The Determination of Organic Phase–Air Partition Coefficient and

Article pubs.acs.org/jced

The Determination of Organic Phase−Air Partition Coefficient and Henry’s Law Constant for Volatile Organic Compounds Using Large Volume Vacuum Headspace Sampling Michael H. Hiatt* U.S. Environmental Protection Agency, National Exposure Research Laboratory Environmental Sciences Division. P.O. Box 93478, Las Vegas, Nevada 89193-3478, United States S Supporting Information *

ABSTRACT: Sampling the gas phase over peanut oil provided the means to determine the oil phase−gas phase partition coefficients (Koa) and the Henry’s law constant (HLCO) at temperatures between 10 °C and 98.5 °C. For the 123 compounds in the study the linear range of natural logarithm of Henry’s law constants by temperature was determined. The enthalpies of phase change of the compounds from the peanut oil were derived from the HLCO over the linear range. The experimental Koa values of the compounds correlated well with those generated when 1-octanol was the organic phase (r2 = 0.97 with intercept at 0).

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INTRODUCTION The partitioning between an organic phase and air (Koa) is an important physical property that is commonly used to model a compound’s behavior in the environment. This property has been used to predict partitioning of compounds in the environment,1−3 movement between air and soil4 and between air and vegetation.5 The Koa can also be used to predict the content of compounds in foods.6,7 Investigations of Koa have been critical to model the movement of compounds in the environment. In this pursuit Harner and Mackay developed a generator column method using 1-octanol to measure the Koa for chlorobenzenes, PCBs, and DDT.8 They further stated the direct measurement of Koa is preferred over calculating the value from Kow and Kwa because of the inherent errors of the calculation approach. Multiple headspace methods have been used to determine Koa for volatile organic compounds (VOCs).9−11 Gas chromatographic retention times have also been used to estimate 1-octanol−air partition coefficients.12−14 Estimation methods using quantitative structure property relationships (QSPRs) such as fragment constant method15 and molecule polarizability16 are also used. Organic phase−air partition coefficients have also been determined using hexadecane,17 plant cuticle,18 hexachlorobenzene,13 and n-octadecane13 with results similar to those for 1octanol. In this work peanut oil is used as the organic phase. Peanut oil was selected as the organic phase as it is a natural oil and has a boiling point (227 °C) higher than 1-octanol (194.5 °C) and is less likely to be volatilized. The boiling point for 1octanol is lower than many of the compounds in this study, and significant migration of the solvent through the apparatus This article not subject to U.S. Copyright. Published 2014 by the American Chemical Society

would be detrimental. In addition, 1-octanol is classified as a health hazard making it a less desirable solvent. It was felt that peanut oil would be a viable surrogate for organic phases in the environment, and that Koa determined using peanut oil would be equivalent to those for other organic phases. There is a paucity of Koa data for VOCs and only 46 of the 123 compounds investigated in this work have reported Koa values. Many of the VOCs are ubiquitous in the environment and modeling their movement through environmental compartments should be fundamental to understanding the movement of chemicals through the environment. This work investigates the Koa of this important group of compounds over a range of temperatures. The temperature-dependent Henry’s law constants (HLCO) are derived from the Koa and are in the solubility form (mole·kg−1·bar−1), and the dimensions match those used for HLC (water) by NIST.19 By obtaining HLCO over a temperature range it follows that the van’t Hoff equation can be used to determine the enthalpy of phase change for compounds in peanut oil.

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EXPERIMENTAL SECTION Vacuum Distiller. A Cincinnati Analytical Instruments model VDC1012 12 port-vacuum distiller (Indianapolis, IN) performed the evacuations in the study (see Figure 1). The headspace of a 100 mL round-bottom flask connected to the distiller with an O-ring connector that is evacuated by opening

Received: November 5, 2013 Accepted: January 16, 2014 Published: January 24, 2014 499

dx.doi.org/10.1021/je400967m | J. Chem. Eng. Data 2014, 59, 499−507

Journal of Chemical & Engineering Data

Article

(40 Pa) at the target temperature for the analysis. The peanut oil was allowed to cool to room temperature, and then 5 uL of a methanol solution containing compounds normally used as internal standards5 and 5 uL of methanol solution containing the balance of analytes were added. The compounds are identified in Table 1 with an ‘I’ immediately following the compound name of the internal standards. The compounds were added to the flask by syringe with the tip contacting the glass flask above (∼0.5 cm) the peanut oil. The sample was immediately sealed by connecting to the distiller using a Teflon O-ring and clamp. The Teflon O-ring seals were tightened past finger tight to minimize leakage through the O-ring connection. The flask containing the sample was immersed in a liquid nitrogen bath. When the liquid nitrogen bath no longer boiled vigorously a vacuum was applied to remove air until the system pressure was below 1 Torr (133 Pa), when evacuation was stopped. The liquid nitrogen bath was then removed and the flask warmed to ambient temperature. After the flask warmed to room temperature, the flask was heated to its target temperature with a heating cozy (Cincinnati Analytical Instruments, Indianapolis, IN) while a magnetic bar was used to stir the inside of the flask. The analytes were allowed to equilibrate between the peanut oil and gas phase for (100−120) min prior to taking the headspace sample while two blanks were analyzed (ports 1 and 2). The peanut oil was stirred at a rate of 1 rotation every 2 s. The temperature settings for the heating cozies were 25 °C, 45 °C, 60 °C, 80 °C, and 98.5 °C (temperature settings ± 0.1 °C). The 10 °C bath was constructed of a 450 mL beaker containing water and cooled by liquid nitrogen. The amounts of peanut oil were varied to moderate the amount of analyte in the headspace. Sample temperatures were maintained with a controller (model 5310, Drews Electronic, Kamp-Lintfort, Germany). The equilibration time had been found to be sufficient for equilibration in studies of spiking fish tissue20 and was used for this study. Prior to sample collection the condenser and transfer lines are under vacuum (valves V1 and V5 open and multiport valve in collection position). The headspace sample is collected by briefly opening the sample port valve (0.003 min). This time was selected so that ∼35 % of the headspace vapors are removed. If the headspace were allowed to completely expand into the condenser and transfer lines (equal pressures), 75 % of the headspace would be transferred. Calibration was performed by analyzing the same amount (Table 1) of each analyte contained in a sample flask without water or peanut oil. Because there was no matrix to help contain the analytes, the flask was precooled in a liquid nitrogen bath and then the addition of analytes to the flask resulted in their condensation on the interior walls. The transfer of the analytes to the cryotrap was accomplished by a 5 min evacuation. Calibration standards were analyzed before and after peanut oil analyses, and their standard error was propagated with each result. Integration was of a single quantitation ion.

Figure 1. Apparatus in sampling mode.

both the sample port valve and valve V1. The flasks are shown as 12 circles at the left in Figure 1, and the sample port valves are shown as smaller circles just above the flasks. The sample port valve for port 9 (lower left corner of Figure 1) is shown in the open position. The condenser was heated to 125 °C to prevent condensation of vapors passing through it to the cryotrap cooled to −150 °C where the vapors are condensed. The evacuation was stopped after 0.003 min by closing the sample port valve. Evacuation of the distiller lines continued for 3 min to complete the transfer of compounds through the distiller to the cryotrap, and then valve V1 was closed. The cryotrap was heated to 110 °C during a 2.5 min transfer with helium carrier to the GC−MS through the transfer line held at 200 °C. The multiport valve (V2) was heated to 200 °C, and all internal transfer lines were heated to 125 °C. The volume of each flask used in the study was determined (weight of water contained) as well as the volume of the sample port to the solenoid valve resulting in a total internal volume of 104.8 mL. All of the flasks used for measuring content of headspace above peanut oil were (100.00 ± 0.05) mL volumes. However, different size flasks were used to determine the impact of headspace volume on recovery of analytes to compensate for actual volumes of headspace when 0.20 to 5.0 g of peanut oil (density 0.91 g/cm3) was added. GC−MS. The vacuum distiller was interfaced to a GC−MS so that the vacuum distillate is transferred directly to the GC− MS. In this study, the GC−MS was a Thermo DSQ mass spectrometer and Trace GC (ThermoElectron Corp., Austin TX). The GC capillary column was a 30 m × 0.25 mm i.d., 1.5 μm film VOCOL (Supelco, Bellefonte, PA). The GC operating conditions were 2.5 min at −35 °C, 40 °C/min ramp to 60 °C, 5 °C/min ramp to 120 °C, and held at 120 °C for 1 min, 20 °C/min ramp to 220 °C and held for 12 min resulting in a GC run time of 34 min. The injection was split 40:1 with a constant flow rate of 1.4 mL/min. The mass spectrometer scanned between 35 and 300 amu at 1 scan/sec. Analyses. Prior to the introduction of analytes an aliquot of peanut oil (Hollywood Enriched Gold, The Hain Celestial Group Inc., Melville NY; cold press process, ≥99.98 % pure) was treated to remove dissolved gases. This was accomplished by adding the peanut oil to the round-bottom flask along with a 1.2 cm microstir bar and then evacuated to a pressure of ∼0.3

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THEORY The solubility form of the Henry’s law constant (HLC) is used by NIST to describe the partitioning between water and air. In this work HLCO is the Henry’s law constant describing the partitioning between an organic phase and air using the same dimensions (mole·kg−1·bar−1). The solubility form of HLC for a compound can be expressed as its concentration in the liquid phase (cl) divided by its partial pressure (p) in the gas phase or 500

dx.doi.org/10.1021/je400967m | J. Chem. Eng. Data 2014, 59, 499−507

ng 250 250 250 250 250 1000 250 250 250 250 2500 250 250 1250 250 250 250 1250 250 1250 250 1250 250 1000 250 1250 250 250 250 250 250 1250 2500 12000 500 5250 250 600000 250 500

compound

1,1,1,2-tetrachloroethane 1,1,1-trichloroethane 1,1,2,2-tetrachloroethane 1,1,2-trichloro-1,2,2-trifluoroethane 1,1,2-trichloroethane 1,1,2-trichloroethane-d3 I 1,1-dichloroethane 1,1-dichloroethene 1,1-dichloropropene 1,2,3-trichlorobenzene 1,2,3-trichlorobenzene-d3 I 1,2,3-trichloropropane 1,2,4-trichlorobenzene 1,2,4-trichlorobenzene-d3 I 1,2,4-trimethylbenzene 1,2-dibromo-3-chloropropane 1,2-dibromoethane 1,2-dibromoethane-d4 I 1,2-dichlorobenzene 1,2-dichlorobenzene-d4 I−V 1,2-dichloroethane 1,2-dichloroethane-d4 I−V 1,2-dichloropropane 1,2-dichloropropane-d6 I−V 1,2-dimethylbenzene 1,2-dimethylbenzene -d10 I−V 1,3 and 1,4-dimethylbenzenes 1,3,5-trimethylbenzene 1,3-dichlorobenzene 1,3-dichloropropane 1,4-dichlorobenzene 1,4-difluorobenzene I−V 1,4-dioxane 1,4-dioxane-d8 I−V 1-methylnaphthalene 1-methylnaphthalene-d10 I 2,2-dichloropropane 2-chloroethanol-d4 I−V 2-chlorotoluene 2-methylnaphthalene

amount added 10 10 10 10 10 10 10 10 10 40 40 10 40 40 10 40 10 10 10 25 10 10 10 10 10 10 10 10 10 10 25 10 10 10 25 25 10 10 10 25

min

temperature range °C 99 60 99 60 99 99 60 60 80 99 99 99 99 99 99 99 99 99 99 99 60 60 60 60 99 99 99 99 99 99 99 80 60 60 99 99 80 99 99 99

max

B 5065.4 4698.9 6353.5 5579.7 5020.0 5130.7 5207.5 5085.4 4153.3 4964.3 5861.2 6004.6 5561.8 5148.5 5671.2 5273.1 4921.4 5048.5 5448.7 5626.1 5290.5 5007.7 5474.8 5011.1 5436.6 5030.1 5546.5 5651.4 5625.3 4658.4 5329.3 4704.8 5853.2 5694.8 4070.1 4361.8 3938.2 5175.9 5735.5 4328.6

A −11.297 −12.491 −14.612 −16.996 −11.652 −12.010 −14.574 −14.816 −10.454 −8.196 −10.828 −13.438 −10.114 −8.955 −12.026 −10.043 −11.109 −11.536 −10.982 −11.531 −14.065 −13.255 −14.226 −12.800 −12.247 −10.982 −12.880 −12.085 −11.735 −10.545 −10.768 −11.973 −15.305 −14.877 −5.025 −5.827 −10.146 −12.405 −12.654 −5.809

ln(mol·kg−1·bar−1)a 0.272 0.155 0.342 0.341 0.295 0.324 0.243 0.300 0.252 0.256 0.330 0.330 0.323 0.258 0.287 0.403 0.259 0.283 0.266 0.295 0.225 0.241 0.172 0.233 0.262 0.307 0.252 0.289 0.283 0.356 0.321 0.261 0.166 0.191 0.606 0.563 0.319 0.462 0.283 0.572

Uline

Henry’s law constant

Table 1. Henry’s Law Constant, log (Kao), and Enthalpy of Phase Change Resultsn

3.83 2.78 4.27 2.10 3.61 3.61 2.61 2.33 2.87 5.03 5.19 4.27 5.07 4.97 4.40 4.68 3.70 3.70 4.52 4.54 2.95 2.89 3.15 3.10 3.96 3.91 3.84 4.34 4.45 3.56 4.44 3.01 3.24 3.19 5.10 5.18 2.69 3.51 4.22 5.14

log(Koa)

b

0.10 0.06 0.13 0.13 0.11 0.14 0.09 0.11 0.10 0.10 0.14 0.12 0.12 0.11 0.11 0.15 0.10 0.12 0.10 0.13 0.09 0.10 0.07 0.10 0.10 0.13 0.10 0.11 0.11 0.13 0.12 0.11 0.07 0.08 0.21 0.24 0.12 0.20 0.11 0.20

UKoa

c

5.42

5.25

5.32f, 5.19h 4.52f 5.10f, 4.94i

501

3.17g, 3.18k

4.32f, 4.46g, 4.16i, 4.45j

5.77

5.79

3.15

3.88 4.40 4.39 3.19 4.40

3.80l 4.27f, 4.12i, 4.45j

3.56

3.83

3.92l

2.77 2.89

2.57

2.78g

3.46

1.86

2.58

Koa

e

cuticle

3.19f

4.47

4.48f, 4.36h

4.44

2.35 2.11

3.69f 2.56f

Koa 3.64 2.69 3.80 2.12 3.29

Koa

d

hexadecane

3.97 2.70g, 2.62f 4.30f

f

Literature Values 1-octanol 41.74 38.89 52.81 45.59 41.63 42.50 42.89 41.79 34.11 41.26 48.72 49.50 46.23 42.79 46.76 43.83 40.55 41.82 44.99 46.76 43.55 41.36 44.95 41.31 44.87 41.64 45.92 46.54 46.36 38.53 44.29 38.67 48.20 46.82 33.83 36.25 32.20 41.97 47.23 35.98

kj·mole−1

ΔH 3.80 3.44 4.77 7.56 4.12 4.52 5.37 6.65 4.23 5.98 7.72 4.60 7.55 6.02 4.01 9.41 3.62 3.95 3.71 5.22 4.98 5.33 3.80 5.15 3.66 4.28 3.52 4.04 3.95 4.97 5.68 4.38 3.67 4.24 10.73 9.97 5.34 6.45 3.95 10.13

UHoa

Journal of Chemical & Engineering Data Article

dx.doi.org/10.1021/je400967m | J. Chem. Eng. Data 2014, 59, 499−507

2-propenenitrile 3,5-dibromotoluene 3,5-ditert-butyltoluene I 4-bromofluorobenzene I−V 4-chlorotoluene 4-methyl-2-pentanone a,a-dichloro-o-xylene I acetonitrile acetophenone acetophenone-d5 I acrylonitrile allylchloride azulene benzene benzene-d6 I bromobenzene bromobenzene-d5 I bromochloromethane bromodichloromethane bromomethane butan-2-one butan-2-one-d5 I carbon disulfide chlorobenzene chlorobenzene-d5 I−V chloroethane chloromethane cis-1,2-dichloroethene cis-1,3-dichloropropene cis-1,4-dichloro-2-butene cyclohexane decafluorobiphenyl dibromochloromethane dibromomethane dichlorodifluoromethane dichloromethane dichloromethane -d2 I diethyl ether diethyl ether-d10 I−V ethylacetate-13C I−V

compound

Table 1. continued

500 11000 5000 1250 250 1000 48750 1000 500 5210 500 250 12500 250 1250 250 1250 250 250 250 1000 2500 250 250 1250 250 250 250 1250 1000 250 1250 250 250 250 250 1250 500 1250 12500

ng

amount added 10 60 60 10 10 10 25 10 25 40 10 10 60 10 10 10 10 10 10 40 10 10 10 10 10 NA 10 10 10 10 10 25 10 10 10 10 10 10 10 10

min

temperature range °C 60 99 99 99 99 99 99 60 99 99 60 80 99 60 60 99 99 60 99 80 80 60 60 99 99 NA 40 60 99 99 60 99 99 99 40 80 80 80 60 60

max

B 5367.7 5810.3 5303.6 5449.9 5663.8 4710.3 4657.7 5481.4 5226.9 6162.1 5206.7 4181.0 6703.7 5210.5 4999.9 4662.4 5235.7 5232.8 4619.9 2977.0 5789.7 5503.6 4501.2 5253.9 5301.9 3927.2 6590.7 5087.1 4580.1 5064.1 4927.2 5791.3 4688.8 4284.9 6801.7 4485.7 4436.7 4287.4 5854.4 5211.7

A −14.817 −9.662 −8.658 −11.826 −12.342 −11.218 −6.642 −15.877 −9.760 −12.740 −14.897 −11.682 −12.219 −13.894 −13.262 −9.641 −11.091 −14.078 −10.908 −8.840 −16.436 −15.120 −12.611 −12.079 −12.311 −8.539 −20.926 −13.815 −10.300 −10.633 −13.319 −11.923 −10.310 −9.918 −21.356 −12.393 −12.189 −12.433 −17.602 −14.439

ln(mol·kg−1·bar−1)a 0.299 0.419 0.321 0.289 0.226 0.277 0.880 0.379 0.347 0.324 0.362 0.366 0.637 0.280 0.168 0.366 0.444 0.237 0.258 0.254 0.468 0.176 0.220 0.259 0.299 0.870 0.447 0.273 0.375 0.442 0.193 0.427 0.266 0.289 0.155 0.321 0.324 0.520 0.393 0.250

Uline

Henry’s law constant

2.74 5.62 5.32 4.16 4.25 3.35 5.26 2.45 4.73 4.80 2.47 2.37 5.82 2.91 2.88 3.96 4.17 2.87 3.35 1.85 2.65 2.81 2.44 3.76 3.73 NA 1.87 2.77 3.55 4.12 2.75 4.62 3.71 3.29 1.99 2.51 2.53 2.20 2.24 2.68

log(Koa)

b

0.11 0.18 0.14 0.13 0.09 0.11 0.38 0.14 0.13 0.14 0.13 0.14 0.28 0.11 0.07 0.14 0.19 0.09 0.10 0.11 0.17 0.08 0.09 0.10 0.13 NA 0.19 0.10 0.14 0.16 0.08 0.19 0.10 0.11 0.07 0.12 0.14 0.18 0.17 0.11

UKoa

c

Literature Values

2.80

2.80g, 2.81l

502

2.19g

2.27g

3.07g

2.75l

2.06

2.84

1.68 1.16

3.64

2.28g 3.70j 1.73f

2.29

2.77g

4.03

1.56

Koa

d

hexadecane

2.31g

Koa

1-octanol

2.53

2.38

2.23

3.52

2.56

2.63

2.40 2.08

2.58

3.04

Koa

e

cuticle 44.25 48.29 44.08 45.07 47.07 38.75 38.71 44.97 43.44 51.22 42.80 34.13 55.72 42.85 41.47 38.96 43.33 43.11 38.11 24.74 47.35 45.79 37.32 43.30 43.69 31.19 54.02 42.01 37.85 42.09 40.59 48.13 38.79 35.37 56.54 36.79 36.37 34.76 48.16 43.11

kj·mole−1

ΔH 6.63 15.82 12.12 4.04 3.16 3.87 15.58 8.40 6.14 7.58 8.01 6.14 24.06 6.20 3.73 5.12 6.19 5.24 3.61 8.23 7.85 3.90 4.88 3.62 4.18 12.15 15.51 6.05 5.23 6.17 4.28 7.56 3.71 4.04 5.37 5.38 5.43 8.72 8.70 5.55

UHoa

Journal of Chemical & Engineering Data Article

dx.doi.org/10.1021/je400967m | J. Chem. Eng. Data 2014, 59, 499−507

ethylbenzene ethylbenzene-d10 I−V fluorobenzene I−V hexachlorobutadiene hexafluorobenzene I−V hexan-2-one hexan-2-one -d5 I iodomethane isopropylbenzene methyl 2-methyl-2-propenoate methyl acetate methyl cyclohexane methyl cyclohexane-d14 I−V methyl tert-butyl ether naphthalene naphthalene-d8 I n-butylbenzene nitrobenzene nitrobenzene-d5 I nitromethane-13C I pentachloroethane pentafluorobenzene I−V p-isopropyltoluene propan-2-one propan-2-one -13C I propionitrile propylbenzene pyridine-d5 I sec-butylbenzene styrene tert-butylbenzene tetrachloroethene tetrachloromethane tetrahydrofuran-d8 I toluene toluene-d8 I−V trans-1,2-dichloroethene trans-1,3-dichloropropene trans-1,4-dichloro-2-butene tribromomethane

compound

Table 1. continued

250 1250 1250 250 1250 501 2500 500 250 500 250 250 1250 250 250 2500 250 500 1250 3250 250 1250 250 500 15500 500 250 62500 250 250 250 250 250 1250 250 1250 250 1250 1000 250

ng

amount added

503

10 10 10 40 40 10 25 10 10 10 10 10 10 10 10 10

10 10 10 40 10 10 10 10 10 10 10 10 10 10 25 25 10 25 40 10 10 10 25

min

temperature range °C 99 99 60 99 80 99 99 60 99 60 80 60 60 60 99 99 99 99 99 60 99 60 99 60 60 60 99 80 80 99 99 99 60 80 99 99 60 99 99 99

max

B 5393.3 5418.4 4961.9 4069.4 4430.2 5557.2 5361.9 4684.3 5269.8 5699.9 4741.1 5498.3 4842.3 5047.3 5548.3 5340.7 5667.3 5422.2 6100.8 5619.5 5409.7 5588.8 5681.3 NA 6567.8 5307.3 5843.8 5095.1 5902.5 5290.4 5667.1 4768.5 5145.5 4648.9 4879.5 4376.5 4905.9 4163.5 5477.7 4940.2

A −12.542 −12.742 −12.977 −5.971 −12.064 −13.481 −12.806 −13.322 −11.526 −15.026 −13.442 −14.634 −12.607 −14.503 −9.970 −9.371 −12.088 −10.229 −12.519 −15.555 −11.235 −15.587 −12.018 NA −19.586 −14.663 −13.145 −11.433 −13.122 −11.651 −12.221 −10.906 −13.862 −12.296 −11.660 −9.977 −13.696 −9.335 −12.147 −10.134

ln(mol·kg−1·bar−1)a

0.300 0.320 0.230 0.155 0.149 0.301 0.195 0.340 0.199 0.300 0.243 0.333 0.212 0.377 1.096 0.321

0.242 0.268 0.188 0.375 0.336 0.409 0.245 0.232 0.246 0.163 0.408 0.144 0.158 0.320 0.276 0.248 0.323 0.262 0.364 0.200 0.449 0.154 0.225

Uline

Henry’s law constant

3.77 3.72 2.95 4.69 2.57 3.60 3.61 2.39 4.03 3.13 2.43 3.01 2.93 2.41 5.11 5.07 4.36 4.81 4.81 2.79 4.36 2.73 4.41 NA 2.42 2.72 4.16 3.81 4.26 4.00 4.30 3.57 2.83 2.79 3.40 3.40 2.55 3.37 4.06 4.15

log(Koa)

b

0.09 0.12 0.08 0.14 0.15 0.15 0.11 0.09 0.10 0.07 0.15 0.06 0.07 0.12 0.11 0.11 0.12 0.10 0.16 0.09 0.16 0.07 0.09 NA 0.13 0.12 0.09 0.07 0.06 0.11 0.08 0.13 0.08 0.13 0.09 0.14 0.08 0.14 0.32 0.12

UKoa

c

Koa

3.26

3.31g, 3.32l

3.75

3.58

3.86

4.23

1.70

4.26

4.46

2.40 5.16

2.11 4.11

3.26

3.78

Koa

d

hexadecane

3.48g 2.79g

2.31g 2.69g

2.52g,2.53k 3.89f

5.19g, 5.37m

2.31g 3.05l

3.68g

3.74

l

Literature Values 1-octanol

2.98 2.43

3.74

2.40

3.40

Koa

e

cuticle

54.62 43.65 48.35 42.35 49.06 43.63 47.10 39.55 42.54 38.06 40.19 36.23 40.62 34.43 45.73 41.07

44.59 44.77 41.04 33.82 36.32 45.65 43.95 38.48 43.63 46.94 38.75 45.14 40.12 41.53 46.11 44.39 47.10 45.07 50.71 46.66 44.77 46.00 47.22

kj·mole−1

ΔH

6.65 7.10 3.21 5.03 4.84 4.20 3.46 4.75 4.41 5.03 3.39 4.64 4.70 5.26 15.31 4.48

3.37 3.74 4.16 8.77 5.64 5.70 3.42 5.13 3.44 3.61 6.84 3.20 3.51 7.09 4.88 4.40 4.51 4.64 8.50 4.42 6.27 3.40 3.98

UHoa

Journal of Chemical & Engineering Data Article

dx.doi.org/10.1021/je400967m | J. Chem. Eng. Data 2014, 59, 499−507

2.80g

3.31

0.12 0.01 0.10 0.09 0.03

UKoa

c

3.10 1.68 2.86 2.08 1.93 0.324 0.020 0.255 0.213 0.071 4082.5 2423.5 5388.5 6580.0 8648.7 trichloroethene trichlorofluoromethane trichloromethane vinylchloride vinylchloride-d3 I

250 250 250 250 1250

10 25 10 10 10

99 80 60 60 40

−9.682 −7.376 −14.608 −20.402 −27.695

Uline compound

ng

min

max

A

B

log(Koa)

b

ln(mol·kg−1·bar−1)a

Henry’s law constant

temperature range °C amount added

Table 1. continued

Article

a The regression constants for temperature dependence of ln(HLCO) described by ln(HLCO) = A + B/K. The uncertainty is the standard error of the line. bThe log of Koa (25 °C) interpolated from the line ln(HLCO) to temperature for comparison to reported Koa at 25 °C cThe uncertainty with log of Koa (25 °C). The uncertainty is the standard error of the line. dFrom reference 17. eFrom reference 18. f From reference 16. gFrom reference 9. hFrom reference 8. iFrom reference 12. jFrom reference 15. kFrom reference 10. lFrom reference 14. mFrom reference 11. nNA: No viable data were collected for the compound.

4.53 0.45 5.65 4.72 2.47 33.16 20.14 44.49 54.67 71.89 2.88 2.48

Koa

i

Koa

3.00

kj·mole−1 Koa

e

cuticle hexadecane Literature Values

1-octanol

d

ΔH

UHoa

Journal of Chemical & Engineering Data

HLC =

cl p

(1)

Rather than measuring the partial pressure of a compound, it was easier to measure the concentrations of a compound in the gas phase cg and knowing the amount of compound in the closed system as well as the volumes of oil and headspace calculate the concentration of compound in the headspace cl. A comparison of the the concentrations in the liquid and gas phase is simpler using the partition coefficient Koa as follows: c Koa = l cg (2) The Koa can then be converted to HLCO as HLCO =

Koa R×T×ρ

(3) 3

−1

−1

where R = 0.00008314 m bar K mol , T = temperature in K, and ρ is the density for peanut oil, 910 kg·m−3. Determining the HLCO at the various temperatures allows a determination of the relationship of ln (HLCO) vs temperature. The determination of enthalpy of phase change from the peanut oil is described by the van‘t Hoff equation ⎡⎛ 1 1 ⎞ ΔHoa ⎤ HLCOT = HLCOθ exp⎢⎜ − θ ⎟ ⎥ ⎣⎝ T T ⎠ R ⎦

(4)

HLCOθ

where is the HLCO at a reference temperature such as 298 K, and θ is the reference temperature, HLCOT is a second HLC at temperature T in K. R is the gas constant 8.314 J K−1 mol−1 and ΔHoa is the enthalpy of phase change from the peanut oil. Equation 4 can be expressed as ln(HLCOT) = A +

ΔHoa T×R

(5)

ln(HLCOθ)

θ

where A is a constant equal to − ΔHoa/T × R. If the relationship of ln (HLCO) to 1/T is a straight line over a temperature range, then eq 5 can be used to solve for ΔHoa over the temperature range.

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RESULTS The volume of headspace taken was calibrated by the fractional amount of compounds that would be transferred from flasks without any matrix in 0.003 min compared to how much could be extracted in 5 min. Only the analytes (internal standards) noted with an ‘I’ following the compound name in Table 1 were used. In order to minimize the impact that compounds with greater measurement errors (primarily higher boiling compounds), only those internal standards whose responses had a precision less than 5 % for both the 0.003 and 5 min evacuations were used in this calculation (19 of 40 compounds and these labeled with a ‘V’ in Table 1). By evacuating flasks of different volumes the recovery of headspace taken in 0.003 min were related to volume. The volume of headspace sampled was determined as 39.3, 39.8, and 42.2 mL for when the 100.0 volume flask contained the 0.2 g, 1.0 g and 5.0 g peanut oil amounts (relative error of 2.7 %). This uncertainty of the volume of sample was included with the determination for the uncertainty for each experimental Koa value. It was observed that when the analytes were held in an evacuated flask (sealed using a Viton O-ring) without water or peanut oil there would be attenuation of response of the higher boiling analytes. By using Teflon O-ring seals in place of Viton 504

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seals the attenuation was almost eliminated. However there remained a minor attenuation, and this was attributed to the Viton seat in the sample port solenoid valve. The memory effect of the system observed was translated as equivalent to 0.003 mL of peanut oil. Therefore care needed to be taken to ensure the minimum amount of peanut oil used was much larger than the organic phase contributing to system memory. The minimum amount of peanut oil used in these studies was 0.20 g. The partitioning of compounds between peanut oil and the gas phase was measured at various temperatures and amounts of peanut oil. For an amount of peanut oil heated to a target temperature, conditions would be optimal for recovery of a subset of the analytes. Because the gas phase is sampled and the concentration of a compound in the oil phase is determined by what is not in the gas phase it was necessary to evaluate how the measurement error for analytes in the headspace impacted the uncertainty in determining Koa and HLCO. The relative uncertainty in Koa relating to measurement error can be described using eq 2 (and HLCO using eq 3) as Koa ± sm =

Table 2. Range for Fractional Amount of a Compound in Headspace Necessary for Determining HLC T °C 10 25 40 60 80 98.5

0.005 0.005 0.005

1g upper

0.63 0.63 0.63

lower 0.005 0.005 0.005 0.005 0.005

0.2 g upper 0.63 0.2 0.2 0.2 0.63

lower

upper

0.005 0.005 0.005 0.005

0.63 0.2 0.2 0.2

a

Lower limit selected as a general fractional amount of compound present in the gas phase that could be assumed free of interference. b Upper limit determined as the fractional amount of compound in the gas phase that could be measured with a 5 % relative error and result in a measurement error of the Koa of 5 % (0.2 fractional amount). For the largest amount of peanut oil, the upper limit reflects a measurement error for Koa of 10 %. Increasing the upper recovery limit was necessary to obtain results for the most volatile analytes at each temperature.

would leave data for only two temperatures or if the standard error of the line using the result was less than 5 % (as measured at 298 °C). A total of 36 of 96 results were eliminated by this test. Because of the problems exhibited with heating 5 g of peanut oil on chromatography and rejection of results, 5 g of peanut oil at 98.5 °C was not investigated. The HLCO for two compounds in the study could not be determined because of contamination in the peanut oil. These were propan-2-one (by a large amount of propan-2-one in the oil) and chloroethane (by an unidentified compound). The natural logarithms of the experimental HLCO values were plotted against temperature to evaluate linearity (Figure 2). The temperature range where the natural logarithm of

(x ± Δx)/Vg (1 − x ± Δx)/Vo

5g lowera

(6)

where x is the fraction of the analyte present in the headspace, Δx is the relative measurement error for x (includes error associated with taking an aliquot of headspace), Vg is the total volume of gas phase, Vo is the volume of peanut oil, and sm is the resulting propagated error. The measurement error associated with measuring the fractional amount of a compound in the gas phase can exaggerate the error in the amount of the compound in the oil phase. As the fractional amount of a compound in the gas phase approaches 1, the relative error for the denominator in eq 6 approaches infinity. To minimize the impact of measurement error on Koa when the fractional amount of a compound in the gas phase exceeded 0.63, Koa was determined for the instance. This limit reflects a relative uncertainty (relating to sm) in Koa of 10 % when the relative measurement error is 5 %. For those instances where a smaller oil volume was taken, the upper limit for a fractional amount of 0.20 for a compound in the gas phase was selected when smaller sample sizes (0.2 and 1 g) would yield relative uncertainty in Koa of 5 % or less. A fractional amount of compound in the gas phase of 0.005 was the lower limit for recovery of analytes in the headspace to minimize interference effects. The limits for the amount of compound in the gas phase are presented in Table 2 by amount of peanut oil and temperature. Some of the most volatile compounds had insufficient data that passed the initial criteria to describe the temperature dependence of HLCO necessitating relaxing of the limits. In these cases the upper limit 0.74 of a compound in the headspace (applied only for the 5 g peanut oil experiments) was used resulting in an estimated 15 % relative uncertainty for HLCO. The determination of HLCO at 80 °C for the more volatile compounds was problematic due to their minimal sorption in peanut oil at that temperature. Therefore an evaluation of the 80 °C HLCO values was performed (by compound) to verify by including them that temperature dependence did not seriously impact results. The test was that the 80 °C data would raise the standard error of the line (natural logarithm of HLCO vs K−1) by more than 15 %. However, a result would not be excluded if

Figure 2. Temperature dependence of HLCO for selected compounds: ■, vinyl chloride; □, benzene; ▲, toluene; ●, 1,2-dichlorobenzene; ○, and naphthalene.

HLCO vs 1/K is essentially linear was determined for each compound. Generally the more polar compounds had greater uncertainty in the determination of their HLCO because of their comparatively less resolved chromatography and interferences. The higher boiling compounds also had greater uncertainties due their minimal responses even in the smaller peanut oil sample sizes. Reconfiguring the distiller used in the study with seals less prone to uptake of organic compounds would allow taking even smaller amounts of peanut oil and minimize the uncertainties associated with the higher boiling compounds. 505

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Figure 3. The ln(Koa) generated for compounds using peanut oil compared to reported for 1-octanol values at 273 K.

Figure 4. Temperature dependence of HLCO for 1-octanol by generator column method8 and for peanut oil by headspace. Data for 1,2dichlorebenzene and 1,2,3-trichlorobenzene by the generator column method are the circle and triangle points, respectively. Data for 1,2dichlorebenzene and 1,2,3-trichlorobenzene from this study are the solid circle and solid triangle points, respectively.

another gas chromatography method14 for benzene, toluene, ethylbenzene, cyclohexane, and methyl cyclohexane provided similar results to this study near 25 °C, but the temperature dependence diverged with higher Koa. The similar temperature dependent Koa trends observed for 1-octanol by the previous studies suggest peanut oil is a similar organic phase for Koa studies. After the determination of the HLCO over a temperature range and identification of its linear range, the enthalpy of phase change of the compounds from the peanut oil can be determined using the van’t Hoff equation (eq 5). The determination of the energies by this method resulted in larger uncertainties than by other approaches and were generally

The experimental results are tabulated in Table 1. Also presented in Table 1 are the compounds’ HLCO converted to log Koa at 25 °C for comparison to literature values. The Koa generated using peanut oil correlate well (r2 = 0.97 with 0 as the intercept) with those reported for 1-octanol (Figure 3). A comparison of the profiles of 1,2-dichlorobenzene and 1,2,3-trichlorobenzene obtained by the generator column method8 to this study’s results indicates a very similar temperature dependence (slope) but that the compounds’ Koa are greater for the peanut oil phase (Figure 4). This comparability was similar to that found by the gas chromatography method,12 and the computed Koa values (1octanol) by the Fragment Constant method.15 The Koa by 506

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(13) Wania, F.; Lei, Y. D.; Harner, T. Estimating Octanol−Air Partition Coefficients of Nonpolar Semivolatile Organic Compounds form Gas Chromatographic Retention Times. Anal. Chem. 2002, 74, 3476−3483. (14) Gruber, D.; Langenheim, D.; Gmehling, J. Measurement of Activity Coefficients at Infinite Dilution Using Gas−Liquid Chromatography. 6. Results for Systems Exhibiting Gas−Liquid Interface Adsorption with 1-Octanol. J. Chem. Eng. Data 1997, 42, 882−885. (15) Li, X.; Chen, J.; Zhang, L.; Qiao, X.; Huang, L. The Fragment Constant Method for Predicting Octanol−Air Partition Coefficients of Persistant Organic Pollutants at Different Temperatures. J. Phys. Chem. Ref. Data 2006, 35, 1365−1384. (16) Staikova; Wania, F.; Donaldson, D. J. Molecular Polarizability as a Single-Parameter Predictor of Vapour Pressures and Octanol−Air Partitioning Coefficients of Non-polar Compounds: A Priori Approach and Results. Atmos. Environ. 2004, 38, 213−225. (17) Hilal, S. H.; Ayyampalayam, S. N.; Carreira, L. A. Air−Liquid Partition Coefficient for a Diverse Set of Organic Compounds: Henry’s Law Constant in Water and Hexadecane. Environ. Sci. Technol. 2008, 42, 9231−9236. (18) Welke, B.; Ettlinger, K.; Riederer, M. Sorption of Volatile Organic Chemicals in Plant Surfaces. Environ. Sci. Technol. 1998, 32, 1099−1104. (19) Sander, R. Henry’s Law Constants. In NIST Chemistry WebBook; NIST Standard Reference Database Number 69; Linstrom, P.J., Mallard, W.G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, http://webbook.nist.gov (retrieved April 30, 2013). (20) Hiatt, M. H. Analyses of Fish Tissue by Vacuum Distillation/ Gas Chromatography/Mass Spectrometry. Anal. Chem. 1997, 69, 1127−1134.

lower values than for those reported for the 1-octanol solvent.8,12

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Table of Henry’s law constants (mole·kg−1·bar−1) by experiment and graphs for each compound of Henry’s law constants by temperature. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Tel.: 702 798 2381. Fax: 702 798 2142. E-mail: hiatt.mike@ epa.gov. Funding

The U.S. Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), funded and performed the analytical research described. Notes

This manuscript has been subjected to the EPA’s review and has been approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. The authors declare no competing financial interest.

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REFERENCES

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