Countercurrent Distribution with Simple Equipment I. R. C. Blck, A. J. Blackman, a n d E. J. Browne University of Tasmania. Hobart. Tasmania, Australia 7001 The separation of a mixture of solutes by sequential partition between two immiscible liquid phases is used in various forms of countercurrent distributi~nl.~, as well as in GLC, HPLC, and other types of liquid-liquid partition chromatography" in the latter, a solid support is employed to keep one phase immobile, but the methods are the same in principle. The countercurrent experiment described here is designed to illustrate this principle with simple equipment that can be made by a glassblower with only modest skills. The apparatus is of the Craig-type' and consists of 10 extraction tubes and 10 transfer tubes (Fie. . . 1) . mounted on a frame. The two phases used are low boiling petroleum and 90% aaueous methanol: the mixture to be seoarated consists of bfnzophenone and diphenyl. These sulkances distribute differently between the two phases in the lint extraction tube. Hy suitahly t~ltingthe frame, the upper phace (petroleum)can he run off into the transfer tube, and thence into thc second extraction tuhe which contains fresh lower phase (nqueous methanol). Mure upper phase is added to the first extraction tube, and the conte&s of the first two tubes are equilibrated; the whole process of decantation and equilibration is repeated nine times. a t the end of which the benzoohenone and diphenyl in the original mixture will be distributed through the ten extraction tubes in a certain oattern deoendine on their partition coefficients in the two phases. ~ h contents k of the tubes are then withdrawn and analyzed by gas chromatography using an internal standard. The experiment is suitable as a class proiect, with each student analyzing the contents of one t u b e . ~the t conclusion of the experiment, the results are correlated and curves are plotted showing the distribution of benzophenone and diphenyl in the upper and lower phases (Fig. 2). This may be compared with the distribution pattern calculated from the partition coefficients. Equipment Class units from an existing Craig countercurrent machine may be used, or the simolified desien illustrated (Fie. I ) can be made u p . ~ h eunits may have any convenient size: in the apparatus used by us, each extraction tuhe, when turned vei%ically,holds 40ml up to the lower side arm. The frame on which the glassware is mounted (Fig. 3) can be swung on pivots by means of a handle, and locked in any position bjr rotating aclamp on the opposite pivot to the handle. The whole frame, including the pivots, can be made of Flexaframk units. In the normal resting position B (Fig. 4), the extraction tubes slant uoward a t an anele about 20' to the horizontal. In oosition A. used during emptying, the tubesslant downward aiabout the same anele. In the transfer oosition D. the tubes ooint vertically upward. Position C is i&mnedia& between B and D and is used during filling operations.
1 Craig, L. C.. and Craig. 0.. "Technique of hganic Chemistry." md Ed.. Welssberger. A,. (Editor). Interscience. New York. 1956. vol. Ill. pp. 171-180. pp. 242-258. lto. Y.. and Bowman. R. L.. J. Chromatog. Sci.. 8, 315 (1970). Pecsok. R. L.. Shields. L. D.. Cairns. T.. and McWilllam. I. G.. "Modem Methods of Chemical Analysis." 2nd Ed.. Wiley. New York, 1976. pp. 44-52. pp. 59-61.p~.85-92.
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Journal of Chemical Education
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I
'Extraction Figure 1. SlmpleCralgtypeunit.
tube
I
Tube No Figwe 2. Diswibution of Benmphenone (K = 0.65) and Diphenyl ( K = 2.5) In Tubes 1-10 after 9 Transfers.
- BBnlOpheMMe in lower phase
..... Dlphenyl in lower phase Diphenyl in upper phase
Experimental Procedure Petroleum (b.p. 40-60°, 1I) is equilibrated with methanol (9M) ml) and distilled water (100 ml) in a separating funnel. Benzophenone (1.00 g) is dissolved in 220 ml of lower phase, and with the apparatus in position C, the solution is introduced into the first extraction tube. together with rinsings from the flask that contained it, keeping the level helow the lower side arm. The remaining extraction tubes are filled with lower phase almost ta the level of the side arms, then the apparatus is gently swung to position D and locked. All 10 tubesare
carefully topped up with more lower phase to the level of the lower side arms with a Pasteur pipet. A graduated cylinder is employed t o measure out 40 ml of upper phase, and about half of this is used to diasolve 1.00 g of powdered diphenyl. With the apparatus in position B, thissolution isadded to the first extraction tube,and the remainder of the upper phase from the cvlinder is used to rinse out the vessel containing the solution into the same tube. Then the plasticstoppers are inserted firmly, and the apparatus is gently rocked between positions A and B for 2 mi". After a further minute or two in position B to allow the phases toseparate, the angle is changed gently to the upright position D in which the upper phase flows into the transfer tube and thence into theadjacent extraction tube by changing to position A. The apparatus is then returned to position B, and 40 ml of fresh upper phase isadded to the first extraction tube through the upper side limb by meansof a funnel. The seauence of ooeretions. includine rockine. ... settline and transfer are rep;ated as bey or^, and lhe pn& is continued until the upper pharr originally in 112brI has reached tube 10. The phasesare then removed from the tubesfor analysis. Withthe apparatus in position B, the stopper from tube 1 is withdrawn, and the apparatus is swung gently to position A with a small separating funnel carrying a filter funnel held under the lipaf the tube. The two phases thus collected are run intoseparate 100-rnl round-bottomed flasks with standard taper joints which will fit directly onto a rotary evaporator. The remaining tubes are emptied in the same way, and the solutions are evaporated carefully to dryness under reduced pressure. A calibrated syringe is used to measure out accurately 100pl of the internal standard, isoamyl sslicylate. into each flask: the contents are dissolved in about 1 ml acetone and transferred quantitatively with Pasteur pipettes t o 5 ml standard flasks, which are then made up t o volume with acetone. T o determine the partition coefficients of benzophenone and diphenyl, 100mg of each are dissolved in a mixture of 40 ml of upper and 40 ml of h e r phase in aseparating funnel.The phases are equilibrated, then separated and evaporated, and after addition of 100 p1 of isoamyl salicylate the constituents are dissolved in acetone and made up to 5 ml as before. To serve as a standard, 40 mg each of benzophenone and diphenyl together with 100pl of isoamyl salieylate are made up to volume with acetone in a 5 ml standard flask. This standard must be made up fresh each day, since solutions of these substances slawlv deteriorate on standing. We have found that tLe GLC analysis can be conveniently carried out on a 2-m column packed with 2% SE30 supported on Gaschrom 4.with an oven temperature of 16OS.About 0.8-1.0p1 of each solution is injected, and the peak heights for better, the areas under the peaks) of diphenyl, isoamyl salicylate, and benzophenone, which emerge in that order,are measured. Then the amountsof diphenyl and benwphenone in each solution are calculated using thedataderived from the standard solution. The analytical results obtained by individual students for the contents of the various tubes are correlated, and the valuesare dotted in aseries of curves (Fie. . .. 2) showine thedistribution of henzopl;enone and diphenyl in hoth upper and lrher phases. The partial sepnmtmn achieved with Ill t u l w wuulrl dead" heeome cmplelt. with a sufficwnt numher ofextm tubes.
-
Fiwe 3. Showinc Two Pairs ol Extraction am( TransferTubes Mounted on Frame. with Handle Left, and Clamp Right.
Figure 4. P~snlons01 e m c t l o n hlbes dwlng cwntercunent cycle.
1 and q = l+K and the binomial can b e written:
Calculation of Solute Dlstrlbutlon If t h e mole fractions of benzophenone in u p p e r and lower phases a r e P a n d a, respectively. the distribution of benzophenone a€ter n t&nsfers is &en by the t e r m s of the exp a n d e d binomial equation':
F r o m this, the a m o u n t of benzophenone in a n y tube a f t e r n transfers c a n b e found. F o r the r t h tube, the a m o u n t will he the rth t e r m of t h i s binomial expansion
( P + q)"= 1
where p
+ q = 1. Since the partition coefficient
I n t h e experiment described, n = 9. T h e distribution of diphenyl c a n be calculated similarly from a knowledge of i t s partition coefficient. T h e calculations can be used to check t h e experimental observations
Volume 58
Number 10 October 1981
815
