Environ. Sci. Technol. 1997, 31, 2998-3003
Measurement of Henry’s Constants of High-Volatility Organic Compounds Using a Headspace Autosampler JIAN PENG* AND AROMA WAN Department of Civil Engineering, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A9, Canada
A method using a headspace autosampler and gas chromatograph (GC) was developed to measure Henry’s law constant of volatile organic compounds (VOCs). Equilibrium of VOCs between gas and liquid was established in the 22mL headspace glass vials sealed with Teflon-faced silicone septa and cramp aluminum caps. Equilibration, sampling, and chemical concentration analysis by a high precision headspace autosampler-GC system minimize sample losses due to volatilization and give accurate results. The method uses several different gas-liquid volume ratios in the headspace vials to create different equilibrium concentrations in the gas phase. With data of the chemical concentration in gas versus the different gas-liquid volume ratio, Henry’s constant is determined by regression analysis of a headspace system equation derived from mass balance principle and ideal gas law. Henry’s constants for benzene, toluene, trichloroethylene, and tetrachloroethylene at 15, 20, 25, 30, 35, 40, and 45 °C were measured with this method and compared with literature data.
Large quantities of volatile organic compounds (VOCs) have been released into the environment during their production, distribution, application, and disposal. Many of them are found toxic to animals, human beings, and aquatic organisms and plants. The growing number of VOCs under present or potential regulation has promoted extensive studies on structure-activity relationships (1) and on various mass transport models in order to determine the distribution of such chemicals in the different environmental compartments. Henry’s law constant is one of the most important physical properties of VOCs to determine the partitioning of the chemical between air and water. It is defined as a proportionality constant between gas partial pressure and liquid concentration of a given chemical at equilibrium under a constant temperature:
KH )
Pi x
(1)
where KH is the Henry’s constant of the chemical (atm), Pi is the partial pressure of the chemical in air (atm), and x is the mole fraction of chemical in the liquid phase. Another common expression used in the environmental literature is
K H′ )
Cg Cl
(2)
where KH′ is the non-dimensional Henry’s constant, Cg is the * Corresponding author telephone: (306) 966-5431; fax: (306) 9665427; e-mail:
[email protected].
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KH′ )
vw K RT H
(3)
where νw is the molar volume of water (L/mol), R is the ideal gas constant, and T is the absolute temperature (K). The values of Henry’s constant are required by many mass transport models that attempt to describe movement of volatile pollutants between the different environmental compartments and by design and performance models of air-stripping processes for the remediation of organic solventcontaminated waters. Unfortunately, these data in literature are incomplete, and the reported values are often scattered considerably. As shown in Table 1, the reported Henry’s constant at 20 °C for benzene varies from 0.0996 (2) to 0.24 (3) and for trichloroethylene (TCE) varies from 0.243 (2) to 0.42 (3). A number of methods to determine Henry’s constant have been presented in the literature. They include (i) calculation from vapor pressure and solubility data (4); (ii) direct measurement of equilibrium concentrations in air and liquid (4); (iii) equilibrium partitioning in closed systems (EPICS) (5-7); (iv) bubbled column technique (4, 8); (v) multiple equilibration of a closed system (9, 10); (vi) quantitative structure-activity relationship techniques (QSAR model) (1); and (vii) wetted-wall column technique (11). Merits and deficiencies of the various methods have been reviewed by Mackay and Shiu (4) and Gossett (6). In this research, a method using a headspace autosampler was developed to measure Henry’s constant.
Method
Introduction
2998
gas phase chemical concentration (mol/L), and Cl is the liquid phase chemical concentration (mol/L). At normal environmental conditions and dilute chemical concentration, KH can be converted to KH′ as
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 10, 1997
A Genesis headspace autosampler equipped with a 50-vial carousel and a Star 3400 CX gas chromatograph (GC) from Varian was used in the measurement of Henry’s constant. The headspace sampler has an oil-free, resistance-heated, jacketed, variable temperature controlled 12-vial sample platen. Temperature in the sample platen can be controlled precisely and uniformly within (0.1 °C. Figure 1 shows the flow diagram of the Genesis headspace autosampler. Glass headspace vials with capacity of 22 mL were partially filled with the prepared chemical solution. The vials were then sealed with Teflon-faced silicone septa and cramp aluminum caps and loaded into the headspace autosampler for a period of time to reach equilibrium at a preset temperature. According to mass balance principle, the total chemical mass added to each vial is equal to the summation of the chemical solute in gas phase and liquid phase at equilibrium:
Cl,oVl ) ClVl + CgVg
(4)
where Cl,o is the initial chemical concentration in the prepared liquid solution (mg/(mL of water)), Vl is the liquid sample volume added into the vial (mL), and Vg is the headspace gas volume in the vial (mL). Substitution of eq 2 into eq 4 gives
Cl,o Vg 1 ) + Cg KH′ Vl
(5)
After equilibrium, the test vials were automatically raised onto the sampling needle, the needle was punctured in, and helium gas filled the vial through the needle, pressurizing the vial to a preset pressure, Ploop ) 0.35 atm gauge. Increase of the
S0013-936X(97)00240-X CCC: $14.00
1997 American Chemical Society
headspace vials during the experiments. Linear regression of Cl,o/A against Vg/Vl gives the slope and intercept:
TABLE 1. Literature Values of Dimensionless Henry’s Constant benzene
trichloroethylene
temp (°C)
KH′
ref
10 10 15 15 20 20 20 20 20 25 25 25 25 29 30 30
0.123 0.142 0.159 0.164 0.0996 0.24 0.189 0.18 0.188 0.234 0.244 0.224 0.216 0.267 0.294 0.289
7 17 7 17 2 3 7 14 17 4 7 15 17 16 7 17
slope )
temp (°C)
KH′
ref
9.6 10 10 15 17.5 20 20 20 20 20 20 24.8 25 25 30 30.0 34.6
0.163 0.232 0.19 0.282 0.265 0.243 0.41 0.350 0.335 0.411 0.42 0.392 0.479 0.471 0.514 0.527 0.591
6 17 18 17 6 2 14 17 18 19 3 6 15 17 17 18 6
total pressure inside the vial does not change the partial pressure of tested compounds since the partitioning of chemicals in the gas phase is governed by the Henry’s law. At low or modest pressures, the effect of pressure on the partitioning between the gas and liquid phase is negligible (12). Increase in gas volume (Vg) inside the headspace vials is negligible due to the slight increase in pressure since the vials were sealed with 3 mm thick septa and strong aluminum caps. The pressure valve V1 was then closed allowing the system to equilibrate. Then the vent valve V2 was opened, and the motor-actuated six-port valve V6 was switched to connect the headspace vial to the vent valve V2 through the 1-mL sample loop. The pressurized gas inside the sample vial exited through the sample loop to the vent until the total pressure inside the vial and the loop was balanced with ambient pressure of Patm. According to the ideal gas law, the chemical concentration in the sample loop is
Cg,loop ) Cg
Patm Ploop
(6)
The valve V6 was then switched to connect the carrier gas through sample loop to the GC. The gas sample in the loop was swept by carrier gas into the GC column, and the concentration was analyzed by GC flame ionization detector (FID). The transfer lines of the autosampler were heated to a temperature of 110 °C to prevent sorption. Normally, the integrated area counts of GC peak for a given chemical is proportional to the amount of chemical solute injected into the FID detector:
A ) kVloopCg,loop
(7)
where A is the integrated area counts of GC peak for a given sample, k is a constant for a given chemical, and Vloop is the volume of sample loop. Substitution of eq 6 into eq 7 and solving for Cg:
Cg )
A Ploop kVloop Patm
(8)
Substitution of eq 8 into eq 5 yields
(
)
Cl,o Ploop Vg 1 ) + A kVloopPatm KH′ Vl
(9)
Equation 9 can be used to determine Henry’s constant of VOCs by using various gas-liquid volume ratios in the
intercept )
Ploop kVloopPatm
(10)
Ploop 1 kVloopPatm KH′
(11)
Henry’s constant is calculated as
KH′ )
slope intercept
(12)
Two principle requirements for a successful application of the method are that the system must reach the equilibrium and the GC detector response must be linear throughout the interested concentration range.
Experimental Section Benzene purchased from Annachemia Ltd. with a purity of 99.9+%, toluene from OmniSolv with a purity of 99.9+%, trichloroethylene (TCE) from Annachemia Ltd. with a purity of 99.9+%, and tetrachloroethylene (PCE) from BDH with a purity of 99.8+% were tested in the study. These four toxic chemicals were listed on the EPA’s Top 50 TRI (Toxics Release Inventory Public Data Release) Chemicals in 1994. A stock solution having a volume fraction of 100 µL/(L of water) was prepared for each individual compound. An Eppendorf 2100 pipet with an adjustable capacity of 10-100 µL was used to transfer 100 µL of each selected chemical into a 1-L Amber glass bottle filled with distilled and deionized water. The glass bottle was sealed tightly by a screw cap with a Teflon-faced septum and stirred with magnetic stirrer for 24 h for dissolution. The stock solution was then stabilized for 24 h in a refrigerator at 4 °C. For the GC FID linear response check, the stock solution was diluted to a volume fraction of 1 µL/L for each compound by transferring 10 mL of stock solution using a Brinkmann Macro-Transferpettor (inaccuracy: -0.09%, range: 2-10 mL) into a 1-L amber glass septum bottle filled with distilled and deionized water. The bottle was stirred for 4 h and then stabilized for 24 h in the refrigerator at 4 °C. Headspace samples were prepared by dispensing 2, 3, 4, 5, 6, 7, 8, and 10 mL of 1 µL/L solution into eight 22-mL headspace sample vials prefilled with 8, 7, 6, 5, 4, 3, 2, and 0 mL of distilled and deionized water, respectively. Distilled and deionized water was transferred using the Brinkmann Macro-Transferpettor, and 1 µL/L chemical solution was transferred using a positive displacement digital bottle top dispenser (maximum inaccuracy: 0.15%, range: 1.0-10.9 mL). This procedure minimizes the possible chemical loss due to volatilization. The concentration of 10 mL of liquid mixture in each headspace vial was 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 100% of the 1 µL/L solution, respectively. For the experiment to determine the Henry’s constant, a similar procedure was used to prepare the test solutions. The stock solution was diluted to 0.5 µL/L by adding 5 mL of stock solutions in 1-L bottles for each compound. The method developed in this study does not require accurate initial liquid concentrations since the dimensionless Henry’s constant is determined by the slope and intercept of linear regression of eq 9. Sample volumes in 1.5, 2, 2.5, 3, 4, 5, 7, and 10 mL of the test solutions were added into the headspace vials using the positive displacement bottle top dispenser to create different gas-liquid volume ratios. Three replicates were prepared for each gas-liquid volume ratio. A single measurement was taken from each vial. Within the set of 24 vials used for the determination of one value of Henry’s constant, it is important that the initial experimental liquid concentration be constant. Filling the vials is probably the
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FIGURE 1. Headspace autosampler flow diagram. most critical experimental step. Using the positive displacement bottle top dispenser eliminated the possible loss by volatilization that could occur to common vacuum type pipets. Special care was taken during the filling. The Teflon tip of the dispenser was inserted to the bottom of the vial to inject the required volume of liquid solution quickly, and the vial was sealed immediately. It took less then 5 s to complete the filling and sealing of each vial. In order to determine the time required for all the headspace samples to reach equilibrium, 10 mL of liquid sample volume was tested at 15 °C with an equilibration time of 5, 10, 15, 20, 25, 30, 35, 40, and 45 min, respectively, since the largest liquid volume and lowest temperature require the longest time to equilibrate (5). The sufficient equilibration time was determined from the result of the equilibration test and used thereafter for GC detector linear response check and measurement of Henry’s constant. Two linear response tests were performed for each chemical, one at 15 °C and the other at 45 °C. Henry’s constant was measured for each chemical compound at 7 different temperatures: 15, 20, 25, 30, 35, 40 and 45 °C, respectively.
Results and Discussion Full equilibrium of the chemical solute between liquid and gas phases is required for the determination of Henry’s constant. Lincoff and Gossett (5) indicated that the time to reach equilibrium in larger liquid volume system is considerably longer than in low liquid volume and that high Henry’s law constants are unfavorable for reaching the equilibrium. Also the lower the temperature, the longer time it needs to equilibrate. In their preliminary experiments, it was found that, without any agitation, full equilibrium was achieved in a few hours for the 100-mL volume liquid system. The sample volumes in headspace vials used in our method are from 1.5 to 10 mL, which is less than one-tenth of the liquid volume in Lincoff and Gossett’s study (4). In addition, a few minutes of mixing at beginning of the equilibration, which is provided by the headspace autosampler, can accelerate equilibration process considerably. Figure 2 shows the result of the equilibration test. For the chemicals tested,
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FIGURE 2. Equilibration test of GC headspace system. it takes around 30 min to reach full equilibrium. Sixty minutes was selected for the Henry’s constant measurement in this study. Linear response of the GC detector to the chemical concentration range tested is another requirement for the successful application of the method. Figure 3 shows that the GC detector responds linearly with concentration for all the chemical compounds tested. The correlation coefficients are all better than 0.998. Gas chromatographic responses to various gas-liquid volume ratios in the headspace vials for benzene, toluene, trichloroethylene, and tetrachloroethylene were measured at seven temperatures ranging from 15 to 45 °C. Linear regression analysis was performed on eq 9 with Cl,o/A against Vg/Vl for each compound, respectively. Figure 4 shows the result of linear regression analysis on trichloroethylene. Each point in Figure 4 represents the average value of the three replicates. The standard deviation over average among the three replications are all within 1%. Henry’s constants were then determined by the slope and the intercept of linear regression results. The correlation coefficients for all the
FIGURE 3. Linear responses of FID detector to selected VOCs.
FIGURE 4. Experimental results of trichloroethylene. measurements are better than 0.9982. The measured dimensionless Henry’s constants for benzene, toluene, trichloroethylene, and tetrachloroethylene and 95% confidence intervals are listed in Table 2. Perlinger, Eisenreich, and Capel (7) have studied the sorption of hydrophobic organic compounds to solid surfaces. They found that the sorption of benzene and toluene to the glass container or to the airwater interface was negligible. A similar reasoning they used in their study can be applied to Figure 4 in this study. If there was noticeable sorption of the tested chemicals to the glass vials or the air-water interface, it would affect eq 9 and the plot of Cl,o/A against Vg/Vl in Figure 4 would not be linear. Temperature affects the Henry’s law constant of volatile organic compounds significantly. The temperature dependence of Henry’s constants can be well predicted by the van’t Hoff relationship (13):
log10 KH ) a -
b T
(13)
The parameters a and b of eq 13 were determined by regression analysis for each compound. The results of the regression are summarized in Table 3. The correlation coefficients are all better than 0.9973. A typical result of the temperature effect on trichloroethylene along with 95% confident interval is plotted in Figure 5, showing very good agreement. Figure 6 gives a comparison of this work with some literature values for trichloroethylene and tetrachloroethylene. The results of this work agreed well with most of literature data for tetrachloroethylene. For trichloroethylene, at moderate to high temperatures, the measured Henry’s constants were lower than those from Kavanaugh (14), from the vapor pressure and solubility data method (18, 22), and from Gossett (5, 6), and were higher than those from the UNIFAC method (18). Burkhard et al. (21) have pointed out that the error in vapor pressure measurement is at best 6%, but may be much larger, a factor of 2-3, which inevitably leads to large errors in estimation of Henry’s constant by using the vapor pressure and solubility data method. As shown in Table 1, the reported Henry’s constants from the different researchers vary considerably. This indicates that it is difficult to measure the Henry’s constants of high-volatility compounds accurately. Large errors could be introduced due to chemical loss by volatilization during the sample preparation, handling, and chemical analysis. High temperature certainly increases the volatilization rate. In the method developed in this study, filling the headspace vials is the only step to involve possible chemical loss by volatilization, and this loss is minimized by using the positive displacement bottle top dispenser to fill the vials within 5 s. The method does not require accurate initial liquid concentrations. Therefore, if there was any chemical loss prior to filling step, it would not affect the final results. In addition, the equilibration, sampling, and chemical concentration analysis were performed by a high-precision headspace autosampler. It is reasonable to believe that the method gives accurate results. The method developed in this study relies on the variation of chemical concentrations in the gas phase over the range of the gas-liquid volume ratios employed in the headspace glass vials. It may not be sensitive enough when dealing with chemicals having low Henry’s values. The sensitivity of the method can be appreciated by examining the relative change in GC detector readings in response to the relative change in the gas-liquid volume ratio. Taking partial derivative of the area count of GC peak, A, with respect to the gas-liquid volume ratio, Vg/Vl,
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TABLE 2. Results of Regression Analysis on eq 9 KH′ ( 95% temp (°C)
benzene
toluene
trichloroethylene
tetrachloroethylene
15 20 25 30 35 40 45
0.153 ( 0.001 0.179 ( 0.002 0.217 ( 0.002 0.268 ( 0.004 0.299 ( 0.003 0.356 ( 0.002 0.398 ( 0.005
0.163 ( 0.001 0.195 ( 0.002 0.244 ( 0.002 0.307 ( 0.006 0.349 ( 0.004 0.424 ( 0.004 0.495 ( 0.005
0.240 ( 0.003 0.291 ( 0.003 0.357 (0.004 0.463 ( 0.011 0.522 ( 0.010 0.634 ( 0.005 0.758 ( 0.012
0.424 ( 0.006 0.513 ( 0.005 0.690 ( 0.005 0.857 ( 0.024 1.059 ( 0.013 1.261 ( 0.016 1.553 ( 0.027
TABLE 3. log10 KH ) a - b/T Correlation benzene toluene trichloroethylene tetrachloroethylene
a
b
R
7.15 7.94 8.19 9.06
1397 1621 1642 1822
0.9973 0.9983 0.9988 0.9991
a
FIGURE 5. Effect of temperature on Henry’s law constant of trichloroethylene. throughout eq 9 gives
-
Cl,o ∂A Ploop ) 2 Vg kVloopPatm A ∂ Vl
()
(14)
b
The gas-liquid volume ratio used in the method varied from (22 mL - 10 mL)/10 mL ) 1.2 to (22 mL - 1.5 mL)/1.5 mL ) 13.7. The relative change in GC response to the relative change in the gas-liquid volume ratio can be estimated from eq 14 as
|/ |( ()
Vg ∂ Vl ∂A ) SR ) A 13.7 - 1.2
12.5 Vg 1 + KH′ Vl
)
(15)
where SR is the sensitivity response of the method. Equation 15 indicates that the sensitivity of the method decreases with decreasing dimensionless Henry’s value. The SR value varies from 0.527 at low liquid volume to 1.116 at high liquid volume if the dimensionless Henry’s value equals 0.1, indicating that the method is sensitive. For the dimensionless Henry’s value equal to 0.05, the SR value varies from 0.371 at low liquid volume to 0.590 at high liquid volume. For chemicals with dimensionless Henry’s values less than 0.05 the method loses sensitivity. Chemicals with dimensionless Henry’s constants greater than 0.05 represent high-volatility compounds (3).
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FIGURE 6. Comparison of this work with literature results for trichloroethylene (a) and tetrachloroethylene (b). The method developed in this work offers several advantages. It requires substantially less equilibration time due to the small liquid volume used in the headspace vials. Once the headspace vials are prepared, the remaining procedures of the measurement such as equilibration, sampling, and chemical concentration analysis are performed by a highprecision headspace autosampler. This reduces manual steps in sample handling to minimum, which in return eliminates other types of errors, and avoids significant sample losses due to volatilization. It also gives consistent results since the temperatures of both liquid and gas phases are controlled uniformly and accurately by the autosampler within (0.1 °C of the designated temperature. The Henry’s constant is determined based on a range of different equilibrium concentrations by using eight different gas to liquid volume ratios and three replicates for each ratio. The method does not require accurate initial liquid concentration of the test sample solutions.
Acknowledgments This work was supported through an NSERC individual research grant and an NSERC equipment grant from the Natural Science and Engineering Research Council of Canada. We are grateful to Mr. D. Fisher, Technologist at the University of Saskatchewan, for invaluable assistance in the environmental laboratory. Author-Supplied Registry Numbers: Benzene, 71-43-2; tetrachloroethylene, 127-18-4; trichloroethylene, 79-01-6; toluene, 108-88-3.
Literature Cited (1) Nirmalakhandan, N.; Speece, R. E. Environ. Sci. Technol. 1988, 22, 1349-1357. (2) Noonan, D. C.; Curtis, J. T. Groundwater Remediation And Petroleum: A Guide For Underground Storage Tanks; Lewis Publishers: Boca Raton, FL, 1990; 250 pp. (3) Thomas, R. G. In Handbook of Chemical Estimation Methods: Environmental Behavior of Property Organic Compounds; Lyman, W. J., Reehl, W., Rosenblatt, D. H., Eds.; McGraw-Hill: New York, 1981; Chapter 15. (4) Mackay D.; Wan, Y. S. J. Phys. Chem. Ref. Data 1981, 10, 11751199. (5) Lincoff, A. H.; Gossett, J. M. In Gas Transfer at Water Surfaces; Brutsaert, W., Jirka, G. H., Eds.; Reidel: Dordrecht, Holland, 1984; pp 17-25. (6) Gossett, J. M. Environ. Sci. Technol. 1987, 21, 202-208. (7) Perlinger, J. A.; Eisenreich, S. J.; Capel, P. D. Environ. Sci. Technol. 1993, 27, 928-937. (8) Makay D.; Wan, Y. S.; Sutherland, R. P. Environ. Sci. Technol. 1979, 13, 333-337.
(9) McAuliffe, C. D. Chem. Technol. 1971, 1, 46-51. (10) Hunter-Smith, R. J.; Balls, P. W.; Liss, P. S. Tellus 1983, 35B, 170-176. (11) Fendinger, N. J.; Glotfelty, D. E. Environ. Sci. Technol. 1988, 22, 1289-1293. (12) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases & Liquids, 4th ed.; McGraw-Hill: New York, 1987; 741 pp. (13) Snoeyink, V. L.; Jenkins, D. Water Chemistry; Wiley: New York, 1980; 463 pp. (14) Kavanaugh, M. C.; Trussell, R. R J. Am. Water Works Assoc. 1980, 72, 684-692. (15) Hine, J.; Mookerjee, P. K. J. Org. Chem. 1975, 40, 292-298. (16) Hansen, K. C.; Zhou, Z.; Yaws, C. L. J. Chem. Eng. Data 1993, 38, 546-550. (17) Ashworth, R. A.; Howe, G. B.; Mullins, M. E.; Rogers, T. N. J. Hazard. Mater. 1988, 18, 25-36. (18) Munz, C. D. Ph.D. Dissertation, Stanford University, 1985. (19) Roberts, P. V.; Dandliker, P. G. Environ. Sci. Technol. 1983, 17, 484-489. (20) Leighton, D. T., Jr.; Calo, J. M. J. Chem. Eng. Data 1981, 26, 382-385. (21) Burkhard, L. P.; Armstrong, D. E.; Andren, A. W. Environ. Sci. Technol. 1985, 19, 590-596. (22) Horvath, A. L. Halogenated Hydrocarbons Solubility-Miscibility with Water; Marcel Dekker, Inc.: New York, 1982; 889 pp.
Received for review March 14, 1997. Revised manuscript received June 22, 1997. Accepted June 29, 1997.X ES970240N X
Abstract published in Advance ACS Abstracts, August 15, 1997.
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