Fritz 5. Allen, William F. Coleman, Gary J. Morrow, and Thomas M. Niemczyk University of New Mexico Albuquerque, 87131
Using NMR to Determine the Boiling Point Diagram for a Non-Ideal Solution
The determination of a boiling point diagram is a classical physical chemistry experiment.1-3 The usual procedures-for determining the mole fractions necessary to prepare the diagram involve measurement of some physical property such as the index of refraction or density with subsequent reference to a calibration curve, or a titration procedure followed by calculation. These procedures are either excessively time consuming or require that computations be performed after the conclusion of the lab. We have devised an experiment which uses the nmr spectrometer for the rapid determinations of mole fraction. Two important advantages are gained by this technique. (1) thus ~ -There -, - ~ -~ is no calibration curve to be ~reoared. . . freeing more time for experiment. If a calibration curve is necessarv for the determination of mole fraction, we feel the student should determine the curve to avoid' systematic errors in instrumentation, sample preparation, etc. which can become important if some standardized curve is used. (2) The rapid determination of mole fractions means that the student can plot the boiling point diagram early in the first lab period and use it for subsequent observations. By employing the boiling point diagram as a tool, we feel we have developed an experiment which leads to real insight into the meaning of the plate concept and the operation of a fractionation column. At the same time the students are made aware of a quantitative use of nmr and develop a better understanding of non-ideality. In our integrated lab course we have used the EM-360 nmr spectrometer for this experiment. It should also be possible to do this experiment using any medium resolution nmr spectrometer. ~
Experimental
The experiment developed has three phases: (I) The determination of the boiling point diagram, (11) The simulation of the operation of a 3-4 plate column by hand, and (111) The determination of the number of theoretical plates in a fractionation column. The spectrometer is used in each of the three parts. To facilitate filling the tubes for the nmr spectrometer we have designed a special boiling flask to be used in a manner similar to that of footnote 1 (See Fig. 1).The liquid sample is introduced into the flask and boiled by the electric element. The vapor is condensed and the condensate is returned to the pot through the three-way stopcock, until equilibrium is attained (3-5 m i d . The stopcock is then turned and a few drops of the distillate are collected directly in the nmr tube. The tube is rapidly capped to prevent evaporation. After the samples are collected and the nmr spectra determined, fractions can be calculated by considering the
Figure 1. Special sampling flask. This flask is designed to be used in the manner of footnote 1. The stopc~ckassembly greatly simplifies the filling of thenmr tubes.
areas under the resonance peaks. In this experiment we have chosen to study the chloroform-acetone system. Both of these liquids have but one type of proton so there is only one resonance for each component. Although the EM-360 has an integrator for determining the areas, we have found that its use is not required because a simple peak height rule gives the necessary accuracy. To determine a mole fraction from a spectrum, the height of the chloroform peak, Hc, and the acetone peak, Ha, are measured. These are then normalized by the number of nmtons resonatine in each
and where the barred quantities indicate the height per proton. Since the peaks for chloroform and acetone are found to have essentially the same width, the mole fractions X, follow directly as -
X,
=
H.
H:.+ H . -
amd
As both the nmr instrument and the boiling points are quite sensitive to impurities we have used Spectrograde solvents in the experiment. The following steps are used to determine the boiling point diagram 1) Prepare by weighing, a solution of chloroform and acetone of
known male fraction. 2) Check the mole fraction by nmr and verify the peak height
rule. 'Daniels, F., Williams, J. W., Bender, P., Alberty, R. A., Comwell, C. D., and Harriman, J. E., "Experimental Physical Chemistry," Seventh Ed., MeGraw-Hill, Book Ca., New Yark, 1970, p. 61. 2Shoernaker, D. P., and Garland, C. W., "Experiments in Physical Chemistry," McGraw-Hill Book Co., New York, 1967 p. 166. %Jasper, J. J., "Laboratory Methods of Physical Chemistry," Haughton Mifnin Company, Boston, Mass., 1947, p. 100. 132
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3) Volumetrieally prepare eleven 50-ml samples with 0.1 mole
fraction intervals. 4) Record the nmr of the samples prepared in step (3). 5) Boil each sample prepared in (3) until a constant boiling tem-
perature is obtained. Record this temperature. 6) Collect a sample of the condensate in a clean, dry nmr tube
and record its spectrum. 7) Determine the male fractions and prepare the boiling point diagram by plotting the mole fraction of acetone in the liquid
and vapor condensate as a function of the hoiling temperature ofthe liquid. The boiling point diagram should he completed before proceedinz to the second portion of the experiment. - In the second phase o i the procedure-the student manually performs a multi-plate distillation. By modifying the p o t ~ ~ o m ~ o s i t to i o ncoGform to the mole fractions oc the previous condensate, the horizontal equilihrium relationships of the boiling point diagram become apparent. This method also demonstrates the process by which fractionation columns operate and the significance of azeotrope in a distillation. The manual distillation is performed as follows 1) Begin with a 50-ml sample on the CHC1.-rich side of the azeo-
--
trope, e.g., Xc 0.75.
2) Accurately determine the mole fractions by nmr. 3) Boil the liquid sample and collect a sample of the condensate. 4) Determine the mole fractions present in the condensate by
nmr. to the oot to eive 5) Add enoueh of the more volatile camnansnt . it thecomp,,s~tiundeterminrdinstep (4,. 6 ) Repeat steps 131, (41, and ~5 several tune5 and each lime plot on the boiling point d~agramthe pointa rorrespunding to lhr liquid and vapor composition.
MOLE
F R A C T I O N CHC13
~n~
The plots which result give the familiar stairstep pattern in the liquid-vapor equilihrium region of the phase diagram. In the last phase of the experiment, the student sets up a n ordinary 'fractionation apparatus using a column packed with steel wool, ceramic saddles, or some other material. Beginning with the same sample as in part I1 the student then proceeds a s follows 1) Boil the liquid and collect the distillate. 2) Record the nmr and calculate the mole fractions. 3) Determine the number of plates in the column from the boiling
point diagram. The experiment is facilitated if the student makes up a large amount of one of the CHCIJ rich fractions in step (I), 3 g. X c = 0.7. This mixture can then be used in the second and third parts without further determinationof mole fraction. Discussion
The experiment has been performed a number of times and typical student data are shown in Figure 2. The data were taken in Albuquerque, New Mexico where atomospheric pressure is considerably less than 1 atm (-630 t o n ) and consequently all of the liquids have low hoiling points. This is reflected in Figure 2 although it.has no important effect upon the understanding of the principles involved. By using the nmr spectrometer a s a means for rapid, accurate, and simple determination of mole fraction we feel we have produced an experiment in which the data collection technique is very basic in concept and introduces the students to a quantitative use of nmr, a technique which they have previously seen applied only a s a n aid to structural determination. As a result of the uncom-
Figure 2. Boiling point data. Typical student data far the acetone-chlorosystem. The circles denote the vapor composition curve, the triangles denote the liquid composition curve, and the square indicates the azeotrope. form
nlicated nature of the data collection mocess. the stu-dents are able to focus their attention on the fundamentals of the non-ideal liquid-vapor equilihrium. Perhaps the most important feature of this experiment is that the hoiling point diagram determined in part I is used to explain observations in the subsequent parts. There are numerous features which can he added to this experiment. Students can he encouraged to study the azeotrope itself and verify that the liquid and vapor have the same composition. They can determine the composition of a liquid unknown by finding the composition of the equilihrium vapor. They can study a minimum hoiling azeotrope such a s the chloroform-methanol system and ohserve that the azeotrope is now in the distillate instead of the pot. Liquids with similar refractive index or other properties could he studied since coincidence of all peaks in nmr soectra is hiehlv- unlikelv. Possihlv. .. some students could devise computer procedures for the determination of activities and activity coefficients. Clearlv, there are many aspects which co"ld benefit from furtheistudy. In the procedure proposed there is minimal room for error. Mole fractions can quite easily be determined from nmr spectra to f2% and hoiling points are easily obtained to this accuracy. These are the only quantities measured. Errors can appear if the nmr tubes are not capped immediately or if the atmospheric pressure varies significantly during the course of the experiment.
-
Acknowledgment
We are indebted to Miss Judy Perea and Miss Sara Shelly for their work on the development of this experiment. We are also grateful for the financial aid from Project Seed of the American Chemical Society.
Volume 52, Number 2, February 1975
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