Laboratory Experiment pubs.acs.org/jchemeduc
Determination of Ethanol in Gasoline by FT-IR Spectroscopy Alfred Conklin, Jr.,* Michael J. Goldcamp, and Jacob Barrett Department of Chemistry, Wilmington College, Wilmington, Ohio 45177, United States S Supporting Information *
ABSTRACT: Ethanol is the primary oxygenate in gasoline in the United States. Gasoline containing various percentages of ethanol is readily available in the market place. A laboratory experiment has been developed in which the percentage of ethanol in hexanes can easily be determined using the O−H and alkane C−H absorptions in an infrared spectrum. Standard solutions of ethanol mixed with hexanes are prepared, and their infrared spectra collected. The areas under the O−H and C−H absorptions are used to prepare a standard curve, which is used to determine the percentage of ethanol. Results for the analysis of ethanol in gasoline from this model ethanol/hexanes calibration system compare favorably to advertised values, as well as to values determined by water extraction.
KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Alcohols, Alkanes/Cycloalkanes, IR Spectroscopy, Quantitative Analysis
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spectroscopic data to the associated features in molecular structures.
reviously both ethanol and MTBE (methyl t-butyl ether) have been used as oxygenates, and thus, absorptions in the 1000−800 cm−1 region of the infrared (IR) spectrum have been used to determine the alkane, alcohol, and ether components of gasoline.1,2 This method is no longer utilized because MTBE has been phased out as an additive to gasoline. Currently, ethanol is the prime oxygenate in gasoline and is produced from ethene, grain (primarily corn), and as a byproduct of beer production.3,4 The most common ethanol gasoline mixture contains 10% ethanol by volume (E10). However, other blends ranging from E5 to E85 are currently under investigation.5 The availability of E85 is generally restricted to areas surrounding ethanol-manufacturing plants. The alcohol content of gasoline can be measured by a number of analytical methods including gas chromatography and NMR, Raman, and infrared spectroscopy.6−10 Because water is not soluble in the mixture of alkanes that constitutes the bulk of gasoline, the O−H absorption in the infrared spectrum of gasoline is directly related to the amount of ethanol it contains. Thus, the O−H absorption in the 3500−3000 cm−1 region and the alkane C−H absorptions in the 3000−2800 cm−1 region can be used to quantitatively determine amount of ethanol in gasoline. Infrared spectroscopy is a common instrumental technique that is taught in many general, organic, and analytical chemistry courses and is routinely used in many chemical analyses for both quantitative and qualitative purposes. In this experiment, students analyze commercially available gasoline for ethanol content using IR spectroscopy. It can be used to introduce students to or enhance students’ experiences with instrumental methods of quantitative analysis, including the generation and usage of calibration curves. Additionally, the experiment can be used to introduce or reinforce the connection of infrared © 2014 American Chemical Society and Division of Chemical Education, Inc.
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EXPERIMENTAL SECTION
Materials and Instrumentation
Ethanol (as “reagent alcohol”: contains 90.25% ethanol, 4.75% methanol, and 5.00% 2-propanol) and hexanes were obtained from Pharmco AAPER. Solutions of 0.0%, 1.0%, 3.0%, 5.0%, 10%, 30%, 50%, and 85% ethanol in hexanes were prepared by volume (e.g., 5.0% = 0.50 mL ethanol and 9.50 mL hexanes). FT-IR spectra were obtained on a Thermo Scientific Nicolet iS10 infrared spectrometer with an AVATAR Multi-Bounce HATR sampler or a Nicolet 210 infrared spectrometer with an ATR sampler. Both FT-IR spectrometers were operated using the Thermo Scientific Nicolet OMNIC software package supplied with the spectrometers. Spectral data were collected in absorbance mode for quantitative determination of peak areas. Calibration Standards
FT-IR spectra of the eight ethanol/hexanes standards were collected. The area under the O−H absorption (from about 3660 to 3045 cm−1) and area under the combined alkane C−H absorptions (from about 3045 to 2760 cm−1) in the infrared spectrum (Figure 1) were determined using the spectrometer’s software. Care was taken to make sure that the defined regions for each peak and the baseline correction by the software were kept consistent for all standards and samples measured. The ratio of the area of the O−H absorption to the area under the combined alkane C−H absorptions was calculated; thus, the Published: April 24, 2014 889
dx.doi.org/10.1021/ed400824g | J. Chem. Educ. 2014, 91, 889−891
Journal of Chemical Education
Laboratory Experiment
Execution
This experiment has been employed at Wilmington College in the general education Chemistry and the Environment course. It is part of a series of laboratory experiments exploring ethanol as a fuel source; this series includes a preceding multiweek experiment involving the conversion of corn to ethanol. A laboratory section totaling about 20 students typically work in small groups (3−4 students) to generate the data for the calibration curve; each student analyzes his or her own gasoline sample. With two FT-IR spectrometers, a 20-student laboratory class usually finishes the entire spectroscopy experiment within about 1.5 h. If only one spectrometer is available, 2−3 h is more typical. We have also used this experiment as part of an instrumental analysis workshop for visiting advanced high school students, in which 8−10 students (working in groups of 2−3) perform this experiment and two other separate instrumental experiments in about 2−3 h.
Figure 1. A representative FT-IR spectrum (absorbance mode) of a standard solution of ethanol (25%) in hexanes.
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signal produced by the alkane portion of the gasoline is treated as a quasi-internal standard. A standard curve of percent ethanol versus the ratio of the areas was then produced for quantitative analysis (Figure 2). The calibration curves that are produced are linear, with typical R2 values of 0.99.
HAZARDS
Hexanes, ethanol, and gasoline are volatile and flammable liquids and are toxins and irritants. All sample handling and preparation should be performed in a fume hood. A cover should be put on the ATR sample plate when spectra are being collected. Waste should be placed in an appropriate container for disposal.
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RESULTS Students in a nonmajors, general education course have generated results fairly consistent with the expected results for commercially available gasoline, with some inaccuracies. The average value for the percentage of ethanol in gasoline generated by a class of approximately 15 students was 14%, with most values ranging from 12−18%.
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DISCUSSION FT-IR methods for the analysis of MTBE in gasoline have been reported previously in this Journal.2 However, MTBE has been phased out as an oxygenate additive to gasoline, making these experiments less practical in the present-day educational laboratory. As ethanol has become increasingly the most important oxygenate in present-day gasoline, experiments for its analysis are more relevant. Several reports in the literature indicate that current ethanol-gasoline blends may have environmental, safety, and health concerns.11,12 Thus, facile methods for the determination of the ethanol content in these fuels are valuable. The commercially available mixture of hexanes was chosen for the preparation of standard solutions in this work because it is a major component of gasoline and is representative of the mixture of alkanes found in gasoline, which are typically between five and 12 carbons. Other common alkanes, such as pentane or heptane, would also work in place of hexanes. Hexane has been successfully utilized in previous work in the analysis of MTBE in gasoline by FT-IR spectroscopy.2 Hexane or mixtures of hexanes are also readily available to most academic institutions at low cost with few outstanding chemical hazards. Additionally, gasoline with no ethanol content is difficult to locate in many areas, and removing the ethanol from gasoline (typically by water extraction, following by drying) for the purposes of preparing standards exposes students and/or instructors to additional hazards (as it contains a variety of
Figure 2. A representative standard calibration curve for ethanol in hexanes.
The percentage ethanol in the gasoline sample can then be calculated by the equation: % Ethanol = [(Area of O−H peak)/(Area of C−H peaks) − (y‐intercept)]/Slope
(1)
Determination of Ethanol in Gasoline
A gasoline sample was obtained from a gasoline station available to the public in Wilmington, Ohio. This location indicated that the gasoline may contain up to 10% ethanol. An FT-IR spectrum of the sample was obtained, and the areas of the O−H and C−H absorptions were recorded. The ratio of the areas was then calculated, and this result was used to calculate the ethanol percentage using the equation from the ethanol/hexane standard calibration curve. The average determined percentage for ethanol in this particular sample of gasoline was 10%, which is consistent with the advertised concentration. 890
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Laboratory Experiment
commercially available gasoline, with common sources of error having been identified. Student responses to the experiment are generally mixed. Independent of the actual analytical results, the experiment serves to educate students about the chemistry of fuels such as gasoline that are used in their everyday lives. The diverse population of students in a nonmajors course has a varied knowledge of the chemistry of gasoline prior to the experiment, with some having little idea of the chemical composition of gasoline. Upon completion of the experiment, students have a greater understanding of the chemistry of modern gasoline.
aromatic hydrocarbons) and consumes laboratory and preparation time. The ethanol source used is typical laboratory reagent grade ethanol, which is denatured with methanol and 2-propanol. These other alcohols, as well as the use of hexanes as a gasoline substitute, contribute to small systematic errors in the analysis. However, for educational purposes, this minor effect is offset by the benefits of practicality and safety. Additionally, while the alkane portion of the standards and the gasoline samples is treated as an internal standard for data analysis, this is not strictly true due to the contributions to the C−H absorption region by the alcohol, itself. However, the degree of error introduced by this approximation is relatively low, especially at lower ethanol concentrations such as those in typical E10 samples. At these lower ethanol concentrations, the signal contributed to the C−H region of the spectrum by ethanol is relatively small. Analyses of several calibration data sets has shown that the limit of detection for ethanol in gasoline is somewhere near 3%. In Figure 2, the signal ratio produced by the 1% sample is indistinguishable from that of just hexanes (0% ethanol). To verify the relative accuracy of this IR method, additional data were obtained for comparison. The percentage (by mass) of ethanol in the same gasoline was obtained by weighing a sample of the gasoline, extracting the ethanol with water, and then reweighing the sample of gasoline afterward. On the basis of the mass difference, an ethanol percentage of about 10.5% by mass was determined. For comparison, a solution of 10.5% ethanol in hexanes by mass is about 8.9% ethanol by volume. This result is comparable to the result as determined by the IR spectroscopic method (10% by volume). It is also notable that the ethanol percentage as determined by this extraction method may be slightly lower than the actual value, given that the FTIR spectrum of the gasoline after extraction showed that a trace amount of ethanol may remain dissolved in the gasoline (although the vast majority is removed). The average value for the percentage of ethanol in gasoline generated by a class of approximately 15 students was 14%, with most values ranging from 12−18%. From the examination of their calibration curves, many students appeared to have varying levels of difficulty preparing their standard solutions properly, as many of these curves had one or more data points clearly off the linear fit. In the hands of more experienced students, more accurate data and results have been obtained. Additionally, from these and other data and results, we have noted that care must be taken to be consistent in defining the region for each peak and the baseline correction applied by the spectrometer’s software in order to obtain accurate results at lower ethanol concentrations.
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ASSOCIATED CONTENT
S Supporting Information *
A document containing experimental instructions, tables for recording data, instructions for the calculation of results, and notes for instructors. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding for this work was provided by Wilmington College. REFERENCES
(1) Iob, A.; Buenafe, R.; Abbas, N. M. Determination of oxygenates in gasoline by FTIR. Fuel 1998, 77, 1861−1864. (2) Gebel, M. E.; Kaleuati, M. A.; Finlayson-Pitts, B. J. Measurement of Organics Using Three FTIR Techniques: Absorption, Attenuated Total Reflectance, and Diffuse Reflectance. J. Chem. Educ. 2003, 80, 672−675. (3) Maslowskye, E. Ethanol From CornOne Route to Gasohol. J. Chem. Educ. 1983, 60, 752. (4) Oliver, W. R.; Kempton, R. J.; Conner, H. A. The Production of Ethanol From Grain. J. Chem. Educ. 1982, 59, 49−52. (5) U.S. Department of Energy, Alternative Fuels Data Center. http://www.afdc.energy.gov/fuels/ethanol_blends.html (accessed Apr 2014). (6) Sanford, C. L.; Mantooth, B. A.; Jones, B. T. Determination of Ethanol in Alcohol Samples using Modular Raman Spectrometer. J. Chem. Educ. 2001, 78, 1221−1225. (7) Leary, J. J. A Quantitative Gas Chromatographic Ethanol DeterminationA Contemporary Analytical Experiment. J. Chem. Educ. 1983, 60, 675. (8) Renzoni, G. E.; Shankland, E. G.; Gaines, J. A.; Callis, J. B. Determination of Alcohols in Gasoline/Alcohol Blends by Nuclear Magnetic Resonance Spectrometry. Anal. Chem. 1985, 57, 2864−2867. (9) Standard Test Method for Determination of MTBE, ETBE, TAME, DIPE, Methanol, Ethanol and tert-Butanol in Gasoline by Infrared Spectroscopy, ASTM D 5845-01; ASTM International: West Conshohocken, PA. (10) Brazdil, L. C. A Versatile Experiment Using Gas Chromatography. J. Chem. Educ. 1996, 73, 1056−1058. (11) Hubbard, C. P.; Anderson, J. E.; Wallington, T. J. Ethanol and Air Quality: Influence of Fuel Ethanol Content on Emissions and Fuel Economy of Flexible Fuel Vehicles. Environ. Sci. Technol. 2014, 48, 861−867. (12) Ma, J.; Luo, H.; DeVaull, G. E.; Rixey, W. G.; Alvarez, P. J. J. Numerical Model Investigation for Potential Methane Explosion and Benzene Vapor Intrusion Associated with High-Ethanol Blend Releases. Environ. Sci. Technol. 2014, 48, 474−481.
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CONCLUSION FT-IR spectroscopy can be used to determine the ethanol content in gasoline using the O−H absorption of ethanol and the alkane C−H absorptions of ethanol/hexane mixtures and gasoline samples. This exercise teaches students about the use of FT-IR for quantitative analysis by constructing a calibration curve from standards and applying it to the calculation of the amount of an analyte in an unknown sample. It is used to introduce and reinforce fundamental ideas about molecular structure and the interpretation of IR spectra of organic compounds, specifically in regards to the assignments of the O−H and C−H adsorptions in the spectrum. Student results are generally close to the expected amount of ethanol in most 891
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