL. 50, NO. 7, JUNE 1978 20.00
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
999
Table 11. Variation of Aqueous Solubility with Temperature, C Solubility dependence o n temperature, Correlation Compound pglkga coefficient" 1. Benzene (0.0247t3 -- 0.6838t2 + 0.3166t + 1 8 3 3 ) x l o 3 0.9443 2. Naphthalene (0.0189t2 + 0.2499t + 13.66) x l o 3 0.9987 3. Fluorene 0.0185t3 + 0.4543t2 + 22.762 + 543 0,9999 4. Phenanthrene 0.0025t3 T 0.8059t2 + 5.4132 i324b 0.9992 5. 1-Methylphenanthrene 0.0080t' 0.1301t2 T 6.8016t + 55.4b 0.994 6. Pyrene - 0 . O O l l t ' + 0.2007t2 - 1.051t + 50.2 0.9997 7. Fluoranthene 0.0072t3 - 0.1047t2 + 4.322t T 50.4 0.9988 8. Anthracene 0.0013t3 - 0.0097t2 + 0.8861t + 8.21b 0.9998 9. 2-Methylanthracene 0.0011t3 - 0.0306t2 + 0.8180t + 2.7gb 0.9988 10. 1,2-Benzanthracene 0.0003t3 - 0.0031t' + 0.1897t + 1.74 0,9991 11. Chrysene 0.0024t2 + 0.0144t + 0.609 0.9982 These equations and correlation coefficients were obtained from fitting solubility vs. temperature data to the Polynomica1 Regression (2" or 3") program supplied with a Hewlett-Packard 9830A Calculator. Data reported previously, in Ref. 25. ~
Table 111. Enthalpies of Solution of Some Aromatic Compounds Measured between 5' and 30" C Enthalpy of solution, kcal/mol Wauc h o pe and Compound This work Schwarz" Getzenb Naphthalene 6.30 i 0.12 5.29 i 0.10 6.10 i 0.28 Fluorene 7.88 i 0.15 6.99 + 0.16 Anthracene 10.46 t 0.10 8.32 t 0.38 1 0 . 4 3 0.8 Phenanthrene 8.32 k 0.14 8.68 i 0.23 7.7 i 0.9 10.10 i 0.17 2-Methylanthracene 1-Methylphen- 9.34 c 0.18 an threne Fluoranthrene 9.52 i 0.38 Pyrene 8.47 c 0.36 11.4 i 0.2 7.3 t 0.2 1,2-Benzan1 0 . 7 1 t 0.25 thracene Chrysene 9.86 I0.24 Reported in Ref. 21. Calculated from solubility data reported in Ref. 2 3 in the 0-30 " C temperature range. 3 50 r
3.20 L
2001
0 00
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NaCl CONCENTRATION (MOLES LITER)
Figure 2. Solubility of phenanthrene as a function of sodium chloride concentration
where So and S, are the concentrations of the solute in fresh a n d salt water, respectively, K , is the Setschenow constant for the sodium chloride solution, and C, is t h e molar salt concentration. T h e solubility of phenanthrene as a function of salt concentration and temperature is shown in Figure 2. T h e value of t h e Setschenow constant is shown to be independent of temperature over the range studied. T h e Setschenow constants determined for the compounds studied in this paper are presented in Table I\'. T h e theory developed by McDevitt and Long (29) for the salting out of liquid hydrocarbons from aqueous solutions predicts an increase in K, with increasing liquid molar volume for nonelectrolyte solutes. The data given in Table I\' shows t h a t this relationship seems t o also be valid for the PAHs.
Table IV. Setschenow Constants for Some Aromatic Hydrocarbons at 25 " C Molar volume," Ks, Limo1 mL/mol Compound Benzene 0.175 F 0.006 89 Naphthalene 0.213 I0.001 125 Fluorene 0.267 t 0.005 153 Anthracene 0.238 i 0.004 160 Phenanthrene 0.276 0.010 160 2-Methylanthracene 0.336 z 0.006 .. 1-Methylphenanthrene 0.211 T 0.018 . . Pyrene 0.286 i 0.003 172 Fluoranthene 0.339 I0.010 175 Chrysene 0.336 i 0.010 194 l12-Benzanthracene 0.354 i 0.002 194 " Reported bv Davis and Gottlieb in Ref. 30. Table V. Correlations of Solubility with Molecular Parameters Molar vol- Molecular Carbon ume,a lengthb Compound No. mL A Benzene 6 89 5.5 Naphthalene 10 125 8.0 Fluorene 13 153 ... Phenanthrene 14 160 9.5 ... 1-Methylphen15 '.. anthrene Anthracene 14 160 10.5 ... 2-Methylan15 ... thracene Pyrene 16 172 9.5 Fluoranthene 16 175 9.4 1,2-Benzan18 194 11.8 thracene Chrysene 18 194 11.8 " Reported by Davis and Gottlieb in Ref. 30. ed by Klevens in Ref. 1 7 .
Molar solubility as (-ln S) at 25 " C 3.77 8.30 11.5 12.1 13.4 15.2 15.9 14.2 13.8 17.0 18.7 Report-
Correlations of Solubility with Molecular Parameters. The aqueous solubility of aromatic hydrocarbons has been shown by Klevens (17) to be related to carbon number, molar volume, or molecular length. These parameters along with molar solubilities (expressed as -In S ) of the compounds studied are presented in Table V. These data show that for each of these parameters, there are several compounds whose anomalous behavior makes accurate extrapolations of solubility from these relationships impossible. For example, anthracene and phenanthrene are structural isomers. They, therefore, have identical carbon numbers and very similar molar volumes. However, their aqueous solubilities differ by
1000
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE
1978
more than a factor of 20. Phenanthrene, fluoranthene, and pyrene have very similar molecular lengths; but their respective aqueous molar solubilities at 25 "C are 5.6 x lo4, 1.0 X lo4 and 6.8 X lo-' mol/L. Tsonopoulos and Pransnitz (19) have reported that the hydrocarbon infinite dilution coefficient, y-,is the appropriate quantity for correlating the aqueous solubilities of hydrocarbons. They, along with Leinonen et al. (15) and Pierotti et al. (20) have successfully correlated y- with carbon number, molar volume, and degree of branching. Recently Mackay and Shiu (18) have correlated the hydrocarbon infinite dilution coefficients of 32 aromatic hydrocarbons (using the super cooled standard state) with carbon number. From this relationship, they derived a parabolic equation from which individual solubilities could be calculated. A comparison of the correlated and experimental solubility values showed that solubilities could be estimated only to within a factor of 3.
CONCLUSION The DCCLC technique is a very rapid, precise, and accurate method for determining the aqueous solubility behavior of sparingly soluble organic compounds. The agreement of PAH aqueous solubilities and calculated solubility parameters, such as AH8, with values that have been previously reported in the literature helps to confirm the validity of this approach. In all cases, replicate solubility measurements of the generated PAH solutions at constant temperature were better than &3% and in most cases better than &l%.The lack of an appropriate physical parameter for accurately extrapolating PAH solubilities, further enhances the utility of DCCLC method. LITERATURE CITED (1) D. MacKay and W. Shiu, Can. J . Chem. Eng., 53, 239 (1975). (2) C.McAuliffe, J . Phys. Chem., 70, 1274 (1966).
(3) C. Sutton and J. A. Clader, J . Chem. Eng. Data. 20, 320 (1975). (4) D. Amo(d, C. Plank, and E. Erickson. Chem. Eng. Data Ser.,3,253 (1958). (5) R. Bohon and W. Claussen. J. Am. Cbem. Soc., 73, 1571 (1951). (6) F. Franks, M. Gent, and H. Johnson, J . Chem. SOC., 2716 (1973). (7) L. Andres and R. Keffer, J . Cbem. Soc., 3819 (1952). (8) T. Morrison and F. Billet, J . Cbem. SOC., 3819 (1952). (9) H. Vermillion, Ph.D. Thesis, Duke University, Durham, N.C., 1939. (10) R. R. Stearns, H. Oppenheimer, E. Simon, and W. Harkins, J . Chem. Phys., 14, 496 (1974). (11) A. HIII, J . Am. Chem. SOC.,44, 1163(1922), (12) M. Hayashi and T. Sasaki, Bull. Cbem. SOC. Jp, 29, 857 (1956). (13) H. Booth and H. Everson, Ind. Eng. Chem.. 40, 1491 (1948). (14) R. Brown and S. Wasik, J . Res. Natl. Bur. Stand., Sect. A., (45),78, 453 (1974). (15) P. Leionen. D. MacKay, and C. Phillips, Can. J . Chern. Eng., 49, 288 (1971). (16) W. Davis, M. Krohl, and G. Clower, J . Am. Chem. Soc., 84, 198 (1942). (17) H. Kelvens, J . Phys. ColloidChem., 54, 283 (1950). (18) D. MacKay and W. Shiu, J . Chem. Eng. Data, 22, 4 (1977). (19) C. Tsonopoulos and J. Prausnitz, Ind. Eng. Chern., Fundam., IO, 593 (1971). (20) C. Pierotti, C. Deal, and E. D w , Id.Eng. Cbem.. Fundam., 51,95(1959). (21) F. Schwarz, J . Chem. Eng. Data, 22, 273 (1977). (22) R. Weimer and J. Pausnitz, J . Chem. Phys., 42, 3643 (1965). (23) R. Wauchope and F. Getzen, J . Chem. Eng. Data, 17, 38 (1972). (24) L. Andrews and R. Keefer. J . Am. Chem. Soc., 71, 3644 (1949). (25) W. May, S. Wasik, and D. Freeman, Anal. Chem., 50, 175 (1978). (26) W. May, S.Cheder, S. Cram, B. Gump, H. Hertz, D. Enagonio, and S. Dyszel, J. Chromatogr. Sci., 13, 535 (1975). (27) W. May, Ph.D. Dissertation, University of Maryland, College Park, 1977. (28) J. Setschenow, 2. Phys. Cbem., 4 117 (1889). (29) W. McDevit and F. Long, J . Am. Cbem. SOC.,74, 1773 (1952). (30) H. Davis and S. Gottlieb, Fuel, 8,37 (1962).
RECEIVEDfor review February 7,1978. Accepted April 3,1978. The authors are grateful to the Office of Air and Water Measurement, National Bureau of Standards, for partial support of this work. This work is from a dissertation submitted in September 1977 to the Graduate School, University of Maryland by Willie Eugene May, in partial fulfillment of the requirements for a Ph.D degree in Chemistry.
High-Efficiency Packed Columns Using Fine Particles of Graphitized Carbon Black Antonio Di Corcia" and Maurizio Giabbai
Istituto di
Chimica Analitica, Universiti di Roma, Rome, Italy
The eflect of particle size of graphitized carbon black (Carbopack) on column efficiency has been investigated. A continuous decrease of the plate height for gas-modified solid columns has been observed by reducing the mean particle diameter from 185 down to 28 pm. As a result, a maximum of 10 000 plates per meter of column has been achieved by equal to 28 k m at a carrier gas velocity of 7 cm/s using a and with a pressure drop of 19 atm. It Is noteworthy that the optimum mobile phase velocity is practlcally unaffected by the reduction of ap. This effect coupled with the use of hydrogen as carrier gas has been exploited in the fast analysis of complex mixtures of practical Interest.
a,
Numerous efforts have been made in order to obtain packed columns with high numbers of theoretical plates. This goal has been obtained by using long columns operating at medium and high pressures (1-6). This approach has some drawbacks. First, long columns under given conditions require a proportionally long analysis time; second, long columns are often inconvenient to prepare and use; third, the increase in column 0003-2700/78/0350-1000$01 .OO/O
length results in a dilution of the sample and thus may cause the undetectability of trace-contained components in complex mixtures. Another approach to achieve high theoretical plate numbers is that of improving column efficiency by varying parameters which are correlate to the plate height. The treatment and interpretation of H1r.i data obtained by varying column parameters demand a choice of rate equation. Since 1940, various developments of concepts and mathematical expressions have been formulated in order to obtain a correct relation of all parameters which can affect the plate height. A simple plate-height equation for gas-solid packed columns, which has been shown elsewhere (7-10) to give a good reproduction of experimental data over a wide range of carrier gas velocity, is:
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