G. Dana Brabson
US. Air Force Academy USAF Academy, Colorado 80840
NMR Analysis of Water-Acetic Acid Solutions
c a d e t s a t the Air Force Academy perform two nuclear magnetic resonance experiments during their instrumental analysis course. The first is designed to acquaint the student with the operation of the instrument, and with the significance of the chemical shift, the spin-spin splitting, and so forth. The second experiment, described below, is designed for a typical analytical nuclear magnetic resonance spectrometer such as the Varian A-60 and has proved to be one of the most popular experiments offered in the instrumental analysis course. This experiment has two specific objectives. First, it is intended to teach operation of the instrument and interpretation of the data obtained. Students are expected to perform all the required operations with the help of carefully written instructions. Second, it is designed to demonstrate a significant principle of chemistry. Theory
Because of the rapid proton exchange between water and acetic acid molecules, solutions of acetic acid in water yield only two nmr resonances; one resonance is due to the methyl protons in the acetic acid molecule while the other resonance represents the time averaged environment experienced by the protons attached to the oxygens in the water and acetic acid molecules.' Moreover, since the water and acetic acid molecules are ionized to only a slight extent, a linear relationship exists between the observed chemical shift of the OHresonance and the fraction of protons which exist in acetic acid molecules. If nl and nz are the number of moles of water and acetic acid, respectively, the fraction of protons that exist in acetic acid molecules, PI, is given by 2% Pz = nl
+ 2nn
This experiment fulfills two objectives. First, i t teaches operation of the instrument and interpretation Presented a t the Southwest Regional ACS Meeting, Austin, Texas, December 4-6, 1968. I GUTOWSKY, H. S., AND SAIXA,A,, J . Chem. Phya., 21, 1688 (1953).
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of the data obtained: students are ex~ectedto Derform all the required operations with the help of carefully written instructions. Second, it demonstrates the fact that the chemical shift of a labile proton is quite sensitive to the environment. Thus, the nmr resonance due to the labile protons in a compound such as an acid, an alcohol, an amine-hydrochloride, etc., can usually be quickly identified by comparing the spectrum of the pure compound with the spectrum of a sample to which a drop of acid has been added. Then the observed chemical shift, Bobs, is given by Sob.
= Pd6aano - 6a,o) f 6a,o
where 8aoAEis the chemical shift of the OH-proton in pure acetic acid and 8,,0 is the chemical shift of the protons in pure water. This linear relationship is not surprising since the average residence time of a proton in an acetic acid molecule, and hence the contribution to the average environment, is a linear function of the fraction of protons that exist in acetic acid molecules. Solutions to be Solution
MI glacial acetic acid
Pre~ored
M1 distilled water
Approximate
Pa
Experimental The student prepares ten solutions as specified in the table. Reagent grade glacial acetic acid is quite satisfactory for this purpose, and may be used without any special treatment. Since the methyl resonanoe (6 = 2.10 ppm) is unaffected by dilution of the acid with water, this resonance mskes a convenient reference and no other reference substance need be added to the solution. After mixing the solutions thoroughly, the student records the spectra of the ten solutions and of one unknown solution provided by the instructor. The eleven spectva can be conveniently placed on one piece of chart paper; if, for each different solution, the sweep offset is changed by a few Hz and
Typical chart recorder trocing for the unknown and 10 known solutionr of acetic acid and water.
the recorder zero is changed by a small amount, the spectra will not overlap.
Results
A typical chart recorder tracing has been reproduced in the figure; note that the ringing, which is quite excellent in all cases, has not been shown for the sake of simplicity. For each data pair, the observed chemical shift is computed as follows
Sob. is then plotted as a function of P,and the Method of
Least Squares is used to determine the best straight
line through the data points. Finally, the concentration of the unknown solution is determined from the graph of Soh, versus P,. I n general, the results are quite excellent; the experimentally determined value of ~HOA., for example, rarely deviates from the literature value by more than 0.05 ppm. The results illustrated in the figure are based on a particularly excellent set of data obtained by two cadets; the probe temperature was 40.5'C. The average deviation of the experimental data points from the Least Squares Line is 0.02 ppm. The value obtained for ~ E O A . is in excellent agreement with the literature value, viz: 6 = 11.372. Cadets work in pairs on this experiment, although a student could easily complete the experiment individually. Each cadet performs the required calculations independently and submits his own laboratory report. The experimental portion of the experiment is easily completed in two hours. Acknowledgment
The author is indebted to Mr. J. Lloyd Pflug who maintains our instrument in peak operating condition and capably assists the cadets when they have difficulties, and to Second Lieutenants R. 0.Rasor and J. J. Tobolski who, while cadets, obtained the data reproduced here. r BHACCA, N. S., JOHNSON, L. F.,A N D SHOOLERY, J. N., "High Resolution Nuclear Magnetic Resonance Spectra Catalog," Varian Associates, Pda Alto, 1962.
Volume 46, Number I I, November 1969
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