Anel. Chem. lW3, 65,3113-3118
alia
Using the Static Headspace Method To Determine Henry’s Law Constants Gary A. Robbins Department of Geology and Geophysics, U-45,345 Mansfield Road, University of Connecticut, Storre, Connecticut 06269-2045
Suya Wang and James D.Stuart’ Department of Chemistry, U-60,215 Glenbrook Road, and the Environmental Research Institute, University of Connecticut, Storrs, Connecticut 06269-3060
A new, accurate, and experimentally simple method has been developed to determine dimensionless Henry’s law constants using the static headspace method. The method appears applicableto a wide range of volatile and semivolatile organic compounds. Themethod worked well even for methyl tert-butyl ether (MTBE), despite its very high water solubility and hence, low Henry’s law constant. The approach developed extends the usefulness of the static headspace method in obtaining real-time, accurate information for assessing environmental problems. INTRODUCTION Volatile organic chemicals are often found as contaminants in ground water and soil. This problem stems, for the most part, from improper disposal, spills, and leakage of gasoline and solvents. A static headspace method has been shown to be effectivein analyzing for these compoundein contaminated water and soil samples.’ The method ie particularly useful in obtaining real-time data in the field using portable gas chromatographs. In this method, the components in the enclosed air space above contaminated water or soil are sampled and analyzed by gas chromatographyafter chemical and thermal equilibration has been achieved. To perform this method accurately, however, a knowledge of the airwater partition coefficients (Henry’s law constants) for the volatile organic compounds, especially in the water matrix in which they exist, is required. Further, knowledge of the Henry’s law constants is important in estimating the environmental behavior of volatile organic contamination and in achieving an effective method of site remediation. In 1981, Mackay and Shiu reported using three different methods to experimentallydetermine Henry’s law constants.2 The first method involved measuring the compound’s vapor pressure and solubility. The second method involved directly measuring the compound‘s vapor and aqueousconcentrations in an equilibrium system. The third method involved using a batch air stripping technique. These methods are not suitable for use when mixtures of compounds are involved, difficult to perform with accuracy at low concentrationstypical of environmentalsamples, or hard to carry out experimentally. In 1984, Lincoff and Gossett reported using the EPICS (equilibrium partitioning in closed system) method.* This ~
~~
(1) Roe,V. D.; Lacy, M. J.; Stuart, J. D.; Robbins, 0.A. A d . Chem. 1989,62,2584-2586. (2) Mackay, D.; Shiu, W. Y. J. Phys. Chem. Ref. Data 1981, 10(4), 1176-1199. 0003-2700/93/0366-3113$04.00/0
method involved measuring the concentration of a single volatile compound in the headspace of two sealed bottles having different liquid volumes. The method required that exact, equal masses of the compound be added to each bottle. They reported dimensionless Henry’s law constants in the range of 0.08-0.9 with precisions of 4-5 % ,expressed as relative standard deviation (RSD). The factor limiting the precision of their method was associated with the inability to deliver equal masses to the two bottles. In 1987, Gossett reported using a modification of the EPICS method which involved carefully weighing the amount of the volatile compound added to both of the sealed bottles and working with ratios of masaes.4 Dimensionless Henry’s law constants in the range of 0.06-18 were reported for 13 commercially-important, C1 and C2 chlorohydrocarbons. Precisions of 3-4% (RSD) were obtained using this method. Although the modified EPICS method offered an improvement in the precision of the Henry’s law constant measurement, it still required knowledge of the exact ratio of the masses added to both bottles. Also, it would be impossible to determine the Henry’s law constant for a compound in an unknown matrix, as would exist for an environmental sample. This paper presents a new method for determining Henry’s law constants, applicame to the static headspace method, in which neither the exact concentration of the volatile compound nor its matrix need be known. Experimentally, this method involves measuring by gas chromatography the equilibrium headspace peak areas of one or more compounds from aliquots of the same solution in three separate enclosed vials having different headspace-to-liquid volume ratios. A plot of the reciprocal of the peak areas versus headspaceto-liquid volume ratios gives a straight line. The slope of that l i e divided by its y-intercept, as determined by linear regression, gives a value for the dimensionless Henry’s law constant. This paper reporta the use of this new method for determining Henry’s law constants for the following, environmentally important organic compounds: benzene, toluene, ethylbenzene, m-xylene and p-xylene (together), o-xylene, and three C2 chlorohydrocarbonsl,l,l-trichloroethane, trichb roethylene, and tetrachloroethylene. In addition, for the first time, a dimensionless Henry’s law constant is reported for methyl tert-butyl ether (MTBE). MTBE is an octaneenhancing additive of gasoline which has been found useful in decreasingthe total volatile organic hydrocarbon emission from automobiles.6 Current EPA regulations allows it to be added to unleaded gasoline up to 11%by volume. (3) Lincoff,A. H.;Goseett,J. M. Iu Gaa Transfer at Water Surfaces; Bmtaaert, W., Jirka, C. H.,EMS.; Reidel: Dordrecht, The Netherlands, 1984; pp 17-25. (4) Gossett, J. M. Enuiron. Sci. Technol. 1987,21, 202-208. (6) Hoekman, S. K.Enoiron. Sci. Technol. 1992,26, 1206-1216.
@ 1993 AmorIan Chemical Socbty
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 21, NOVEMBER 1, 1993
THEORETICAL DEVELOPMENT
Static Headspace Method. The static headspace method requires that chemical and thermal equilibrium be achieved within the enclosed sampling vessel when solutes are present at low concentrations. Under these conditions, Henry’s law (eq 1) applies to the vapor-liquid system, where Pi is the
Pi = HPXq
(1)
partial pressure of component i in the vapor phase in units of atm, X ~isJa mole fraction of the component i in the liquid phase, and Hpis the Henry’s law constant of component i at agiven temperature in units of atm. Under ideal gas behavior, the partial pressure of a component i in the vapor phase, Pi, is related to the mole fraction of that component in the vapor phase, Xig, times the total pressure of the vapor phase, Pt, as expressed by eq 2. Since most headspace gases are mixtures
Pi = Xi& of several volatile components and solvent vapor, the total vapor pressure is the sum of the partial vapor pressures.6 Because the concentrations in the liquid phase are usually very low, the partial pressures in the vapor phase are sufficiently small that they may be expressed by Dalton’s law:
Pi = N,&T/V,
(3)
where N h is the number of moles of component i in the headspace vapor phase at equilibrium, R is the ideal gas constant in units of 0.082 05 atm L/mol K, Tis the temperature in K, and Vb is the volume of the headspace vapor phase in L.
A mole balance is expressed in eq 4, where Ni is the total (4) moles of component i in the enclosed container, Niwe is the moles of component i in the water phase, and N b is the moles of component i in the vapor phase, both at chemical and thermal equilibrium. If both sides of eq 4 are divided by V,, the volume of the water phase, a concentrationbalance equation results:
Ci, = Ciwe+ N d V ,
Hi = C d C , Substituting eq 8 into eq 6 and rearranging gives
Ciwo C b ( l / H i + V,! V,)
i in the headspace phase in molar gaseous units. Henry’slaw constant, Hp,in terms of atm., may be converted to a value of Henry’s law constant, Hi, in dimensionless terms as shown by eq 7,’ where k is the conversion factor from mole (7)
fraction to molarity for 1 L of water. This factor would vary from 55.35 at 298 K to 54.84 at 323 K. H is the symbol for the Henry’s law constant in units of atm ms/mol. A dimensionless Henry’s law constant, Hi, may be defined as (6) Umbreit, G. R.; Grob, R. L. J. Enuiron. Sci. Health 1980, A15(6), 42H66. (7) Cowen, W. F.; Baynes, R. K. J. Enuiron. Sci. Health 1980,A15(5), 413-427.
(9)
Note that eq 9 relates the initial water concentration, Ciwo, to the equilibrium headspace phase concentration, Ch. Dividing eq 9 for an unknown by that for a standard solution yields eq 10.
(CiwJUnLl(CiwJ8td = [ ( C h ) d / ( C h ) s a l[(UHi + V,!Vw)uk/(l/Hi
+ V,!Vn),~l (10)
If the Henry’s law constant, the temperature, the sample matrix, and the ratio of the volumes of the headspace, Vb, to volume of the liquid phase, V,, are the same for both solutions, then eq 10 simplifies to (11) ( C , , ) ~ (ci&a = (Ci&&/ (CiIAtd The ratio of headspace concentrations, as expressed in eq 11, is equivalent to the ratio of peak areas reported from the,gas chromatagraph. Hence, the aqueous concentration of an unknown solution may be directly determined relative to the concentration of a standard. New Method for Determining Henry’s Law Constants. This paper reports the development of a new method for measuring Henry’s law constants. The method involves measuring the concentrationof component i in the headspace of three vials having different headspace-to-water volume ratios. Aliquots of the same initial solution are wed to fill the three vials to meet this requirement. A significant advantage of the present method is that it does not require knowledge of the exact initial concentration of component i in the water phase; it only requires that the solutions in each of the three vials be identical. This makes it possible to measure Hi in real samples as well as in a known aqueous standard. Rearranging eq 9 gives eq 12. Since C b is directly pro-
1/Ck
where Ci,, is the initial concentration of component i in the water phase in molar units and Ci,, is the equilibrium concentrationof component i in the water phase. Multiplying the second term of the right side of eq 5 by V,! Vb gives eq 6, where C h is the equilibrium concentration of component
Hi = HdkRT = H/RT
the ratio of the equilibriumconcentrationbetween headspace phase and liquid phase at a specific temperature, as expressed in eq 8.
(l/Ciwo)(l/Hi+ VdV,)
(12)
portional to the peak area of the GC response, PA, (Le., PA = R C h , where R is a responsefactor),eq 12 can be expressed by eq 13. Equation 13 predicts that a plot of 1/PA vs V d V,
1/PA = [(l/~Ciwo)(l/HJl + [(l/RCiwo)(V,!Vw)l
(13)
willgiveastraight linewithaslopeof 1IRChanday-intercept of l/(HJ3Ciwo). If the slope is divided by the y-intercept, a value of Hi can be directly obtained, as is expressed by eq 14.
Hi = slope/y-intercept
(14)
The slope and intercept may be obtained by linear regression of eq 13. Hence, Hi may be determined without a priori knowledge of R or Ci,. Temperature Dependence of Henry’s Law Constant. The laws of thermodynamics predict that Henry’s law constant varies with temperature and pregsure. If the pressure is kept constant, the temperature dependence of Henry’s law constant can be expressed by
H = exp(A - B / T ) (15) where H is the Henry’s law constant in units of atm m3/mol,
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
Tis the temperature in K, and A and B are constants for the experimentaldata. Over a narrow temperature range in which the heat of vaporization of the solute from water remains constant, there is a linear relationship between the natural logarithm of the Henry's law constant for a given compound and the reciprocal of the absolute temperature. EXPERIMENTAL SECTION Instrumentation. Separationswere performed on a capillary column gas chromatograph (Model 5890A, Hewlett-Packard). A split/splitless injector was used in the splitless mode. A short, ca. 1-cmplug of silanized glass wool was inserted about half-way down the splitless liner to prevent septa corings from entering the capillary column.' A 30-m-long, 0.55-mm4.d. megabore capillary column having a 3.0-pm f i i of methyl silicone, DB-1 (J&W Scientific), was used. The helium flow rate through this capillary column was 5-7 mL/min. The column's eluent was passed in series first through a photoionization detector (PID) (Model 52-02A, HNU Systems) equipped with a 10.2-eV lamp and then to a flame ionizationdetector (FID)(Hewlett-Packard). The PID and FID outputs were connected to separate recording integrators (Model 3396A, Hewlett-Packard). Due to the better long-term reproducibility,only data from the FID will be reported in this paper. Sample Preparation. A concentrated standard (stock solution) was prepared as follows: about 8 mL of methanol (purge and trap grade) was placed into a 10-mLground glass volumetric flask equipped with a polyethylene stopper. The flask was allowed to stand unstoppered for about 10 min to allow the alcohol-wetted surfaces to dry. The flask was then stoppered and weighed to the nearest 0.1 mg. In succession,the following volumes of pure, analyzed-reagent grade compounds were added just below the methanol level into the flask by appropriate syringes: 200 pL of methyl tert-butyl ether (MTBE), 100 pL of benzene (BEN), 110pLof toluene (TOL),120pL of ethylbenzene (ETH), 60 pL of m-xylene and p-xylene (MPX) (these two compounds coeluted from the DB-1 capillary column), 150 pL of o-xylene (OXY), 150 pL of l,l,l-trichloroethane (TCA), 200 pL of trichloroethylene (TCE),and 150pL of tetrachloroethylene (PCE). To obtain a more exact concentration value for each compound, the weight gained after each volume was added was obtained. The resulting solution was carefullydiluted to volume with methanol. This concentrated stock solution,made monthly, was stored at 4°C. A one-tenth concentrated stock solution was also made by diluting 1.00mL of the concentrated standard with methanol in a 10-mL volumetric flask. Daily, diluted aqueous standards (workingsolution)were made from the stock solution. The concentrated stock solution,(usually 25.0 pL) was added to 250 mL of room temperature equilibrated distilled/deionized water. This resulted in an aqueous working standard solution of approximately 880 pg/L (ppb), in terms of the aromatic compounds,i.e., BEN,TOL, ETH, MPX, and OXY. This standard is referred to as the '880 ppb standard". Proper safety procedures, avoidanceof worker exposure,and appropriate disposalof the toxicaromatic and chlorohydrocarbonecompounds should be followed. Static Headspace Method. The 880 ppb standard was used to fill three or four 40-mL volatile organic analysis (VOA) vials (Supelco Inc., Part No. 2-3283). These vials were capped with a silicone/poly(tetrafluoroethylene)-facedsepta (Supelco, Part No. 2-3281) and holed screw caps (Supelco, Part No. 2-3292M). The volume of each VOA vial was predetermined by obtaining the difference in weight between the full and empty vial and then dividing that weight of the water by the known density of water at the laboratory's room temperature. For the determination of Henry's law constants, vials were selected having volumes of 40.00 f 0.30 mL. After the vials were fiied with the 880 ppb standard but before the headspace was developed, each vial was placed in a 25.0 0.2 "C water bath in an upside-down position for at least 5 min in order to reach temperature equilibrium. To generate the headspace, two needles, each 22 gauge, were used with the vial in an upright position. The first needle, 1.5 in. (3.8 cm) in length, was inserted so that only its tip passed
*
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Table I. Comparison between the Calculated Concentration and the Experimentally Obtained Concentration Using the Static Headspace Method at 26 "C. t-test (n = 15, DF 14, tdt 2.98) compd C d c avC, SD MID(%) talc conclb 877 32.8 3.74 BEN 874 0.354 same TOL 949 963 54.5 5.66 0.995 same 1052 ETH 1030 137 13.0 0.622 same 1037 143 MPX 1009 0.758 same 13.8 15.3 OXY 1309 1376 211 1.23 same TCA 1974 1998 268 13.3 0.347 same TCE 2916 2938 76.8 2.61 1.11 same PCE 2435 2350 189 8.04 0.307 same MTBE 1444 1452 78.9 5.43 0.393 same a Key to abbreviations used: t-test, 99% confidence level, twotails;" n, number of experiments; DF = degrees of freedom. tdt, critical t value;" C d , calculated concentration value, ppb; a! Cp., average experimental concentration value; SD, standard dewahon (here, of 15 experimental concentration values); RSD, relative standard deviation; talc, calculated t value, talc = (av ,C Cdc)N/2/SD.b Conclusion.
Table 11. Method Detection Limits (MDL) for the Static Headspace Method at 25 "C Using the Flame Ionization Detector. L i t = 1.94.6 n = 7. DF 6 compd av C, SDe MDLd BEN TOL ETH MPX OXY TCA TCE PCE MTBE
7.76
37.1 21.7 20.7 22.6 53.6 13.0 10.6 77.6
0.230
0.446
2.35 5.22 4.29 9.87 1.04 1.94 1.79 4.87
1.21
2.69 2.21
5.09 0.53 1.00 0.93 2.51
a See Table I footnote for key to abbreviations used. * tdt,critical value of the t-test at 95% confidence level, one-tail. SD on seven experimentalconcentrationvalues. d Method detection limit, t&D (in ppb).12 f
through the septa and entered the vial. This needle was used to allow for the introduction of fresh air into the vial. The second needle, 3.5 in. (8.9 cm) in length, (Part No. 7307,Spinal-Tap Needle, Popper & Sons, Inc.), was inserted through the septa to the bottom of the vial. This long needle was attached to a LuerLock plastic syringe,and a 10.0-, 30.0-, or 36.0-mL volume of the liquid phase was removed from each of the vials. Preliminary studies had indicated the need to use a long needle so as not to allow air bubbles to pass through the solution. These air bubbles were shown to cause significant increases of the volatile organic compounds in the headspace due to air stripping. After the appropriate volume of headspace was generated, each vial WM shaken for 2 min in an upside-down position in order to hasten and achieve phase equilibrium. The vi& were again placed in the water bath for at least another 5 min to reobtain temperature equilibrium. The samples were then ready for GC analysis. Gas Chromatographic Analysis. At the time of GC anely~is, a 200 pL sample of the headspace was taken using a 2 5 0 4 gastight, valve-locking microsyringe (Scientific Glass Engineering, Part Nos. 010507 and 0315243, respectively). Previous experiments had shown that it was necessary to withdraw samplea of the headspace using a valve-locking syringe, since the encloeed headspace, after shaking and equilibration, had a slight poeitive pressure. The valve-lock assured the transfer of a representative sample of the solution's headspace without loss of volatile components. The valve on the gas-tight syringe was not opened until the syringe needle had been inserted into the splitless liner of the GC's injection port. The following GC oven temperature program was used initial temperature of 40 "C, initial time of 1 min; the column temperature was then raised at a rate of 8
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
Table 111. Comparison at 28 O C of the Static Headspace Method (A) to the EPICS Method (B)As Reported by Ashworth et al.’U results of Henry’s law constant (dimensionless) compd method avHi SD RSD (%)
Hi30.2 15, r = 1 .OO
=!h
BEN
TOL
ETH MPX
Y 1
I
TCA
8
TCE
/ 0
2
4
6
OXY
PCE
Pharo Ratio
MTBE
12.00
b , L .
HirO.02 14, rr0.999
11.10
(b
I
8
A (n = 3) B (n = 8) A (n = 3) B (n = 8) A (n = 3) B (n = 8) A (n = 3) B ( n = 8) A (n = 3) B (n = 8) A ( n = 3) B (n = 8) A (n = 3) B ( n = 8) A ( n = 3) B ( n = 8) A ( n = 3) B (n = 0)
0.216 0.216 0.263 0.263 0.318 0.322 0.298 0.304 0.204 0.199 0.718 0.704 0.420 0.392 0.697 0.724 0.0216 NRb
1.00 x 10-8 1.02 x 10-2 3.06 x 10-8 1.58 x 10-8 2.08 X 10-8 9.57 x 10-9 1.54 X 1.00 x 10-2 1.51 X 4.20 X 10-8 6.45 X 9.67 X 1W 3.64 x 10-2 5.23 X 10-8 1.75 X 1.16 X le2 2.08 X l(r
0.46 4.12 1.16 0.60 0.65 2.97 5.17 3.29 7.40 2.11 8.98 1.37 8.67 1.33 2.51 1.60 0.96
NRb
NRb
t-testd F-teste compd DF Fdc F&t concl DF talc tdt 2.31 BEN 7,2 104 8 0 39.4 dlff 2.26 TOL 2,7 3.75 6.54 same 9 0 ETH 7,2 21.2 39.4 same 9 0.695 2.26 MPX 2,7 2.28 6.54 same 9 0.776 2.26 4 0.565 2.78 6.54 diff OXY 2,7 12.9 2 0.374 4.30 6.54 diff TCA 2,7 44.5 2 1.33 4.30 TCE 2,7 48.4 6.54 diff PCE 2,i 2.28 6.54 same 9 3.03 2.26
concl
same same same same same
same same diff
See Table I footnote for key to some of the abbreviations used. F-test, 95% confidence interval,two-tailsall d t-test, 95% confidence interval, two-tails.ll a
* NR = not reported.
Phaw Ratio Figure 1. Representativeplots showing the new method of obtaining dhnenslonless Henry’slaw constants: (a) benzene and (b) methyl tet7butyl ether (MTBE). (Each data point represents the average of three independent values.)
OC/min to a final temperature of 125 O C for the standard compounds or 190 OC for the environmental samples.
RESULTS AND DISCUSSION Static Headspace Method. The calibration plots obtained using the static headspace method for the 10 components studied were all highly linear with correlation coefficients close to 1.00. Each calibration plot consisted of seven points arranged over three orders of magnitude, e.g., from 8.74 to 8740 ppb for benzene, and run in triplicate. A phase ratio of 10/30(headspace volume/liquid volume) and a temperature of 25 “C was used. A BASIC program was used to perform a Q-test on outlying results and to calculate the average, standard deviation, and relative standard deviation (RSD) of each set of data. Linear least-squares best fits of the data were also performed. The slope of the MTBE calibration line was considerably lower than the others, as would be expected given its very high water solubility. Its solubility is reported to be 43 g/L,8 in comparison to benzene’s solubility of 1.80g/Lgor o-xylene’s of 0.120 g/L.l0 The high degree of linearity between the (8)Stephenson, R. M. J. Chem. Eng. Data 1992,37,80-95. (9)Poulaen, M.; Lemon,L.; Barker, J. F. Enuiron.Sei. Technol. 1992, 26,2483-2489. (10)Pereonalcom”icationfromSpittler,T. M.,U.S.Environmental Protection Agency, Region I Laboratory, Lexington, MA, 1993.
headspace peak area and aqueous concentration demonstrates the usefulnessof the static headspace method for quantitation. It also verifies that the response factor in eq 13 is, indeed, constant over the aqueous concentration range of interest. In order to verify the accuracy of our manual static headspace method, 15 analyses of the 880 ppb standard were run over a 1-month period. Table I summarizes the results of the two-tailed t-test.” At the 99% confidence level,there was no significant difference between the average experimental concentration and the known concentration for each of the 10compounds analyzed by the static headspace method. Table I1 summarizes the method detection limit (MDLP for each of the compounds studied using the static headspace method with the flame ionization detector (FID). In general, MDLs in the range of 0.4-10 ppb were obtained for the volatile aromatics and MTBE, while values of 2 ppb were obtained for TCA, TCE, and PCE. It should be noted that later eluting compounds were found to have higher MDLs, as their peak widths (standard deviations) tended to increase. New Method for DeterminingHenry’s Law Constants. In the process of experimentally verifying that eq 13 can be used to determine a dimensionless Henry’s law constant, it was found that it became important to work with a wide range of headspace volume- to-liquid volume ratios. Results were best when headspace volume measurements were derived by weighing each vial before and after the appropriate volume (11) Miller, J. C.;Miller, J. N. Statistics for AnuZytical Chemistry, 2nd ed.; John Wiley & Sons: New York, 1988;pp 54-57,216-217. (12)Definition and Procedure for the Determmnation of the Method Detection Limit-Rev. 1.11.FederaZRegister,Part 136,U.S. Government Printing Office: Washington, DC, 1988;Appendix B, 40 CFR, Chapter 1 (July 1, 1988 edition), pp 510-612.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, lQg3 3117
Table IV. Henry's Law Constant (of Benzene) Measured by the Static Headspace Method on Ground Water Samples some sample parameters concn ECb Hi sample PA& designation (dimensionless) (ppb) bS) UPA)c (%) MMWll MMW6 MMW7
0.214 0.208 0.222
sample ground water 800-ppbstandard
14 150 324
avHi 0.215 0.216
3 3
7.02 X 109 1.00X 10-9
a
3.26 0.46 ~~
t-test.
DF F d
Fdt
concl
DF
tdo
2,2
39.0
diff
2
0.244
49.3
0.1 0.01 1
600 338 269
~
BEN
t ~ t concl
0.30
same
4.30
0.31
~~
~
Table V. Henry's Law Constant at Different Temperatures Measured by the Static Headspace Method av Hi, dimensionless (n = 3) compd
25'C
30%
40 'C
45 O C
50 OC
BEN TOL ETH
0.216 0.263 0.318 0.298 0.204 0.718 0.420 0.697 0.0216
0.272 0.337 0.409 0.357 0.253 0.877 0.528 0.929 0.0480
0.342 0.438 0.640 0.574 0.424 1.03 0.644 1.18 0.0862
0.467 0.545
0.541 0.585
MPX
OXY TCA TCE PCE MTBE
NAa
NAa
0.602 0.408 1.36 0.857 1.46 0.139
0.610 0.437 1.55 0.990
NAa 0.154
NA = not analyzed.
of the liquid phase had been removed. Volume ratios of typically7.00,3.33, and 0.333 were found to give a sufficiently wide range of the compounds studied to allow meaningful regression analyses to be performed. Representative plots of data obtained using the method for benzene and methyl tertbutyl ether, respectively, are shown in Figure l a and lb. Table I11 presents a comparison of the Henry's law constants obtained here by the new method with those reported by Ashworth et al.,13 using the EPICS method. Appropriate F- and t-tests were run. There was excellent agreement for the Henry's law constants between both methods except for PCE, whose Henry's law constant showed that the averages were significantly different a t the 95% confidence level. For the first time, we report a value of 0.0216 for the dimensionless Henry's law constant of MTBE, measured a t 25.0 OC. This average value is based on three separate trials with a precision of 0.96 3'% (RSD). It should be noted that this Henry's law constant is over 1order of magnitude lower than values of 0.2-0.4 obtained for the volatile aromatics and chlorohydrocarbons studied. A Henry's law constant was determined for benzene at 25.0 "C using samples obtained from three ground water monitoring wells. The samples came from a site contaminated by gasoline. The results are summarized in Table IV. Values in the range from 0.208 to 0.222 were obtained. The (13)Ashworth, R. A.; Howe, G.B.; Mullins, M. E.;Rogers, T. N.
J. Hazard. Mater. 1988,18,25-36.
0.33
0.34
0.3s (E-2)
b H-oxp( 18.4-7666/T), r-0.085
confidence interval, two-tails.11*t-test, 95 % confidence interval, twO.tails.11
0.32
117 (K)
See Table I footnote for key to some of the abbreviations used.
bEC,electricconductivity. P A h , benzenepeakareaofGCreaponse. X(PA), summation of peak area of all GC peaks. F-test, 95%
a
H=0~p(7.14-3689/T),r-0.987
comparison of Hi values n SD RSD (5%)
F-test' compd
a ,
r 3
-
