A Laboratory Method for the Determination of Henry's Law Constants of Volatile Organic Chemicals Keith C. Hansen, Zhou Zhou, and Carl L. Yaws Lamar University, Beaumont, Texas 77710 Tejraj M. Aminabhavi Karnatak University, Dharwad, India 580 003 In the development of concepts in physical chemistry the dilute solution has played a very important role. Consequently, both in theory and experiment, the study of very dilute solutions has been given a prominence perhaps even greater than it intrinsically deserves. The various laws of dilute solution are really laws of the infinitely dilute solution. It remains for experiment or theory to show how far into the range of finite concentration these laws may be applied without material error. There are several of these laws which, although discovered independently, may be shown to be thermodynamically deducible from one another: van't Hoff'slaw of osmotic pressure, the laws for the lowering of vapor pressure and freezing point of a solvent obtained by Raoult and van't Hoff, and Henry's law for the partial pressure of a solute. These are related through thermodynamics, and if any one of these laws is based on empirical fact, then others will follow (I). I n this paper, we shall consider the application of Henn's law. which in its orizinal .. form states that the Dartial pressure of a solute is proportional to its concentration. This conceDt was established and tested by Willlam Henry in 1803 i n a series of measurements of thk dependence oh pressure of the solubility of gases in liquids. Henry's law is thus applicable to dilute solutions involving gas-liquid, liquid-liquid, and solid-liquid systems. In most standard textbooks, the Henry's law constant H is conveniently expressed as a ratio of partial pressure Pi in the vapor (in various units such as Pa, atm, or tom) to the concentration C;in the liquid (also in various units such as mole fraction and mass or mole concentration) so that
The most commonly used measures of concentration are mole fractionx and amount of substance concentration (expressed in mol m3), which yields H in the units of Pa m3 mol-'. Recently, there is a growing interest in the study of Henry's law constants, which describe the equilibrium partitioning between a liquid and a gas phase to predict the fate of volatile organic chemicals (VOC'S) in the environment (2). Thus, the Henry's law constant, referring to the airlwater equilibrium is one of the most important parameters governing the distribution of chemicals in the environment. In a two-phase system containing air and water, volatile organic chemicals will continue to volatilize from or dissolve into liquid until chemical potential (or fugacity) is equalized and equilibrium is reached between the two phases. Chemicals exhibiting low values of H will tend to accumulate in the aqueous phase, whereas those with high values of H will partition more into the gas ~ h a s eBecause . air and water are the maior com~onentsof ihe modern ecosphere and water is considered to'act as the link b6,tween all its other compartments, knowledge ofH is
very important in assessing the environmental risks associated with a chemical. In this paper, the laboratory measurements of Henry's law constants are reported for 10 chemicals spanning a wide range of chemical structures and volatilities. An innovative static headspace method referred to as equilibrium partitioning in closed systems (EPICS) was nsed to measure Henry's law constant. Measurements were carried out over a temperature range of 26-45 "C, and the results were correlated with a temperature-regression equation. Techniques To Measure Henry's Law Constants Experimentally, Henry's law constants have been determined by two different approaches. In one method known as the static method, the measurement of equilibrium concentrations is carried out in a closed svstem ( 3 . 4 ) .In the other, called the dynamic method, an open-flow &stem usine water air exchange is used (5).Both these methods req&e the determination of gas concentrations. In the chemical literature, several modifications of these methods have been used to obtain the values ofH. These include indirect methods like partial-pressure measurements (6, 7) and direct methods including the air- and water-phase concentrations (8).Mackay et al. (9)introduced a novel gas-purging technique to obtain Henry's law constants for several hydrophobic compounds; a purge-vessel technique has also been described by Oliver (10). The EPICS Method The EPICS method nsed originally by Gossett and Lincoff (11)is an attractive procedure to determine Henry's law constants of volatile organics. This method is based on a closed system of mass balance for a given VOC distributed between liquid and gas phases in contact. A component balance takes the general form
where M is total organic mass in the system (mol); CI is the liquid-phase organic concentration (mol/m3);C, is the gasphase organic concentration (moWm3);VIis the total liquid volume (m3); and V, is the total gas volume (m3). If the same mass of VOC is introduced into two closed containers (i.e., sealed septum bottles) containing different volumes of pure water, an expression similar to eq 2 may be written for each system. Equating these mass-balance expressions and introducing Henry's law to substitute for the liquid-phase VOC concentrations gives an equation relating Henry's law constant to headspace concentrations and known volumes as (11)
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Injection volume (pL) Figure 1. Detector response vs volume for imn%lP-dichloroethylene. where H i s a dimensionless parameter (m3of liquid)/ (m3of gas), and the subscripts 1and 2 identify the two closed systems. Thus, the only experimental information needed to determine H is the ratio of gas-phase concentrations (C,I /C@)in the two systems. Thus, absolute concentrations are not necessary because any proportional measure of concentration, such as GC peak areas, yield the desired headspace concentration ratio in eq 3. In practice, the EPICS method involves measurement of multiple pairs of highnow volume closed systems to obtain several independent estimates of H. For example, if three pairs are used, then three replicate headspace measurements will be obtained for each of the two liquid volumes. Thus, the six values can be paired in all permutations to calculate nine estimates of H. Typically, an arithmetic average of these estimates is taken to be the true value ofH, with the associated values reported. Experimental Protocol A total of 10 volatile organic compounds, ranging from halogenated liquids, aromatics, and alkanes, have been used for experimentation. A saturated stock solution for each component is prepared by adding an amount of organic solute slightly in excess of the solubility limit to a volume of pure distilled water. All stock solutions are prepared in 250-mL amber bottles and allowed to equilibrate for at least two weeks for preparation of EPICS samples. For compounds less dense than water, the stock solution is transferred to a 250-mL separatory funnel the day before using. Solution is then withdrawn from the bottom of the separatory funnel for making the EPICS samples. In preparing the EPICS samples, three pairs of 250-mL amber glass bottles are filled with 20 mL and 200 mL of distilled water. The same volume of saturated stock solution is then added to each of the six septum bottles. The solution volume added is dependent upon solubility of VOC under investigation. This was to ensure that the studies were conducted in the region where Henry's law is obeyed (i.e., very dilute solutions). For high concentration stock solutions, 1mL was chosen as the lower limit for addition to the EPICS bottles. For low concentration stock solutions, 10 mL was chosen as the upper limit to be added. The bottles are sealed with a silicone rubber septum cap 94
Journal of Chemical Education
with a poly(tetrafluoroethy1ene)liner facing toward the bottle headspace. These liners are used only once to prevent absorption of test compound into the silicone rubber once the liner has been pierced. The loaded bottles are shaken vigorously by hand and then placed in a constanttemperature water bath for about 24 h and shaken regularly. After equilibration, headspace samples are withdrawn from the bottles via gas-tight syringe and injected into a Varian 3300 gas chromatograph equipped with a flame ionization detector (FID). The GC conditions are adjusted for retention times within 1 min. The principle requirement for the successful application of the EPICS method is that the instrument response be linear through the concentration range of interest. Therefore, the instrument response was examined in the beginning of the experiment with three compounds: trans-1,2-dichloroethylene,toluene, and cyclohexane. These compounds were chosen because they represent the types of compounds selected,that is, halogenated compounds, aromatics, and alkanes. The linearity of the detector response was confirmed using a sample with a constant headspace concentration and then injecting varying volumes of samples. The detector response was found to be linear with respect to volume iniected as shown tvoicallv in Firmre 1for trans-1.2-dichlo.. roethylcne. For other con~poundssuch a linearity was also observed with a correlation coefficientof meater than 0.95 in all cases.
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Discussion of Results H e d s law constants for 10 organic liquids included halogenatkd compounds, aromatic liquids, &d alkanes in dilute aqueous solutions. These compounds were selected becausethey reflect a complete range of H values to be determined. The measurements were made at three temperatures ranging from 25 to 46 "C. The Henry's law constants given in Table 1for some representative chemicals compare well with the literature data. In general, as seen in Table 2, &methylnaphthalene, methylcyclohexane, 2-methylcyclohexane, and n-heptane exhibit higher values of H as compared to the halogenated Table 1. Comparison of Henry's Law Constants for Some Halogenated Compounds compound
Temperature
H (kPa m3 mol )
'C
this study carbon tetrachloride 27.6 35.0 45.0 iransl,2-dichloroethylene
26.2 35.0 46.1
1.1,l-trichloroethane 26.3 35.0 44.8 1,I ,2-trichloroethane 26.2
35.8 44.8
lit (I;?
Table 2. Henry's Law Constants and Standard Errors compound
temperature
"C
bromobenzene
A
compound bromobenzene carbon tetrachloride trans-12-dichloroethylene I,I .I-trichloroethane 2-methylnaphthalene 124-trimethylbenzene cyclopentane mheptane moctane
0.256 f 0.003
30.0
Table 3. van't Hofl Parameters and Standard Errors
H (kPa m3 morl )
carbon tetrachloride
I .l.l-trichloroethane
trans-1,Pdichloroethylene
B
16f 1.11 13 f 0.74 11 f 1.96
-5341 f 346 -3553 f 230 -3396 f 602 3120 f 93 -1234 f 44 -4298 f 686 3351 f 633 -3730 f 68 -8014f 1617
0.10~ 1.97 2.61 9.38
11 f 0.30 1.58 7f0.14 0.70 14 f 2.24 6.44 14 f 2.03 7.63 6 16 f 2.22 9.41 30 f 5.25 20.30
roethylene is shown in Figure 2. The data of Ashworth et al. (12) and those of Gossett (15) are also included in this figure. The parameters A and B as obtained by the linear regression analysis along with the standard errors are given in Table 3.
2-methylnaphthalene
1.2,4-trimethylbenzene
Conclusion cyclopentane
methylcyclohexane
2-methylhexane
mheptane
26.0 35.8 45.0
compounds. The Henry's law constant for bromobenzene is very small, but high for n-heptane. With most liquids, the values of H tend to increase with an increase in temperature except for 2-methylhexane for which the values decrease with a rise in temperature. The values of standard errors are also reported in Tables 1 and 2. A good agreement is generally seen between the present data and those of Ashworth et al. (22). The temperature dependence of Henry's law constant h a s been studied (13, 14) using the vanstHoff relation of the form
91.294 f 0.986 121.083f 1.412 193.024-t1.170
-
L
I
The EPICS is an attractive methodology for a direct determination of Henry's law constants of volatile organics. It is inherently a simple and accurate technique that uses the standard gas chromatographic headspace analysis; the absolute gas-phase concentrations are not needed for the analysis. In general, the data obtained in this study agree reasonably well with the literature findings, and the method requires no special apparatus. In addition, the method is unencumbered by equilibration problems that limit the utilitv of other methods. The constants re~orted may have numerous environmental applications, such as in studies related to contamination of drinkine water with volatile organics. For a classroom teaching, this experiment demonstrates a number of concepts discussed in analytical chemistry courses. With some background in chemical thermodynamics and operational skills on gas chromatography, students can carry out this experiment. Effective educational use of the method and economv of the time can be achieved I
--
--
I
I
----
E, -
I
Ashworth et al
-
-Present work
5
-
--
-
1.0
2
Q
B
"
5
-
bet1
B
---
-
-
-
A lnH=-+B
T
(4)
0.1 00031
I
I
OW32
00233
I
OW34 Linear behavior was obtained I/T(K) in all cases. A typical van't Hoff plot for trans-l,2-dichlo- Figure 2. Typical van't Hoffplot for trans-12-dichloroethylene.
I
O.M)35
I
OJYl36
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by assigning different experiments to different pairs of students. At the end of the experiments, a session may be conducted to discuss and cnmparc results from each group ofntudents. Finallv. re",because the actual GC ex~eriments quire no extensive background, everyone in a laboratory section can carry them out without much difficulty. Moreover, these experiments demonstrate the full utility of the laboratory techniques in understanding the environmental pollution problems. Acknowledgment
The authors are thankful to the Robert A. Welch Foundation, Houston, Texas for financial support of this study. T. M. Aminabhavi thanks the administrators of Karnatak University, Dhamad, India for permission to use the summer vacation of 1991 to participate in this program.
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Journal of Chemical Education
Literature Cited 1. Adamson,A W A M b d ofPhysiml Chomisf'y;Aeademic:NeaYork, 1979. 2. Burrnard, L. P: Andre", A W.:h b o n g , D. E. Enuimn. sei. nchnol. 1985,19, C"rLG", """
3. Puns. 0.R.: Reusrutz. J. M I d E w . Chpm. Fundam, 1971.10.658.
7. Westcott, J. W: Simon, C. C.;Bidleman, T. P.Enuimn. 9ei. Tochnd. 1981,lI. 137% 1378. in the 8. Murphy, T. J.;Pobjoloanyk J. C.;Mullin, M. D. JnPhysMlBehoui~~afPCBg G m f h s : Maekay, D.. et el., Eds.; Ann Arbor Science Ann A r k , MI, 1983; Chapter 3, pp 49-58. 9. M a c h % D.; Shin, W Y.; S u b l a n d , R PEnuimn. Sci. lbchnd. IW9,13,333337. 10. Oliver, B. G. Chemphshs 108.5,14, 1087-1106, 11. LincoE, A H.; Oossett, J.M. The Determination of Henry's Constant. for Volatile Organic. by Equilibrium Partitioningin Closed Systems"InBrutraert, W.; Jirka, G. H., Eda.: Oos lionsferot Wolw Surfom, 1F25: D.Reidel, 1984. 12. Ashworth, R. A: Howe, G B.; M d i m , M. E.; Bogem, T. N. Amen Inst. Chem.Eng. 1986, Summer National Meetmg, Aug 1986, Boston, M A 13. Nirmalakhandan,N. N.: Speeee, R. E. Enuimn. Sei. l k h n d . 1888.22, 1349-1357. 14. Schoene, K: Steinhanses, J.: Freneeaius Z.AM1. Cham. 1965,321,530443. 16. Gassen, J.M. Enuimn. Sci. lkhnol. 1981.21,202-208.
