In the Laboratory
Measurement of Henry’s Law Constants Using Internal Standards A Quantitative GC Experiment for the Instrumental Analysis or Environmental Chemistry Laboratory Chang Ji,* Susanne M. Boisvert, Ann-Marie C. Arida, and Shannon E. Day Department of Chemistry and Biochemistry, Texas State University–San Marcos, San Marcos, TX 78666; *
[email protected] One of the most important processes that affect the chemical transport in the environment is the partitioning of substances between air and water, which can be quantitatively accounted for by the Henry’s law constants, H, in units of Pa m3mol–1. The basic concepts with regard to Henry’s law are usually covered in undergraduate general, physical, and environmental chemistry courses. However, few experiments have been incorporated in the curriculum of undergraduate laboratory courses to accurately determine H, owing to the difficulties of the measurement. Most commonly, the Henry’s law constants are estimated from other thermodynamic data, such as vapor pressure and solubility (1). On the basis of gas chromatographic (GC) analysis, static headspace methods have been used in the undergraduate laboratory to measure the values of H for a variety of volatile organic compounds (2, 3). Although the methodology is simple, the experiments may incur large relative errors for species having low H values (3). The internal standard method serves as an attractive approach in instrumental analysis, especially in quantitative chromatography. The technique offers high precision for the measurements by eliminating the uncertainties introduced by sample injection. Several internal standards have been applied to study some real-life samples by using GC (4) or HPLC (5) in undergraduate laboratory and the corresponding calculations have been discussed (6). The method has also been used for quantitative IR (7) and NMR (8, 9) experiments in instrumental analysis laboratory. Recently, an internal method has been developed to measure the dimensionless Henry’s law constants (Hi) for compounds highly soluble in water (10). This approach is elegantly simple as the knowledge of many experimental variables is not required. Random errors such as those associated with GC injection volumes can also be minimized and the technique works particularly well for compounds that have Hi values less than 10–3. In this article, the internal standard method is described to determine Hi for some low molar mass alkyl nitriles. The measurements have been completed by undergraduate students, who were motivated by the contents of the experiment, which is relevant to real-world environmental issues. The students will not only reinforce their knowledge of Henry’s law and the internal standard method but also learn to analyze real experimental data by calculating various standard deviations and creating graphs for linear regression. The experiment is most appropriate for instrumental analysis or environmental chemistry laboratory but may also be placed in physical chemistry laboratory.
Experimental Methodology A mixture containing similar quantities of the analyte (A) and the internal standard (I) is used to prepare the dilute standard (organic solvent) and aqueous solutions. The aqueous solution is placed in a closed system to reach the thermodynamic equilibrium and the headspace samples as well as the standard solution are subject to GC analysis. The dimensionless Henry’s law constant of the analyte (Hi,A) can be calculated by
R H i, A 1 H i , I R2
(1)
where R1 and R2 are the GC peak area ratios (analyte-to-internal standard) for the headspace and the standard solution, respectively, and Hi,I is the known dimensionless Henry’s law constant of the internal standard. The detailed derivation can be found in the literature (10) or in the online supplement. The exact knowledge of the substrate concentrations, the GC injection volumes, as well as the gas and the liquid phase volumes in the closed system is not required in this approach, which gives it advantages over the previously reported methods. The procedure for sample preparation is very simple. The Henry’s law constants H in units of Pa m3 mol‒1 can be easily obtained by multiplying Hi by RT (R = 8.314 Pa m3 mol‒1 K‒1). The limitation of this method is that it only works for compounds having low Hi values. Chemicals All alkyl nitriles were purchased from Alfa Aesar or Aldrich and used as received. Pentane (EMD Chemicals) was used as the solvent for the standard solutions. The in-house deionized water was used without further purification to prepare the dilute aqueous solutions. Equipment An Agilent Technologies model 6890N gas chromatograph equipped with a flame ionization detector was utilized to quantify the compounds in the headspace samples and in the standard solution. The GC injector was used in the splitless mode and the analytes were separated on a 60 m × 0.25 mm (i.d.) capillary column (SA-WAX, Sigma-Aldrich) with carbowax as the stationary phase. A toluene solution containing 5% dimethyldichlorosilane was used to chemically deactivate the entire inside surface of the 40 mL sample vials (Supelco) to prevent the adsorption of
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 7 July 2008 • Journal of Chemical Education
969
In the Laboratory Table 1. Dimensionless Henry’s Law Constants, H i, Determined by the Internal Standard Method for Some Nitriles
Compound
H i /103 5 °C
10 °C
15 °C
20 °C
25 °C
Acetonitrile, CH3CN
0.32 ± 0.03
0.38 ± 0.03
0.52 ± 0.03
0.65 ± 0.03
0.79 ± 0.04
Propionitrile, CH3CH2CN
0.41 ± 0.04
0.53 ± 0.04
0.75 ± 0.04
0.95 ± 0.04
1.17 ± 0.05
Butyronitrile, CH3(CH2)2CNa
0.47 ± 0.04
0.62 ± 0.04
0.85 ± 0.03
1.14 ± 0.04
1.48 ± 0.05
Isobutyronitrile, (CH3)2CHCN
0.87 ± 0.08
1.16 ± 0.08
1.59 ± 0.07
2.08 ± 0.08
2.70 ± 0.10
Valeronitrile, CH3(CH2)3CN
0.49 ± 0.04
0.68 ± 0.05
0.92 ± 0.04
1.27 ± 0.05
1.65 ± 0.07
Isovaleronitrile, (CH3)2CHCH2CN
0.76 ± 0.07
1.03 ± 0.07
1.43 ± 0.06
1.90 ± 0.07
2.55 ± 0.09
Trimethylacetonitrile, (CH3)3CCN
2.28 ± 0.20
3.00 ± 0.20
4.18 ± 0.16
5.47 ± 0.20
7.27 ± 0.27
a
Data taken from ref 10.
the analytes. After addition of the dilute aqueous solutions, the vials were capped with silicone/poly(tetrafluoroethylene)-faced septa and holed screw caps (Supelco) and kept in an open water bath, of which the temperature was controlled by a BIO-RAD cooling module refrigerated water circulator. A 500 μL gas-tight, valve-locking microsyringe (model 500 R-V-GT, Scientific Glass Engineering) was used to withdraw the headspace samples and inject them into the GC. Various Eppendorf micropipets were used to prepare the solutions. Experimental Procedure Equal volumes of the alkyl nitriles, in which butyronitrile was used as the internal standard (10), were combined to make a mixture. The dilute standard solution was prepared by adding 2 μL of the mixture to approximately 25 mL of pentane. An appropriate quantity (5 μL for each nitrile) of the mixture was added to 250 mL of water to prepare the dilute aqueous solution. The aqueous concentrations were low enough for the nitriles to obey Henry’s law (1). Additionally, the interactions among the analytes should be insignificant and not affect the Hi values. Aliquots of the dilute aqueous solution were transferred into the sample vials, which were then placed in the water bath. The headspace-to-liquid volume ratios were maintained at less than 1. Sufficient time was allowed for the establishment of thermodynamic equilibrium in the vials. Both the headspace samples and the standard solution were analyzed by GC. The GC responses must be examined to be linear throughout the concentration range of the samples (11). Hazards General laboratory safety procedures, including the wearing of safety goggles and gloves, should be followed at all times. All organic chemicals involved in this experiment are considered hazardous. The alkyl nitriles are flammable and toxic liquids. Pentane is harmful by inhalation, ingestion, or skin absorption. It is an irritant and narcotic in high concentration. Results and Discussion The dimensionless Henry’s law constants, Hi, for butyronitrile at various temperatures have been recently reported by our research group (10). The Hi values for six low molar
970
Table 2. Dipole Moments of Selected Nitriles in Gas Phase Computed by Cerius2 Molecular Modeling Software
Compound
µ/D
Butyronitrile
3.17
Isobutyronitrile
2.93
Valeronitrile
3.55
Isovaleronitrile
3.28
Trimethylacetonitrile
3.09
mass, highly water soluble alkyl nitriles were measured by the students using butyronitrile as the internal standard. The experiments were conducted at five different temperatures and the results along with the corresponding standard deviations are summarized in Table 1. Not surprisingly, the Hi data for acetonitrile and propionitrile agree well with the literature values (10). There is a gradual increase in Hi for the homologous series of nitriles and this trend has been previously discussed and explained (12). A discrepancy in Hi values between butyronitrile and isobutyronitrile was observed and that could be qualitatively justified by the polarities of the two compounds as the more polar compound will have stronger interaction with water molecules and thus give lower Hi. Similar disparities can also be explained for valeronitriles. The theoretical dipole moments of the nitriles in gas phase calculated by Cerius2 molecular modeling software (version 4.10, Accelrys, Burlington, MA) are presented in Table 2. The temperature dependence of Hi follows the van’t Hoff relationship:
ln H i
A B T
(2)
The parameters A and B can be readily obtained by plotting ln Hi as the function of 1/T. The Hi values determined in this study were used by the students to create the van’t Hoff plots for all the nitriles (Figure 1) and linear regression analyses were
Journal of Chemical Education • Vol. 85 No. 7 July 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory trimethylacetonitrile isobutyronitrile isovaleronitrile valeronitrile butyronitrile propionitrile acetonitrile
5
6
ln Hi
also carried out. In all cases, the correlation coefficients (r) were greater than 0.99. The values of A and B along with the standard errors are given in Table 3. The experiment appeared to be attractive to the students because the results have applications in environmental chemistry. The students also found it easy to prepare the solutions and samples as the concentrations and volumes are not critical in the internal standard method. The project provides the learning opportunities for students to use the popular ChemStation software to control the GC and run data analysis. Moreover, the students can reinforce their knowledge of data treatment and evaluation by computing and carrying out standard deviations, creating graphs by using Excel or SigmaPlot, and generating linear regression lines with the corresponding equations.
7
8 3.3
3.4
3.5
3.6
3.7
(1/ T ) / (10ź3 Kź1) Figure 1. van’t Hoff plots for seven alkyl nitriles.
Acknowledgments Table 3. van’t Hoff Parameters for Alkyl Nitriles
The authors thank Gary Beall for the theoretical calculations of the dipole moments. This work was supported by the Research Enhancement Program at Texas State University–San Marcos.
Compound
Literature Cited 1. Mackay, D.; Shiu, W. Y. J. Phys. Chem. Ref. Data 1981, 10, 1175–1199. 2. Hansen, K. C.; Zhou, Z.; Yaws, C. L.; Aminabhavi, T. M. J. Chem. Educ. 1995, 72, 93–96. 3. Ramachandran, B. R.; Allen, J. M.; Halpern, A. M. J. Chem. Educ. 1996, 73, 1058–1061. 4. Rice, G. W. J. Chem. Educ. 1987, 64, 1055–1056. 5. Newirth, T. L.; Srouji, N. J. Chem. Educ. 1995, 72, 454–456. 6. Magee, J. A.; Herd, A. C. J. Chem. Educ. 1999, 76, 252. 7. Veening, H. J. Chem. Educ. 1966, 43, 319–320. 8. Peterson, J. J. Chem. Educ. 1992, 69, 843–845. 9. Clarke, D. W. J. Chem. Educ. 1997, 74, 1464–1465. 10. Ji, C.; Evans, E. M. Environ. Toxicol. Chem. 2007, 26, 231– 236. 11. Hansen, K. C.; Zhou, Z.; Yaws, C. L.; Aminabhavi, T. M. J. Chem. Eng. Data 1993, 38, 546–550. 12. Buttery, R. G.; Ling, L. C.; Guadagni, D. G. J. Agric. Food Chem. 1969, 17, 385–389.
A
B
Acetonitrile
–3888 ± 196
5.91 ± 0.68
Propionitrile
–4451 ± 192
8.21 ± 0.67
Butyronitrile
–4816 ± 73
9.64 ± 0.25
Isobutyronitrile
–4727 ± 59
9.95 ± 0.21
Valeronitrile
–5065 ± 65
10.59 ± 0.23
Isovaleronitrile
–5032 ± 47
10.90 ± 0.16
Trimethylacetonitrile
–4843 ± 86
11.32 ± 0.30
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jul/abs969.html Abstract and keywords Full text (PDF) with links to cited JCE articles Supplement
Student handouts
Instructor notes including the derivation of eq 1
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