A QuantitativeGas Chromatographic Ethanol Determination A Contemporary Analytical Experiment James J. Leary James Madison University, Harrisonburg, VA 22807
This note describes a gas chromatographic experiment for the quantitative determination of volume wercent ethanol in water ethanol solutions. Since its introdukion in 1974, approximately 40 analytical chemistry students per year have successfully performed this experiment. The samples studied contain only two components, ethanol and water. For this reason, a simple calihration curve is very well suited for the accurate determination of volume percent ethanol over a wide concentration range. Experiments illustrating the use of other methods for quantitative gas chromatographic determination have also recently been described: internal standard plus calibration curve ( I ) , standard addition plus corresponding plot (21, and relative response factor methods ( 3 , 4 ) .The accuracy and precision of the results of this experiment are both on the order of 2.0%. The precision with which students can measure the peak areas off the chromatograms is the limiting factor. Experimental
Four standards are prepared in 50-ml volumetric flasks by diswensinz 10.00. 20.00. 30.00, and 40.00 ml of ethanol with a h k e t and diluting to &lume'with water. Volume % ethanol = (volume ethanolltotal volume) * 100. Approximately 1WLof the standards and unknowns are infected into the chromatopra~h.Sarn~lesize and attenuation are varied so that all peaks are at least 40 mm high. Chart speed is adjusted such that the water peak is about 20 mm wide a t half height. The experiment has always been performed using 'I4 in. od. X % meter Porapak' T1 columns, 80-100 mesh, installed in Gow Mac gas chromatographs equipped with thermal conductivity detectors; however, any chromatograph with thermal conductivity detectors could he used.2 A calibration curve is constructed usine area ethnnoll1area ~ -,~ ~ ethanol + area water) as the dependent variable and volume percent ethanol as the independent variable. In addition to the experimentally determined points, the calihration curve should include the known noints (0.0) . . and 1100.1). . . . The volume percent ethanol in the unknowns is determined via the calibration curve. (Unknowns are prepared in the same manner as the standards.) Students are told that n roof is by definition two times the volume percent ethanol at 60°F (5).They are also told that real world samples should readily suggest themselves, and that they are welcome to analyze their own samples of alcoholic beverages and compare their results with the concentrations of ethanol given onthe labels. ~
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Results and Discussion
VII I l t h l l l l
Area ethanollWea ethanol
+ area water) versus volume percent ethanol
seeing straight line calibration curves. There are at least two reasons for the curvature. First, a thermal conductivity detector is a mass sensitive device; therefore, ratio of peak areas becomes approximately proportional to weight percent ethanol. In general, weight percents and volume percents are not linearly related. Second, a curve m i ~ hhe t anticiwated because any concentrated solutions, volumes are not additive, meaning that volume (total) f volume (ethanol) + volume (water). A final point involves an extension of this experiment in which students have used least squares computer programs to fit the data associated with this experiment. Because the slope of the curve is changing slowly, and because there are no inflection points, a second order polynomial of the form y = a*. h r 2 is a very acceptable model for the curve. No y intercept term is necessary because the curve must pass through the point (0,O). In the case of the data presented in the figure, the resulting line is given by the following equation, ~ ~ ~ ~ .
+
where y is the area ratio and n is the volume percent ethanol. Acknowledgment
The author wishes to acknowledge R. C. Atkins for providing the instrument operating instructions used a t JMU; T. G. Leahy, superintendent, A. Smith, Bowman Distillery, Sunset Hills, VA, for much helpful advice concerning methods for the preparation of standards and alternate methods for the determination of volume percents; and F. A. Palocsay, with whom I co-authored a meeting presentation ( 6 ) on a virtually identical take-off experiment applicable at the freshman level.
The figure illustrates a typical calihration curve. The fact Literature Cited
iz) van~ t t * R., E. and van~
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Porapak" is a registered trademark of Waters Associates, Inc., Maole Street. Milford. MA. ? lnsrr~ments equpped wjlh flame#on,?allondetectors cannot oe "SW oecau5e F D s 00 not respond to walcr
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t tR.~L.,, J. CHEME ~ ~57,230 ~ . (1980). . (31 Karasek,F. W..DeDecker, E. H.,Tiernay.J. M . , J CHEM. EDUC.51.816 (1974). 14) Pacer. R.A,. J. CHEM.EDUC.. 53,582 (1976). 15) Hawley, Gessner C., "The Condensed Chemical Dietionsrv: 8th ed.., Van Nostrand ReinholdCo.,New York. 1971, p. 732. (6) Pelocsa%F.A.,sndLesm, J. J.,psper2d7Soufbeast-Southwe~tRegj jdAcSMeeting, Memphis,TN,
1975.
Volume 60 Number 8 August 1983
675
