A Convenient Method for Comparison of Efficiency
of Fractionating Columns Robett H. Higgins Fayetteville State University. Fayetteville, NC 28301 Fractional distillation is a technique usually introduced in the organic chemistry laboratory. a mixture of . Typically, .. two components is separated hv simple and hy tractional distillation with the degree of separation being estimated by construction of plots of boilingpoint as a function of the volume collected. We have found it pedagogically useful to demonstrate the ability of various fractionating columns (both packed and unpacked) to resolve the mixture into individual com~onents(from d o t s of fraction comnosition versus total volume collected) and t o correlate the resolving Dowers of column oackines to their "hold-up" volumes. In order to deteimine ihe composition of the various fractions, one must perform chemical analyses on the fractions. Common analytical. methods (proton magnetic resonance spectroscopy, infrared spectrophotometry, gas chromatography, etc.) require substantial investments in training time (at a time when these methods generally have not been introduced) and in instrumentation (each of these instruments is rather expensive and several would be necessary for even moderatelv sized oreanic chemistrv lahoratorv classes). " Analysis by refractive indices is convenient (although often neglected). with the reauisite construction of .. ranid . (even . calibration curves-see figure), inexpensive, accurate, and easilv taueht.' 0;r initial attempt to employ refractive indices for analysis of fractions collected during the separation of an ethyl acetate-hutyl acetate mixturez suggested that relatively good separation of the components was achieved even in simple distillation, while use of any fractionating column afforded nearly complete separation of the components. I t was necessary to employ materials with more closely matched boiling points to assess the resolving power of the fractionating columns. An ethyl acetate-propyl acetate mixture was selected, although some loss in accuracy3 of the method results since these esters have indices of refraction that are not u,idely different. 1)istinct differenres in fraction rom~ositiunwrsus total volume of distillilte for the fractionating columns were observed, while all columns provided much better separation of the components than did simple distillation.
' Skoog, D. A.; West; D. M. Princl~lesoflnslrumenlalAnalysis, 2nd ea.; Saunders. Phl adelphla. 1980; p 356. 'Roberts, R. M.; G lbeR. J. C : Rooewald. L. B.: W ngrove. A. S. Modern Experimenlal Organic Chemlslry. 4th ed.; Saunders: Pniladelphia. 1985: pp 105-109. Refractive indices were determined to f0.0002 % on an Abbe refractometer maintained at 20 "C. inaccuracies caused by temperature fluctuations were minimized by controlling the temperature by use of water circulated through the refractometer from an external constanttemperature(f0.02 "C) bath. Willard, H. H.; Merritt, Jr.. L. L.; Dean. J. A,; Settle.Jr., F. A. lnstrumentalMelhodsofAnalysis,6thed.; D. Van Nostrand: New York, 1981: p 406. Fractionating columns used were approximately 50 cm in length. Commercial Vigreux and six-section Snyder columns were used as supplied, while Hempei columns were packed with 6-mm glass beads. 6-mm glass helices, 6-mm Raschig rings, or porcelain saddles. 810
Journal of Chemical Education
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% EtOAc Vs Calibration curves for analysis by refractive indices.
Students were instructed to work in pairs. Each pair or team of students preparedsolutions which were about 25,50, and 75 vol % propyl acetate in ethyl acetate. The refractive index of each of these solutions and that of each of the pure components was determined, and plots of refractive index as a function of com~ositionwere constructed. Each team also performed simple'and fractional distillations on >0-mLsam~ l e s o f 50:SO a mixture of these aretates utilizing their choice bf fractionating ~ o l u m n Distillations .~ were conducted in 100-mL $ 24/40, magnetically stirred, round-bottom flasks, which were heated wkh hemispherical heating mantles. Students were instructed to collect 3-mL fractions. Distillation was terminated when the not volume was estimated to he 23 mL. The pot residue volume was determined after allowing the svstem to cool. The "hold-un" of the column was determined by subtracting the sum bf the pot residue and the volume distilled from the initial volume (50 mL). Data from each team was shared with other teams such that each student had access to all data necessary the analysis. . to perform . Students were asked to rank the effectiveness of columns in achieving resolution of the acetates and also to investigate any possihik correlation between the "hold-up" volumeand the effectiveness of each column. Acknowledgment The assistance of Christine A. Minch and Nancv J. Lvons. two undergraduate students who did most of theinitial ex: perimentation, is gratefully acknowledged.
