A Ternary Phase Diagram for a Less Hazardous System Barbara A. Udale and John D. wells' La Trobe University, P.O. Box 199, Bendigo, Victoria 3550, Australia Ternarv "nhase . diaerams are used frequently in extractive metallurgy, materials science, and other areas of applied chemistry Many practical examples involve solid-liquid equilibria, but the principles can be illustrated more quickly in a laboratom exneriment with a nartiallv miscible liauid system. One of the most popular experiments, based on the acetic acid-chloroform-water system (141, is no longer acceptable in undergraduate programs becanse chloroform is a suspected human carcinogen2 (5).Furthermore, the laboratory work is somewhat mundane because the whole experiment is based on the results of titrations. In searching for a n alternative system we had several criteria in mind: the reagents should consititute a minimal health hazard: the assav should make use of a " orocedure . modern instrumental method; and only inexpensive materials and apparatus should be necessary. The n-propanol-n-heptane-watersystem meets the criteria admirably. The experiment described below follows the general procedure used ( 1 4 ) for the acetic acid-chloroform-water system, except that the compositions of pairs of phases i n equilibrium are obtained from gas chromatographic data. All mixtures are prepared by weight, and compositions are expressed a s percent by weight. The coexistence curve a t 25 "C is found first by titrating 20%, 40%, 60%, and 80% aqueous solutions of propanol with heptane, and then solutions of 20%, 40%, 60%, and 80% propanol in heptane with water. The end point of each titration is taken a s the first perceptible permanent cloudiness, indicating the appearance of a second phase in the mixture. The compositions of the mixtures a t the end points lie on the co&istence curve, which can be extended to the hentane-water axis bv use of the solubilities (7)of water in heptane (0.01%) and heptane in water (0.0002%). Next a set of tie lines is added to the phase diamam. Four mixtures in the two-phase region(propanoi:heptane:water 40:40:20. 35:35:30. 20:45:35, and 15:45:40) are equilibrated a t 25 "C with gentle mixing (an inclined turntable works well), and then samples of the separated phases taken for analysis. Propanol is measured by gas chromatography, with n-butanol as a n internal standard. Samples-and standards of 15%, 30%, 45%, and 60% propanol in acetone-are diluted 1in 10 with a solvent mixture containing 5% butanol, 10% heptane, 10% water, and 75% acetone-we use a Varian Aeroaanh Model 940 gas chromatopranh with a Carbowax 20 @coiumn and a flame ionization detector. The column is a t 50 "C, and both injector and detector are a t 150 'C. The propanol concentration is estimated from the ratios of the peak heights for propanol and butanol. Assumingthat the upper layer in each case is the heptane-rich phase, and that all the layers lie on the wexistence curve, the compositions of the pairs of phases can be plotted on the phase diagram. The tie lines should pass through the p i n t s that r e p resent the overall compositions of the o r i d mixtures. The phase diagram in the figure was obtained by this method. I t is similar to those for a large number of alcohol"
A
' To whom correspondence should be addressed.
The acetic acid-benzene-watersystem (6)suffersfrom the same problem, with respect to benzene (5).
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Journal of Chemical Education
heptone
weight percent
water
Phase diagram for the propanol-heptane-watersystem, showing exThe perimental points forthe coexistence curve (*) and tie lines (0). points near the mid-points of the tie lines represent the overall cornpositions of the two-phase mixtures. hydrocarbon-water systems studied by Washburn and coworkers (8,9) in the 1930's and 1940's. We avoided benzene and toluene because they are carcinogens (5)and selected propanol over lower alcohols because the chromatographic separation was better. The gas chromatographic assay requires only a basic instrument. The heptane and water added to the acetone solvent ensure that the gross compositions of the solutions iniected onto the column are similar, which serves to redice interference effects. Many variations are possihle-both to the components studied (other alcohols and hydrocarbons are obvious choices) and to the analytical method used for the compositions of the pairs of phases in equilibrium. Washburn and co-workers used a refractometric method (81, b u t i t is slower than the method described here. M i u r e s can be prepared (and dilutions made) by volume, but the data then must be converted to a weight basis by the use of densities. In particular, it is necessary to measure the densities of each of the eight layers in the second part of the experiment. The use of an inexpensive top-loading balance (10 mg sensitivity is adequate) eliminates the need for density measurements. Although we use volumetric burets for the titrations, and hence need to know the densities of water and heptane, it would not be difficult to substitute weight burets, thereby eliminating all volumetric measurements. Literature Cited ,,
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3. Daniels, F.;Williams. J. W ;Bender P.; Aiberty, R. A ; Cornwell, C. D. Erperimnrol Physieol Chsmistry. 6th ed.; New York:McCraw-Hill, 1962; pp 121-124. 4. Findlay, A. Procficol Physlcol Chemistry, 8th ed., rev. J. A. Kitchener; Landon: Langman, 195< pp 132-1334, 5. Lewis, R. J., Sr Hoiordous C h e m i d s Dpsk Refem-; New YorL:Van Nosirand Reinhold, 1991. 6. Wilson. J.M.;Newcombe,R. J.;Densm eoi Chemistry: Oxford: Pergamon. 1962; pp 4 6 4 8 :d.:SoiuhrlclvDoioSeries. %I. 37:H3 9. washbum; E.R.;Beguin, A. E.; ~ ~ ~ k f 0. ~ E. r dJ .Am. ~ h & Soe. 1989. 61, 1 6 9 6 1695: Simonsen. 0. R.:Waahbum. E. R. J Amer Chem. Sm.1946. 6 8 . 2 3 6 2 3 7 : and references t h e n " .
