Addison Ault
Cornell College Mount Vernon, Iowa
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I
Fractionating Column Efficiency Quantitative experiment in elementary organic chemistry laboratory
Experimental work in organic chcmiatry continues to become more quantitative and to depend more upon iustrumental methods of analysis. Wo t)clieve that thesc t ~ r n d sshould hc rcflected in thl: experiments offe~edin the introductory organic coursc. 1Jor this reason, we have introdnccd an experimental determination of the efficiency of a fractionating column which involves making quautitativr analyscs with the aid of the gas chromatograph. One way in which the efficiency of a fractionatiug column may be estimated is as follows.' Using a binary mixture, the column is operated under total reflux until equilibrium is established. At this time, samples of the condensate and of the pot rcsiduc are analyzed. It is reasonable that the greater the degree of enrichment of the distillate in the more volatile component of the mixture, the greater is the cfficieney of the column iu effecting the separation. The efciency may be expressed in another way by using the results of the analyses in conjunction with the boiling point diagram for the system (see Fig. 1). If, for cxample, the mole fraction of the more volatile component of the distillate is Xz and that of the pot is XI, the total efficiency of the system can he said to be equivalent to three successive simple distillations, or equd t o three theoretical plates. Smce one theoretical plate must be attributed to the pot, the efficiency of the column is theu two theoretical plates. I n the experiment to he described, the method is similar to this, but the column is not run under total reflux; instead, the mixture is distilled very slowly. The efficiency which will obtain under these conditions will be less than that which would be reached under conditions of total refluxa However, it will morr nearly be that which would he expected to obtain during an actual distillation. Experimental Instructions
The student is instructed to distil slowly an approximately three-to-one mixture of tolucne and benzene, using some kind of fractiouating column, until the distillate is no longer cloudy (duc to water in the system). The rate of distillation is then further reduced, if possible, and after a few minutes a small sample of the distillate is collected. The source of heat is immediately removed, and the apparatus is allowed to cool. In a few minutes, a small sample of the contents of the pot is taken. The samples are then taken to the instructor or the assistant who subjects I WEI~SBERGER, A., Editor, "Technique of Organic Chenristry," Interscience Publishers, h e . , New York, 1951, Vnl. IV, 1,. 32. ' WEISSBERGER, p. 51.
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them to gas chromatographic analysis. I'rorn the chromatographic traces, the studcnt is expected to ralculatc the mole fraction of benzene in each sample m d then, referriug to the boiling point diagram for I ~ c ~ ~ c n e t o l u e(Fig. n e I), to dctermiue the efficiency of thc column expressed as theoretical plates. While there are other methods of aualysing the data,3 we helieve that this is the onc most likely to give the of what is .. ~ o .i uon~ inside student an auureciation . the fractionating colnmn.
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0
0.4
0.2
0.6
XI
0.8
1 .O
X? Mole Fraction Benzene
Figure 1. Typicol student rerulh (XI, residue; XS, distillate) indicated on the boiling point diagram for the system benzene-toluene o t a total p e l sure of 750 mm Hg. The diogrom was constructed from data of RosanoR, Bacon, and Schulre.'
Since, under the couditio~~s of the analysis, the peak areas of the chromatographic trace are proportional to the weights of benzene and toluene in the sample, the ratio of peak areas gives the weight ratio of benzene to toluene. This may be co~ivertrdto the mole ratio and thence to the mole fraction. Gas Chromatographic Analysis
The analyses have been carried out u s i ~ ~ag5-ft X 'jrin. Apiezon "L" column (20% on 30/60 firebrick) in the Aerograph A-90-P2 gas chromatograph (Katharometer detector) using a Sargent Model-SR 1-mv recorder. The following conditions were found to give excellent results: column temperature, 215" C; sample sizr, 1.1 p l ; hclium gas pressure, 30 lh, and flow rate p. 38. WEISSBERDER,
ROSANOEF, M. A,, BACON, C. W., J . .I n,. Chent. Soc., 36,1993 (1914).
AND
Scrru~z~, J. F. W.,
adjusted to give a retcutiou time for toluene of about 40 sec; attenuation, factor of 32; chart speed, 1 in./min. Analysis under these conditions of mixtures of known composition indicated that the peak areas (determilled I,y multiplyillg the height by the width at half height) were proportional to the weights of benzene and toluene in the sample. The width of a peak was conveniently vstimated by use of vernier calipers. The fact that it can take less than two minutes to analyse both of a student's samples means that this is a practical procedure even for relat,ivcly large laboratory classes.
We llave been quite pleased with the results of this attempt to develop a quantitative experiment based on an instrumental method of analysis. Students are able to duplicate their results easily, and can determine the dependence of the efficiency of a column on variables such as the length and type of packing used in the column. On the basis of student comments, we believe that the experiment gives them much better understanding of the process of fractional distillation than do the more conventional fldistillatioll exPC& ments." The instrument used is one whose basic principles of operation are easily understood by thc student.
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