In the Laboratory
Acetone and Ethyl Acetate in Commercial Nail Polish Removers: W A Quantitative NMR Experiment Using an Internal Standard David W. Clarke Department of Chemistry and Physics, Northwestern State University, Natchitoches, LA 71497 While the use of proton nuclear magnetic resonance (1H NMR) to elucidate the structure of organic compounds is widespread, its potential as a quantitative technique is largely ignored in instrumental analysis textbooks. Several experiments for use with continuous wave instruments that demonstrate the ease with which it can be used quantitatively have appeared in this Journal (1–5), but only one applies the technique to a commercial product (1). This experiment provides an opportunity for students to use an NMR spectrometer and an internal standard to determine the concentration of the primary ingredient in commercial nail polish removers. The major ingredients in the two nail polish removers used in this experiment, as listed on the bottle, are acetone/water and ethyl acetate/alcohol (the identity of the alcohol is not listed). The unique feature of NMR that allows its straightforward application to quantitative problems is the signal intensity’s direct proportionality to the number of nuclei producing the signal. Unlike in other spectroscopies (IR and UV-vis), this is true regardless of the molecule containing the nuclei. An internal standard can be substituted for calibration curves that require analyzing a series of samples containing authentic material. The use of the internal standard is further simplified because both the standard and the analyte generate a signal proportional only to the number of protons present (number of molecules present multiplied by number of protons on each molecule). There is no need to determine detector response factors, as is necessary when using internal standards with other instrumental methods (like GC and HPLC).
that the internal standard and analyte peaks be of approximately the same intensity. This requires that students choose an analyte peak to integrate and take into account the number of protons responsible for its production. After recording the NMR spectra and performing the necessary integrations, the amount of analyte in the standards can be readily determined.
Analysis of the Nail Polish Removers If recoveries for the standards are satisfactory (~100%), analysis of the nail polish removers follows. To maintain equal analyte and standard peak intensities, the amount of nail polish remover used must be increased because the removers are not pure acetone or pure ethyl acetate. By examining the ingredient list on the bottle, students are able to deduce that the analyte probably represents at least 50% of the remover. After adjusting the amount of nail polish remover by this factor, analysis of the samples is carried out and the amount of analyte is calculated.
Experimental Procedure The experiment is divided into three basic parts: acquisition of known spectra and selection of an internal standard, preparation and analysis of standard solutions, and analysis of the nail polish removers.
Figure 1. NMR spectrum of acetone nail polish remover with CH2Cl 2 as an internal standard.
Acquisition of Known Spectra and Selection of Internal Standard Students take spectra (a 60 MHz continuous wave instrument is sufficient) of ~20% solutions of acetone [δ 2.0, (s, 6H)], ethyl acetate [δ 1.2 (t, 3H), δ 1.9 (s, 3H), δ 4.0 (q, 2H)] and nail polish remover in deuterated acetone (acetone-d6). They make peak assignments and practice taking integrations. Methylene chloride is suggested as a possible internal standard and its desirability is discussed by the class. Students are then encouraged to develop their own procedure for the analysis. Preparation and Analysis of Standard Solutions The students test their proposed procedure by preparing solutions of known acetone and ethyl acetate concentration to analyze as though they were nail polish remover samples. To minimize errors in integration it is desirable W Supplementary materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/Issues/1997/Dec/ index.html .
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Figure 2. NMR spectra of (top) ethyl acetate nail polish remover with CH2Cl2 as an internal standard and (bottom) pure ethyl acetate.
Journal of Chemical Education • Vol. 74 No. 12 December 1997
In the Laboratory Table 1. Results Test Solution Amount (g) 0.487
Solution 100% acetone c
CH2Cl2a Acetone-d 6b (g) (g) 0.712
0.592
80% ethyl acetate d
0.242
0.717
0.607
Remover: acetone-based e
0.370 0.320 0.324 0.260 0.230
0.780 1.054 0.941 0.700 0.715
0.580 0.662 0.640 0.607 0.620
Remover: ethyl acetate–based e
Analyte Recovered (g)
(%)
0.469 0.480 0.469 0.188 0.186 0.299 0.265 0.270 0.122 0.107
96 98 96 97 96 81 83 83 43 46
aInternal
standard. b Solvent. cThree aliquots of the same sample were analyzed. The remainder of the solution was ethanol. Two aliquots of the same sample were analyzed. e These are not corrected for the percent recovery. If this were done the values would be slightly higher. d
Results and Discussion For one spectrometer and a class of six students (with no NMR background), the full experiment required two laboratory periods of 3 hours each. Even with the limited number of trials performed, the high recoveries (96–98%) for the standards indicate that the method can be applied to the nail polish removers with relative confidence (Table 1; Figs. 1, 2). Comparison of the experimental concentrations of acetone (82%) and ethyl acetate (44%) with the actual values was not possible because the manufacturer declined to disclose those concentrations. The students seem to enjoy this experiment, as it involves the analysis of familiar analytes (acetone and ethyl acetate) in a familiar product (nail polish remover). The experiment
allows for both qualitative interpretation (peak assignments are made and the unknown alcohol is identified) and quantitative calculation (the amount of each analyte is calculated). The combination of these two aspects appears to give students a much better understanding of the concept of NMR and its potential uses. Literature Cited 1. Markow, P. G.; Cramer, J. A. J. Chem. Educ. 1983, 60, 1078– 1079. 2. Peterson, J. J. Chem. Educ. 1992, 69, 843–845. 3. Phillips, J. S.; Leary, J. J. J. Chem. Educ. 1986, 63, 545–546. 4. Wallace, T. J. Chem. Educ. 1984, 61, 1074. 5. Peterson, T. H.; Bryan, J. H.; Keevil, T. A. J. Chem. Educ. 1993, 70, A96–A98.
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