In the Laboratory
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Using Nuclear Magnetic Resonance Spectroscopy for Measuring Ternary Phase Diagrams Jennifer K. Woodworth, Jacob C. Terrance, and Markus M. Hoffmann* Department of Chemistry, State University of New York College at Brockport, Brockport, NY 14420; *
[email protected] General phase behavior and in particular ternary phase diagrams are a traditional basic topic in thermodynamics for the undergraduate curriculum of physical chemistry (1–10). While it is desirable to keep laboratory experiments of traditional thermodynamic content in the corresponding laboratory curriculum, this is becoming increasingly difficult owing to the growing need to additionally incorporate experiments that employ modern experimental techniques, especially in the area of spectroscopy. Two noteworthy efforts to keep the ternary phase diagram experiment as a viable laboratory experiment have recently appeared. Udale and Wells replaced the standard ternary system of water, acetic acid, and dichloroethane with the less hazardous ternary system of water, n-heptane, and 1-propanol (2). Karukstis et al. used both absorption and fluorescence probes to obtain tie line compositions using UV–vis and fluorescence spectroscopy (1). Sparked by the idea of Karukstis et al. to employ modern spectroscopic techniques for a classic thermodynamics experiment, we modified the ternary phase diagram experiment for the water, n-heptane, 1-propanol system to incorporate nuclear magnetic resonance (NMR) spectroscopy. NMR spectroscopy is arguably the primary analytical tool for chemists, and it is therefore highly desirable to incorporate NMR spectroscopy widely into the undergraduate curriculum (11). In the laboratory, students may first obtain several data points of the phase equilibrium line by the usual titration method (2). As a minor modification, instead of starting with a single-phase mixture of two components and adding the third component until cloudiness is observed, we let our students begin with preassigned compositions close to the phase boundary. If the student observes two phases then more 1propanol is added until the phase boundary, that is, cloudiness, disappears, and if a single phase is observed, more water or n-heptane is added until cloudiness appears. With regard to sample preparation for tie line measurements using 1H NMR, samples need to be well mixed before they are allowed to settle and reach phase equilibrium. Samples from the top and bottom layer are then drawn using a general purpose syringe with a blunt end 20-gauge stainless steel needle and transferred into capillary tubes typically used for melting point determination in the organic chemistry laboratory. W The sampling size is small compared to the overall sample size: a few tenths of a milliliter are drawn from a two-phase mixture of about 3-mL total volume. It is advisable to draw first from the top layer to disturb the equilibrium as little as possible during sampling. The filled capillary tubes are then placed into NMR tubes that are filled with deuterated solvent. If so desired, one could omit the somewhat expensive deuterated solvent and acquire proton NMR spectra unlocked, as is the case for the proton NMR spectra shown in Figure 1. However, the presence of the deuterated solvent makes the sample a “normal” NMR sample suitable for uswww.JCE.DivCHED.org
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ing an NMR autosampler setup, if available. This would allow convenient spectral acquisition outside of lab class time, which might be necessary for larger class sizes. The 1H NMR spectra obtained from the bottom and top layers of five samples labeled A–E are shown in Figure 1. The analysis of the NMR spectra, undertaken outside the laboratory hours, actually touches on a number of basic concepts. Students need to first assign all spectral peaks as shown in Figure 2, which reinforces the chemical shift concepts they
Figure 1. 1H NMR spectra of biphasic samples A–E of 1-propanol/ water/n-heptane ternary system: (left) from the water-rich bottom layer and (right) from the n-heptane-rich top layer. Peak assignments a–g are provided in Figure 2.
n-propanol
peak d H2 C
peak g H3C peak b water
peak a OH C H2 peak c H
H O
peak e H2 C
n-heptane H2 C peak f C H2
H3C
H2 C C H2
peak f CH3
Figure 2. Peak assignment for 1H NMR spectra of 1-propanol/water/n-heptane spectra shown in Figure 1.
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have learned in organic chemistry. In particular, the chemical shifts of water and the hydroxyl proton of 1-propanol are not constant because the chemical shifts for these protons are very sensitive to hydrogen bonding, with larger chemical shift values (less shielded) for increased hydrogen bonding (12,13). At high enough water and alcohol concentrations hydrogen exchange is fast and the water and OH-resonances may coalesce into one resonance (13, 14), as is the case for the bottom layer of sample E in Figure 1. For correct quantitative analysis of the NMR spectra, it is essential that students understand that the presence of multiple species gives rise to a spectrum that shows signals of all of the species present. Specifically, the intensities of the spectral signals are weighted by the concentration, that is, mole fractions of each species in the solution, as well as the number of chemically equivalent protons. The mole fractions are thus obtained from the integrated intensities of the peaks. This is not necessarily obvious to the inexperienced student. Students may further overlook the redundancy of spectral information. For example, 1-propanol displays three resonance signals each representing the same mole fraction of 1propanol in the solution. Students may also have difficulties with converting the mole fractions to mass fractions, even though this is a straightforward manipulation. To confirm that their analysis scheme is correct, students analyze an NMR spectrum obtained from a sample of known composition, that is, from the single-phase region of the ternary phase diagram. Students manipulate the raw NMR data starting from the free-induction-decay (FID), which includes phasing and baseline correction. Phasing and baseline correction both need to be done properly to obtain accurate peak integration, especially for spectral signals of low intensity. Finally, students may also take a spectrum of the 1-propanol stock solution to measure any water impurity present when calculating the overall sample composition. Some student generated results from the titration procedure (solid down-triangles) and the analysis of the NMR spectra (solid up-triangles) are shown in Figure 3. The n-heptane-rich tie line endpoint compositions are from the top layer and the water-rich tie line compositions are from the bottom layer of samples A–E. Agreement with literature data included in Figure 3 is reasonable. The overall compositions of the five samples A–E fall also reasonably well on the tie lines, thus confirming the validity of the NMR method. Hazards 1-Propanol and n-heptane are both flammable and irritants. Chloroform-d is a cancer suspect agent and a mutagen. Although the experiment only uses small quantities of these substances, they should be handled in a hood. Any syringe needles used should have blunt ends, minimizing risk of inadvertent piercing through skin. Acknowledgments Supported by the National Science Foundation (DUE0408617), this report is an outcome of undergraduate labo-
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Figure 3. Ternary phase diagram of the 1-propanol/water/n-heptane system showing phase boundary measurement results from titration (solid down-triangles) and tie line measurement results from 1H NMR analysis (solid up-triangles) of samples A–E (solid circles). Literature values are included from ref 1 (open squares) and ref 2 (open circles).
ratory curriculum development to widely incorporate NMR spectroscopy. JCT was supported by the ACS SEED program. WSupplemental
Material
A handout for students and several notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Karukstis, K. K.; Avrantinis, S. K.; Boegeman, S. L.; Conner, J. N.; Hackman, B. M.; Lindsay, J. M.; Mandel, A. L.; Miller, E. J. J. Chem. Educ. 2000, 77, 701–703. 2. Udale, B. A.; Wells, J. D. J. Chem. Educ. 1995, 72, 1106. 3. Combs, L. L.; Lynn, G. W. J. Chem. Educ. 1995, 72, 608– 609. 4. Stead, R. J.; Stead, K. J. Chem. Educ. 1990, 67, 385. 5. MacCarthy, P. J. Chem. Educ. 1986, 63, 40–42. 6. Clare, B. W.; Hefter, G. T.; Kloeden, P. E. J. Chem. Educ. 1985, 62, 690. 7. Splittgerber, A. G.; Hansen, J. C.; Brue, M. D. J. Chem. Educ. 1985, 62, 382. 8. MacCarthy, P. 1983, 60, 922–928. 9. Clarke, J. R. J. Chem. Educ. 1974, 51, 255–256. 10. Francis, A. W.; Smith, N. O. J. Chem. Educ. 1969, 46, 815– 820. 11. Davis, D. S.; Moore, D. E. J. Chem. Educ. 1999, 76, 1617– 1618. 12. Hoffmann, M. M.; Conradi, M. S. J. Am. Chem. Soc. 1997, 119, 3811–3817. 13. Silverstein, R. M.; Webster, F. X. Spectroscopic Identification of Organic Compounds, 6th ed.; Wiley: Hoboken, New Jersey, 1998; pp 163–168. 14. Hoffmann, M. M.; Conradi, M. S. J. Supercrit. Fluids 1998, 14, 31–40.
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