Environ. Sei. Technol. 1987, 27, 202-208
charges from the Oak Ridge Y-12 Plant.
6 0 ppb Hg UPSTREAM
Acknowledgments
A ppb Hg DOWNSTREAM 5
E. R. Rogers, Jr., and P. Berlinski are acknowledged for their design assistance. R. J. McElhaney is also acknowledged for his discussions and advice. R. E. Carroll is recognized for helping prepare and edit the manuscript.
4
R e g i s t r y No. Mercury, 7439-97-6; water, 7732-18-5.
e”
%
Literature C i t e d
3
(1) “Testimony a t a Joint Hearing of the Subcommittee on 2
i
+-++A
P-+-
*y
1
1 0
2
4
6
8
10
12
~
1 14
/
1 16
1 18
1
1
1
20
TIME (hl
Figure 5. Upstream/downstream ppb Hg at East Fork Poplar Creek.
concentration fluctuations in the stream. The downstream peak at 17:OO h is thought to be the 1O:OO h peak exiting the pond.
Conclusions The instrument described in this paper provides important stream monitoring data on mercury concentrations on a continuous basis. Continuous data of this type are needed to understand and allow control of mercury dis-
Energy Research and Production and the Subcommittee on Investigations and Oversight of the U S . House Science and Technology Committee on the Impact of Mercury Releases at the Oak Ridge Complex”; July 11, 1983. (2) Reece, J. “Preliminary Aquatic Survey of East Fork Poplar Creek and Bear Creek, 1974”; Environmental Protection Branch, Safety and Environmental Control Division, Oak Ridge Operations, USAEC: Oak Ridge, T N , 1974. (3) Elwood, J. W. ORNL/CF/77/320, Oak Ridge, T N , 1977. (4) Van Winkle, W.; Counts, R. W.; Dorsey, J. G.; Elwood, J. W.; Lowe, V. W.; McElhaney, R.; Schlotzhauer, S. D.; Taylor, F. G.; Turner, R. R. ORNL/CF/82/257, Oak Ridge, TN, 1982. (5) Methods For Chemical Analysis of Water and Wastes;US. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, March 1983; EPA60014-79-020. (6) Hatch, W. R.; Ott, W. L. Anal. Chem. 1968, 40, 2085.
Received for review March 25, 1986. Accepted August 25,1986.
Measurement of Henry’s Law Constants for C, and C2 Chlorinated Hydrocarbons James M. Gossett Environmental Engineering Department, Cornell University, Ithaca, New York 14853
A modification to the EPICS procedure for measuring Henry’s constants is proposed, wherein the original assumption of equal solute mass additions to bottle pairs has been eliminated via a gravimetric accounting, resulting in increased precision. The modified procedure was applied to 13 volatile C1 and C2 chlorinated hydrocarbons at temperatures from 10 to 35 “C, representing a range in dimensionless Henry’s constant (H,) of from 0.05 to 1.8. The mean coefficient of variation (CV) in Henry’s constant was 3-4% ; however, by employment of greater differentials in liquid volumes within bottle pairs, CV may be reduced to an estimated 2.5-3.5%. Precision deteriorates markedly at very low Henry’s constants (Le., H , < 0.1). Dilute aqueous mixtures of volatile solutes (with methanol also present) were employed in these measurements of Henry’s constant. Comparison with previous results from singlesolute assays supports the use of mixtures-at least within the range explored here. The effects of ionic strength on apparent Henry’s constants were evaluated for six of the compounds, with KC1 concentrations up to 1.0 M. The practical impact of ionic strength appears to be minimal. Salting-out coefficients ( k ) ranged from 0.107 to 0.213 L-mol-’ a t 20 “C. Introduction
The partition coefficient relating air and aqueous concentrations of a volatile substance is commonly referred 202
Environ. Sci. Technol., Vol. 21, Nu. 2, 1987
to as Henry’s constant. Values of Henry’s constant are required by design and performance models of air-stripping processes for the renovation of organic solvent contaminated waters, as well as by transport models that attempt to describe movement of volatile pollutants in the unsaturated zone of subsurface environments. Unfortunately, reliable values of Henry’s constant are often not available for solutes of environmental concern. Mackay and Shiu (I) have presented a comprehensive review of common methods for measuring Henry’s constants-along with their respective merits and deficiencies. There are three basic methods cited by Mackay and Shiu: (i) use of vapor pressure and solubility data; (ii) direct measurement of air and aqueous concentrations in a system at equilibrium; (iii) measurement of relative changes in concentration within one phase, while effecting a near-equilibrium exchange with the other phase. The first method suffers from lack of reliable solubility data. For example, reported solubility values (25 “C) for dichloromethane range from 13 200 to 19 400 mg/L (1). Additionally, in some instances, the vapor pressure of the w a t e r - s a t u r a t e d organic substance-the vapor pressure that should properly be used in the estimation of Henry’s constant-differs significantly from that of the p u r e organic, which is the value usually tabulated. The second method is difficult to carry out with accuracy at the low concentrations typical of environmental levels. The third
0013-936X/87/0921-0202$01.50/0
0 1987 American Chemical Society
method, typified by the batch air stripping procedure of Mackay et al. (2), occasionally suffers from experimental difficiencies in achieving adequate approach to equilibrium (3). Recently, a novel approach to measurement of Henry’s constant has been presented (3). This method, termed EPICS (Equilibrium Partitioning in Closed Systems), requires no special apparatus and obtains results from measurement of gas headspace concentration ratios from pairs of sealed bottles possessing differing liquid volumes. Since merely relative-rather than absoluteconcentrations are required, the EPICS technique retains the primary advantage of the commonly used batch air stripping technique (2) but is unencumbered by equilibration problems. This paper presents (i) a modification to the EPICS procedure that offers the potential for significant improvement in precision; (ii) a thorough evaluation of EPICS precision and the factors influencing it; (iii) measured Henry’s constants vs. temperature (10-35 OC)-with resulting regression equations-for 13 chlorinated C1 and C2 hydrocarbons of environmental concern; and (iv) data concerning the impact of ionic strength on effective Henry’s constant values-with salting-out coefficients-for six of the compounds, with KC1 concentrations from 0 to 1.0 M. Derivation of Equations Modified EPICS Equation. The EPICS procedure, as originally formulated (3), is based upon the addition of equal masses of a volatile solute to two sealed serum bottles that are identical in all respects except one: they possess differing solvent (water) volumes. The resulting ratio of the two headspace concentrations is used to compute Henry’s constant. Results (3) suggest that this method is capable of yielding values of Henry’s constant with a precision (coefficient of variation, CV) averaging 4-5% for compounds with “dimensionless” Henry’s constants, H, [(mol/L gas concentration)/ (mol/L aqueous concentration)], between 0.06 and 0.9-the full range previously evaluated. The limiting precision was said to be that associated with attempted delivery of equal solute masses to the bottle pairs. In this paper, the original EPICS formulation is modified in an attempt to improve precision. In essence, the assumption of equal solute masses in the individual bottles comprising an EPICS system is eliminated. Instead, differences in mass due to imperfect, volumetric additions are accounted for through gravimetric means. The total moles (M) of a volatile solute added to a serum bottle will be partitioned at equilibrium according to M = CwV, + CgVg= Cg(Vw/H,) + CgVg= Cg[(Vw/Hc)+ VgI (1) where C, = concentration of solute in the water (mol/L), C, = concentration of the solute in the gas (mol/L), Vw = volume of water in the bottle (L), V, = volume of headspace in the bottle (L), and H, = Henry’s constant (dimensionless). If two bottles are prepared with differing liquid volumes, V, and V, then eq 1will hold for each, and we may write (2) M1 = ~,l[(VWl/H,) + Vgll M2 = Cg,[(Vw,/H,) + Vg2l
(3)
In the original EPICS formulation, Ml = M2 (or at least it was so assumed). However, one may refrain from making this assumption and, instead, divide eq 2 by M1 and eq 3 by M2. The left-hand sides of each will now be
unity, allowing the two to be equated
(4) Solving for H,then yields
or
H, =
- rVw1 rVg1 - v g 2
vw2
where r = (Cg1/Ml)/(Cg2/M2).The original EPICS formulation (3) is merely a simplification of eq 6, for the special situation in which Ml = M2 and r = C 1/Cg2. Evaluation of H,using eq 6 does not actualfy require that Ml and M2be known-only that their ratio be known. This is an important point. It means that if a stock solution of a solute is used to prepare EPICS bottles, the actual concentration of the stock need not be known; for example, a gravimetric measure of the relative quantity of the stock added to the two EPICS bottles suffices. The earlier EPICS method (3) was most limited by the imprecision associated with attempts to add equal stock volumes to EPICS bottles. Volumetric measures are far less precise than gravimetric measures. If one attempts to inject equal volumes of a stock solution to two bottles, gravimetric analysis of the stock masses injected (via weighing of a syringe or bottle just before and just after injection) will, in general, detect differences. Equation 6 allows such differences to be incorporated into the determination of H,, and by this elimination of a major contribution to total variance, greater precision in H, should be realized. Measurement of Activity Coefficients. A further modification of the EPICS procedure allows the measurement of activity coefficients for volatile solutes ( 4 , 5 ) . Suppose two serum bottles are prepared with identical liquid volumes: one containing distilled water plus the solute; the other containing the solute in a mixture for which the solute’s activity coefficient is desired. Equation 4 applies, and a rearrangement of it yields (7) (Vw/H,) + Vg = r*[(Vw/H,*) + Vgl where r* = (Cg;/Ml)/(Cgo/M0),Cgl and Cgo= gas-phase concentrations in the bottles with the nonideal and ideal solutions, respectively (mol/L), Ml and Mo = quantities of volatile solutes added to the two bottles, respectively (mol), H, = dimensionless Henry’s constant for the ideal system, Hc* = dimensionlesseffective Henry’s constant in the nonideal system = yH,, and y = the applicable activity coefficient for the nonideal system. Making the substitution of H,* = yH, and rearranging yield y = r*Vw/[Vw+ (1 - r*)HcVg] Equation 8 allows the determination of activity coefficients for a solute in several different, nonideal systems (yielding different values of Cgl/Ml),with a single, reference (ideal) system (giving Cgo/Mo). Experimental Methods Volatile Organic Compounds Studied. The following 13 volatile organic compounds were selected for study: tetrachloroethylene (ultrapure 99+ % , Alfa Products); trichloroethylene (>99.5%, stabilized with 0.01 % triethylamine, Fluka AG Chemicals Fab.); 1,l-dichloroethylene [>99.9% (GC), Riedel-De Haen AG]; cis-l,%diEnviron. Sci. Technol., Vol. 21, No. 2, 1987 203
chloroethylene (97%, Aldrich Chemical Co.); trans-1,2dichloroethylene [>99.9% (GC), Riedel-De Haen AG); vinyl chloride (GC standard, 0.2 mg/mL in methanol, Supelco Inc.); l,l,l-trichloroethane (purified, inhibited, Fisher Scientific); 1,l-dichloroethane [>98% (GC), Fluka AG Chemicals Fab.]; chloroethane (GC standard, 0.2 mg/mL in methanol, Supelco Inc.); carbon tetrachloride (AR, J. T. Baker); chloroform (ACS, preserved with 0.75% ethanol, Mallinckrodt); dichloromethane (distilled in glass, pesticide grade, Burdick & Jackson); chloromethane (GC standard, 0.2 mg/mL in methanol, Supelco Inc.). Preliminary Concerns Relating to Use of Organic Mixtures. Henry's constants were determined for each of the 13 compounds; however, for the sake of experimental convenience, the EPICS assays were conducted with aqueous-phase mixtures containing from three to six of the volatile solutes. Methanol was also present in the systems, since it was used as a solvent in preparation of the stock mixtures that were injected into the EPICS bottles. The use of solute mixtures, and the presence of methanol, might cause concern. However, previous research (5) indicated no mutual effects of organic mixtures on the Henry's constants of tetrachloroethylene, trichloroethylene, l,l,ltrichloroethane, chloroform, and dichloromethane in an aqueous mixture of the five a t up to a total mixture concentration of 375 mg/L. The possible effect of methanol's presence on the Henry's constant of a typical, volatile solute was investigated with trichloroethylene (TCE) in the presence of four different, aqueous methanol concentrations. Six 158.8-mL glass serum bottles were prepared containing 100 mL of 0, 0.1, 0.5, 1.0,2.0, or 5.0% (v/v) aqueous methanol solution. A 0.1-mL volume from an aqueous stock solution of TCE (1100 mg/L) was injected into each serum bottle. After equilibration at room temperature on a wrist-action shaker for 1 h, the headspace concentration of TCE was measured in each. Results showed 71.1, 70.3, 70.7, 73.0, 71.0, and 66.9 relative GC peak area units, respectively. It is concluded that only at the highest methanol concentration (5.0% or 39.6 g/L) is there any significant decrease in Henry's constant for TCE. This is an enormous concentration of methanol relative to the 155-2630 mg/L methanol concentrations used in the EPICS assays described below. Thus, the use of stock, compound mixtures dissolved in methanol is considered to be justified at the relatively low concentrations employed here. Aqueous Mixtures Employed in EPICS Assays. Three different aqueous mixtures of compounds (with methanol present) were used in experimental determinations of Henry's constants. Mixture A contained tetrachloroethylene, l,l,l-trichloroethane, trichloroethylene, chloroform, dichloromethane, and 1,l-dichloroethane (each at added concentrations from 1.9 to 3.3 mg/L), with methanol present at 155 mg/L. Mixture B contained carbon tetrachloride, 1,l-dichloroethylene, and trans-1,2dichloroethylene (each a t added concentrations from 1.4 to 2.9 mg/L), with methanol present a t 159 mg/L. Mixture C contained chloromethane, vinyl chloride, and chloroethane (each at 0.042 mg/L added concentration), with cis-1,2-dichloroethyleneat 2.4 mg/L and methanol a t 657 mg/L. All indicated concentrations are those added-i.e., prior to partitioning into the headspace-to high-volume (100-mL) EPICS systems. Added concentrations to low-volume (25-mL) systems were 4 times as large. The compounds employed in a particular mixture were selected on the basis of analytical convenience (Le., they possessed sufficiently separate GC retention times). Six 204
Environ. Sci. Technol., Vol. 21, No. 2, 1987
different stock solutions (in methanol) were used to prepare the three aqueous mixtures in the EPICS serum bottles. EPICS Procedure. For each aqueous mixture, Henry's constants were measured in six 158.8-mL serum bottles. Three contained 100-mL liquid volumes; the other three, 25 mL. Bottles were prepared as follows: distilled water (25 or 100 mL) was pipeted to each serum bottle, the bottles were sealed with Teflon/rubber septa and aluminum crimp caps, a 0.1-mL gas-tight syringe with a 5-cm side-port needle was used to deliver approximately 20 pL of the appropriate stock solution(s) to each bottle. Precise determination of the quantity of stock injected was made gravimetrically: the 0.1-mL syringe was weighed (to the nearest 0.00001 g) just prior to, and just after, injection to the serum bottle. The six EPICS serum bottles were then incubated (inverted and submerged) for 18-24 h a t each of four desired temperatures (10.0,17.5, 24.8, and 34.6 "C-all f0.1 "C), in a reciprocating shaker bath (approximately 60 cycles/min with a displacement of 1.2 cm), and analyzed by headspace GC as described below. [Note: Preliminary studies indicated that as short an incubation period as 1h is sufficient to achieve equilibrium in shaken serum bottles. No differences were apparent between bottles incubated for 1, 18, or 48 h. Studies also indicated that losses of volatile solutes are minimal if bottles are incubated in an inverted position, with their liquid contents in contact with the septa-even when septa are repeatedly punctured for headspace sampling (@.] For each of the 13 compounds-and at each temperature evaluated-nine possible pairings of triplicate high and low liquid volume EPICS bottles provided nine possible estimates of Henry's constant in accordance with eq 6. These nine were then used to compute a mean and a percent coefficient of variation [ % CV = 100(SD/mean)] for each compound and temperature. Determination of Activity Coefficients. The effect of ionic strength on apparent Henry's constant was evaluated for mixture A with eight 158.8-mLserum bottles that each contained 100 mL of water at different concentrations of KC1 (0, 0, 0, 0.1, 0.3, 0.5, 0.7, and 1.0 M). Carefully measured masses of stock were added to these bottles in the above-described manner. After incubation for 18 h (in the shaker bath) a t 20 "C, headspace samples were analyzed by GC. Activity coefficients were calculated with eq 8 (and the presumption was made that the system without added KC1 was sufficiently dilute with respect to volatile solute concentrations as to be effectively "ideal"). Salting-out coefficients were determined by plotting loglo (activity coefficient) vs. ionic strength, in accordance with the model lOg1oy = kl (9) where k = salting-out coefficient (L/mol) and I = ionic strength (M). Headspace Analysis of Volatile Compounds. Headspace concentrations of volatile organic compounds in equilibrated EPICS bottles were measured by gas chromatography on a Sigma 2000 GC (Perkin-Elmer), coupled with an LCI-100 laboratory computing integrator (Perkin-Elmer), A 2.44-m by 3.2-mm stainless-steel GC column was used, packed with 1% SP-1000 on 60/80 Carbopack-B (Supelco, Inc.). Injector temperature was 200 "C; detector temperature, 250 "C; carrier flow (He) was 40 mL/min, and a flame ionization detector was employed with hydrogen and air flows of 40 and 400 mL/min, respectively. The following temperature programming sequence was used: 60 "C ( 2 min); 20 deg/min to 150 "C (with no hold); 10 deg/min to 200 "C,with a 4.2-min hold
Table 11. Temperature Regressions for Henry's Constantn
Table I. Measured Values of Henry's Constant vs. Temperature compd tetrachloroethylene
temp, "C
9.6 17.5 24.8 34.6 trichloroethylene 9.6 17.5 24.8 34.6 1,l-dichloroethylene 10.0 17.5 24.8 34.6 cis-1,2-dichloroethylene 10.3 17.5 24.8 34.6 trans-1,2-dichloroethylene 10.0 17.5 24.8 34.6 vinyl chloride 10.3 17.5 24.8 34.6 l,l,l-trichloroethane 9.6 17.5 24.8 34.6 1,l-dichloroethane 9.6 17.5 24.8 34.6 chloroethane 10.3 17.5 24.8 34.6 carbon tetrachloride 10.0 17.5 24.8 34.6 chloroform 9.6 17.5 24.8 34.6 dichloromethane 9.6 17.5 24.8 34.6 chloromethane 10.3 17.5 24.8 34.6
H,
%
H,
m3-atm.mol-'
CV'
0.294 0.492 0.723 1.116 0.163 0.265 0.392 0.591 0.548 0.800 1.069 1.451 0.0741 0.111 0.167 0.216 0.181 0.277 0.384 0.545 0.631 0.811 1.137 1.420 0.328 0.502 0.703 0.987 0.107 0.163 0.230 0.321 0.280 0.355 0.456 0.613 0.567 0.883 1.244 1.823 0.0645 0.103 0.150 0.223 0.0498 0.0549 0.0895 0.129 0.168 0.245 0.361 0.491
0.00682 0.011 7 0.017 7 0.028 2 0.00378 0.00632 0.00958 0.014 9 0.012 7 0.019 1 0.026 1 0.036 6 0.00172 0.00265 0.00408 0.005&5 0.00420 0.00660 0.00938 0.013 8 0.014 7 0.019 3 0.027 8 0.035 8 0.00761 0.012 0 0.017 2 0.024 9 0.00248 0.00389 0.00562 0.00810 0.00651 0.00846 0.011 1 0.015 5 0.013 2 0.021 1 0.030 4 0.0460 0.00150 0.00246 0.00367 0.00563 0.00115 0.00131 0.00219 0.00326 0.00391 0.00584 0.00882 0.012 4
3.75 1.28 4.81 1.63 3.52 1.32 3.81 2.89 1.86 2.11
2.86 2.50 3.59 2.44 6.32 5.04 1.76 2.27 2.07 1.75 0.96 3.48 4.39 1.48 2.10 0.67 3.81 3.48 3.40 1.15 4.13 3.54 3.90 3.62 5.84 4.44 5.15 4.20 3.92 3.42 6.16 1.96 3.75 3.76 19.2 19.9 17.4 2.37 9.36 6.65 2.75 5.07
"Percent coefficient of variation = 100(SD/mean).
at this temperature. Retention times ranged from 1.6 (chloromethane) to 14.5 min (tetrachloroethylene). The sampling procedure was as follows. A serum bottle to be assayed was turned upright from its normal, inverted position in the shaker bath; only the top 1 cm of the bottle's neck protruded above the surface of the bath. A 1.0-mL gas-tight syringe (Pressure-Lok, series A-2) with a push-button valve mechanism was used to obtain a 0.5-mL headspace sample. This syringe was fitted with a 5-cm (0.71-mm 0.d. X 0.46-mm i.d.) side-port needle. The syringe needle was inserted through the bottle septum; a 0.5-mL sample was drawn in; the push-button valve was closed; the syringe needle was withdrawn from the bottle and inserted through the GC septum; and in closely synchronized steps, the push-button valve was opened, and the sample was immediately injected. (Use of a syringe
compd tetrachloroethylene trichloroethylene 1,l-dichloroethylene cis-1,2-dichloroethylene trans-1,2-dichloroethylene vinyl chloride l,l,l-trichloroethane 1,l-dichloroethane chloroethane carbon tetrachloride chloroform dichloromethane chloromethane
H = exp(A - B I T ) A B r2 12.45 11.37 8.845 8.479 9.341 7.385 9.777 8.637 5.974 11.29 9.843 6.653 9.358
4918 4780 3729 4192 4182 3286 4133 4128 3120 4411 4612 3817 4215
0.996 0.996 0.994 0.979 0.994 0.987 0.995 0.994 1.000 0.995 0.996 0.951 0.990
nBased upon studies from approximately 10 to 35 "C. H , m3. atm-mol-'; T, K.
with a push-button valve was necessary because headspace pressures would generally not equal atmospheric pressure because bottles were sealed at atmospheric pressure at ambient temperature, but incubated at differing temperatures. Since what was desired was a measure of the gaseous concentration of the compound as it existed i n the serum bottle, it was necessary to inject it at serum bottle concentration-and that meant at serum bottle pressure.) The combined analytical imprecision (associated solely with the processes of headspace sampling and GC injection, coupled with any imprecision in the GC response-but neglecting variance associated with initial addition of volatile compounds to bottles and subsequent losses) was estimated by performing replicate headspace analyses on individual bottles. The CV averaged 1.0%.
Results and Discussion Henry's Law Constant vs. Temperature. Measured mean values of Henry's constant are contained in Table I, along with observed CVs. The mean of the CV values is approximately 4.3%. However, examination of the data indicates the presence of three atypically large CVs: those for dichloromethane at the three lowest temperatures. If these are omitted, the mean CV is reduced to approximately 3.4%. Discussion of precision is reserved for a later section of this paper. The temperature dependence of Henry's constant has been well modeled with the classical, van't Hoff equation for temperature's effect on an equilibrium constant (3). Accordingly, results from linear regressions of In H vs. T 1 (Hin m3-atm/mol; T in K) are shown in Table 11. It is apparent from the regression rz values that this model fits the data well. The results reported here are based upon studies in which mixtures of compounds (with methanol) were used. Earlier studies (3) employed the EPICS technique with single, volatile solutes in aqueous solution. Comparisons are possible for only five compounds: tetrachloroethylene, trichloroethylene, l,l,l-trichloroethane, chloroform, and dichloromethane. For all except dichloromethane, agreement between the two studies was excellent, as exemplified by Figure 1, in which data for l,l,l-trichloroethane are shown. This supports the use of dilute, aqueous mixtures of volatile solutes in the measurement of Henry's constant via the EPICS procedure. It is tempting to dismiss the lack of agreement between current and former measurements of Henry's constant for dichloromethane (Figure 2) as merely the result of the relatively poor precision encountered with this compound in both studies. However, the deviations do not appear Environ. Sci. Technol., Vol. 21, No. 2, 1987
205
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--
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-
0.01
-
0
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e
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f
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Tetrachloroethylene T r i c h I o r o s t h y I enChloroform
-
i
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> z
w
~
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~
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"
"
"
"
15
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~
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25
20
"
30
"
"
"
~ 0.0
35
0.0
0.1
0.2
Flgure 1. Temperature dependence of Henry's constant for 1,1,1trichloroethane. Present results indicated by asterisks and earlier results (3)by circles.
F
-p
0.003
$
0.6
0.7
0.8
0.9
1.0
0.15
0.002
-
0.0015
-
m
E
c
0.5
0.2
0.0025
E
-
0.4
Figure 3. Effect of ionic strength on activity coefficients of tetrachloroethylene, trichloroethylene, and chloroform.
*
1
+ D
0.3
I O N I C STRENQTH ( M K C I I
TEMPERATURE Idea C l
0. I
t
9 8
i
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> 5.0E-4
-
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'
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TEMPERATURE (deg
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to be random. It is therefore possible-though in this investigator's view, unlikely-that the use of mixtures may have affected the activity of dichloromethane. An alternative explanation exists: It was apparent that the integration system employed with the GC had a difficult time handling the data from dichloromethane peaks, which eluted with significant tailing. To avoid incorrect base-line assignment to the peaks of other compounds in the mixture, the base-line sensitivity was necessarily adjusted to a value that tended to cause omission from integration of the final portion of the dichloromethane peak. Since the peak width was relatively independent of the mass of dichloromethane present, the omission of a portion of the peak tail would cause a greater percent error in the estimated area of smaller peaks than of larger ones. In terms of the EPICS procedure, C,/M (for dichloromethane) would have been underestimated to a greater degree in the EPICS bottles with the larger V,. The ultimate effect would be the systematic underestimation of Henry's constant for this compound. Reliable values of Henry's constant for the compounds investigated in this research do not otherwise exist in the literature. Mackay and Shiu (1) and Goldstein (6) have compiled values from a number of sources; wide variations are evident. For one compound in particular-vinyl chloride-some reported values of Henry's constant are at least an order of magnitude higher than measured in this study. It is difficult to determine the reason. The use of solute mixtures and the presence of methanol are not satisfactory explanations-measurement of Henry's conEnviron. Sci. Technol., Vol. 21,
No. 2 , 1987
'
1
~
0.0 " 0 .~0
0J . 1
0.2
35
Flgure 2. Temperature dependence of Henry's constant for dichloromethane. Present results indicated by asterisks and earlier results (3) by circles.
206
~
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
I O N I C STRENDTH < M KCII
Flgure 4. Effect of ionic strength on activity coefficients of l , l , l trichloroethane, l,l-dichloroethane, and dichloromethane. Table 111. Salting-Out Coefficients (20 "C)
compd
k , Lemol-'
r2
tetrachloroethylene trichloroethylene l,l,l-trichloroethane 1,l-dichloroethane chloroform dichloromethane
0.213 0.186 0.193 0.145 0.140 0.107
0.994 0.999 1.000 1.000 0.999 0.998
stant at 34.6 "C was repeated with a gaseous standard of vinyl chloride (100 ppm in N2,Scott Specialty Gases) to effect equal mass additions to EPICS bottles containing only distilled water. Results (H, = 1.29; CV = 3.5%) were in substantial agreement with earlier measurements performed with mixtures (Table I). In the absence of a convincing argument to the contrary, the author is forced to conclude that the values reported here are accurate. Henry's Law Constant vs. Ionic Strength. The effects of ionic strength (using KC1) on the activity coefficients of tetrachloroethylene, trichloroethylene, chloroform, l,l,l-trichloroethane, 1,l-dichloroethane, and dichloromethane are depicted in Figures 3 and 4. The respective salting-out coefficients are summarized in Table 111. Results suggest that salinity must reach rather substantial values to exert significant impact. For example, in the case of the most sensitive compound investigated (tetrachloroethylene), ionic strength must exceed 0.2 M (KC1) to cause a greater than 10% increase in apparent Henry's constant. Precision and EPICS Technique. The previous publication (3),in which the EPICS procedure was first
advanced, contained little discussion of precision associated with the technique. A potential user should understand which factors contribute most significantly to resulting precision in Henry’s constant measurements. It is equally important to define the manner in which precision varies with the magnitude of Henry’s constant itself, allowing delineation of the practical range over which the EPICS technique may be usefully employed. The observed variance in measured H, is the result of variances associated with each of the several variables used in its calculation (see eq 6). For a parameter that is a function of several other variables, y = F(xl, x2, ...,xn), the applicable relation governing variances is ayy)
(aF/dxl)2a2(x1)
Q
-
Observed Predicted
+ ( d F / d X , ) 2 2 ( X z ) + ... + (dF/dx,)2a2(xn) (10)
where a2Cy) = the variance in y, a2(x1)= the variance in xl,etc. This equation is valid only if the variances in the independent variables are unrelated (7). The direct application of eq 10 to eq 6 would therefore be invalid because a functional relationship exists between each bottle’s liquid and gas volumes: v, = v b - Vw,where v b = the total volume. Thus, u2(Vg) = u2(Vb) + a2(Vw). The EPICS formulation used to calculate H, (eq 6) may be restated to eliminate this difficulty:
v w z - rVw1 Vb(r - 1)- rVw, + Vw2 (11) where v b = the total volume of each EPICS serum bottle (158.81 mL). Equation 11 can then be used to estimate variance contributions: a2(H,) (dH,/dVb)2U2(Vb) + (aHc/dvw,)zU2(vw,) + ( ~ H , / ~ V , Z ) ~ ~+~ (dHc/dr)2a2(r) (V~~) (12) where a2(r)
r2~[.(C,1)/C,1I2 + [a(C,z)/CgzI2 + [dM1)/M1I2+ [dMz)/M2I2J(13)
Estimates were made of the variances in vb, Vwl, Vw2,cgl, Cg2,M l , and M z to evaluate their relative contributions to variance in H,. The variance in serum bottle volume was estimated by filling six randomly selected bottles with water and weighing them; a2(vb)= 0.454 mL2. The variances in Vwl and Vw2were also determined gravimetrically; az(VwJ a2( Vwz)= 0.01 mL2. The variances in Cgl and Cg2should properly include only analytical imprecision [i.e., associated with headspace sampling, injection, and GC response; uncertainty in masses of volatile compounds contained in the EPICS bottles are separately accounted for by a2(M) terms]. Accordingly, variances in C, were estimated from replicate samplings/analyses from a given EPICS bottle. Results indicate that, while the standard deviation in C, varies directly with C,, the coefficient of variation (i.e., u/mean) is relatively constant. Thus, [U(CJ/C,]~N 1 X for both Cgl and Cg2. The uncertainty in mass of volatile solute within EPICS bottles, a2(M), may have several sources: imprecision associated with mass addition; possible leakage of volatile solute; possible sorptive losses. Their combined effect was estimated with C,/M data [actually (GC peak area)/(mass of solute stock added to bottle) data] from replicate bottles. The coefficient of variation in C / M averaged 1.35% for both high- and low-volume EPfCS bottles. Given that a2(C,/M)/(Cg/M)2 - u2(C,)/C,Z + u2(M)/I@ and that an
00
05
10
15
20
Henrys Constant (dimensionless)
Figure 5. Precision of modified EPICS procedure as a function of dimensionless Henry’s constant-observations and predictions based on propagation of variance.
independent estimate of u2(Cg)/C,Z= 1 X lo4 exists, then a2(M)/M2 N 8.2 X [Note that this estimate of 2(M)/Mis “flawed in a minor way by the use of a2(C,)/C,2 = 1X which considers only analytical contributions to a2(Cg)among replicate bottles. Contributions from variances in bottle and liquid volumes are neglected. However, as shown below, analytical variance indeed exerts a far greater impact than do these volume variances.] The above variance estimates were used in conjunction with eq 12 and 13 to estimate the expected % CV in H, as a function of H,and also to examine the relative contribution made by each parameter’s variance. Figure 5 shows the expected % CV in H, vs. H,, on the basis of this analysis, along with observed values from Table I. While there is certainly scatter to the data, it would appear that the analysis of precision provided via eq 12 and 13 is in substantial agreement with the data: The EPICS technique loses utility at low values of Henry’s constant. An exact lower limit of practical application cannot be generally specified; it depends upon one’s needs for precision. The analysis of precision also indicates that the overwhelming contributor to variance in H, is the variance associated with r. For example, at H , = 0.5, the last term in eq 12 has a value of 3.6 X the first three terms are 3 X lo-’, and 1.5 X reapproximately 1.1 X spectively. Analysis via eq 13 demonstrates that uncertainties in both C, and M contribute approximatelyequally to the variance in r. Analytical precision (associated with sampling and GC analysis) and the precise addition of volatile solutes to EPICS bottles are equally critical to overall precision of the EPICS technique. This paper has presented a modification to the EPICS procedure wherein the original assumption of equal solute mass additions to EPICS bottle pairs has been eliminated in favor of a gravimetric means of accounting for imperfect, volumetric additions. The purpose of this modification was to increase precision in measurements of Henry’s constant. The mean CV observed in this study was in the range of 3-4% for H,values from about 0.1 to 1.8. The original procedure ( 3 )yielded a mean CV value (4-5%) only slightly higher. This comparison, however, does not take into account differences in bottle and liquid volumes between the two studies. In this investigation, liquid volumes of 25 and 100 mL in 158.81-mL serum bottles were employed. Previous Environ. Sci. Technol., Vol. 21, No. 2, 1987
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studies (3) used EPICS pairs with a greater difference between high and low liquid volumes (11and 101 mL in 120.25-mL serum bottles). Greater precision in either the original or the modified EPICS procedure can be realized as the ratio between high and low liquid volumes is increased. Through an analysis of precision (via eq 1 2 and 13), the modified EPICS procedure is predicted to achieve mean CV values between 2.5 and 3.5% if used with the same bottle and liquid volumes as employed in the previous study. [Use of a less-than-desirable differential between liquid volumes in this study has a simple explanation: The determination of Henry’s constants was only incidental to a research project in which GC calibration factors for the compounds were desired at liquid volumes of 25 and 100 mL in 158.81-mL serum bottles (8).] It should be emphasized that if one’s major objective is to accurately measure Henry’s constant, then use of more disparate liquid volumes is recommended.
S u m m a r y and Conclusions The proposed modification to the EPICS procedure for measurement of Henry’s constant appears to offer the potential for a significant increase in precision. The modified procedure was employed to determine Henry’s constants for 13 volatile C1 and C2 chlorinated hydrocarbons over a temperature range from 10 to 35 “C, representing a range in dimensionless Henry’s constant of from 0.05 to 1.8 (Table I). The mean CV in Henry’s constant was 3-4% ; however, by employment of greater differentials in high and low liquid volumes within EPICS bottle pairs, it is estimated that the CV can be reduced to a mere 2.5-3.5%, as compared with the 4-5% CV observed with the original EPICS method. The precision achievable with the EPICS procedure deteriorates markedly for compounds with very low Henry’s constants (Figure 5). An exact lower limit of practical application cannot be generally specified; it depends upon one’s needs for precision. The major contributors to imprecision in the modified EPICS procedure are the variances associated with headspace sample analysis and knowledge of the relative solute masses contained in incubated bottles. These two variances make roughly equal contributions to the overall variance in measured Henry’s constant. The effects of ionic strength on apparent Henry’s constants were evaluated for six of the compounds, with KCl concentrations up to 1.0 M. The practical impact of ionic strength appears to be minimal. Salting-out coefficients ( k ) ranged from 0.107 to 0.213 L-mol-l at 20 “C (Table 111). Even in the case of the compound exhibiting the greatest ionic strength dependence (tetrachloroethylene), ionic strength would have to exceed 0.2 M (KC1) to cause a greater than 10% increase in apparent Henry’s constant.
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In these studies, aqueous mixtures of volatile solutes (with methanol also present) were generally employed in the determination of Henry’s constants. Comparison with results from previous studies in which aqueous solutions of single solutes were employed suggests that the use of mixtures is justified-at least within the range explored here. An investigation of methanol’s impact on the apparent Henry’s constant of trichloroethylene showed no effect at 2% (v/v) MeOH, while a 5% MeOH solution depressed H, by about 7 % .
Acknowledgments The experimental portion of the investigation was conducted by the author while a Visiting Professor in the Environics Division Laboratories of the AFESC, Tyndall AFB, Florida. Thanks are extended to Bob Olfenbuttel, Tom Walker, and staff for their support and hospitality. Registry No. Tetrachloroethylene, 127-18-4;trichloroethylene, 79-01-6; 1,l-dichloroethylene, 75-35-4; cis-1,2-dichloroethylene, 156-59-2; trans-1,2-dichloroethylene,156-60-5; vinyl chloride, 75-01-4; l,l,l-trichloroethane, 71-55-6; 1,l-dichloroethane,75-34-3; chloroethane, 75-00-3; carbon tetrachloride, 56-23-5; chloroform, 67-66-3; dichloromethane, 75-09-2; chloromethane, 74-87-3.
Literature Cited (1) Mackay, D.; Shiu, W. Y. J. Phys. Chem. Ref. Data 1981, 10(4), 1175-1199. (2) Mackay, D.; Shiu, W. Y.; Sutherland, R. P. Enuiron. Sci. Technol. 1979,13, 333-337. (3) 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. (4) Lincoff, A. H. M.S. Thesis, Cornel1 University, 1983. (5) Gossett, J. M.; Cameron, C. E.; Eckstrom, B. P.; Goodman, C.; Lincoff, A. H. Mass Transfer Coefficients and Henry’s Constants for Packed-Tower Air Stripping of Volatile Organics: Measurements and Correlations; Engineering & Services Laboratory, U.S. Air Force Engineering and Services Center: Tyndall AFB, FL, 1985; Report No. ESL-TR-85-18. (6) Goldstein, D. J. Air and Steam Stripping of Toxic Pollutants; U.S. Environmental Protection Agency. U S . Government Printing Agency: Washington, DC, 1982; Report NO. 68-03-002, Vol. 11. ( 7 ) Snedecor, G. W.; Cochran, W. G. Statistical Methods, 7th ed.; Iowa State University Press: Ames, IA, 1980. (8) Gossett, J. M. Anaerobic Degradation of C1 and C2 Chlorinated Hydrocarbons; Engineering & Services Laboratory, U.S. Air Force Engineering and Services Center: Tyndall AFB, FL, 1985; Report No. ESL-TR-85-38.
Received for review May 20, 1986. Accepted September 8,1986. This research was supported jointly by the U S . Air Force Systems Command (via its University Resident Research Program) and the U.S. Air Force Engineering and Seruices Center ( AFESC) .
