M. Verzele and N. Schamp University of Ghent Ghent, Belgium
Countercurrent Distribution Apparatus for a simple experimenf
The technique of countercurrent distribution as developed mainly hy Craig1 is now firmly established in most research laboratories. This modem separation method is based on the different partitioning of the different components of a mixture in the two phases of a liquid/liquid system. The heavier of the phases is stationary and divided over a series of extraction tubes or cells. The lighter phase of each cell is moved after each equilibration by a simple movement to the adjoining cells while the first cell is supplied with fresh lighter phase. The whole operation of agitating the cell train for the extraction and demixing of the phases and moving the upper phases one cell forward isusually called "a transfer." The different components of the mixture, put into the first cell a t the start of the distribution, advance through the train a t different rates and are in this way separated. The principles underlying this technique of CCD are described extensively in the reference cited.' The apparatus is commercially available, but is rather expensive. A simple machine, giving good results, can be built as follow^:^ Fifty cells are made of glass following the design of Figure 1. Part A holding the heavy phase a t each
l!b Figure 1.
Assembly of thetrain.
transfer is calibrated to 20 ml =t0.5 ml. This can be done by fillina the cell, ulacina i t in the transfer posi-
' CWG, L. C., AND CRAIG,D., "Technique of Organic Chemistry," Vol. 3, 2nd ed., Interscience Publishers, Inc., New York, 1956, p. 149. See also: CRAIG,L. C., J. Bid. Chem., 161, 321 (1945); CRAIG, L. C., AND POST,O., Anal. Chem., 21, 500 (1949). a EDITOR'S NOTE: See also CHRISTIAN, S. D., THIS JOURNAL, 36, 195 (1959).
tion (95' turn to the right), and measuring the volume withheld in compartment A. Too large or too small cells can be adjusted by reducing or blowing out the end part of A. Tube E permits easy sampling with a pipet and is closed with a plastic cap. Fresh solvent feed a t each transfer in the first cell i s obtained with a setup like that depicted in Figure 2.
Figure 2.
Frerh rolvedfeed.
Part C determines the volume of the fresh solvent feed (in this case also 20 ml) ; D is a reservoir glass tube inserted through the train and should hold a t least one liter of the lighter solvent. The cells are mounted on a "Dexion" rack providing 120 cm axis length and the cells are attached with any convenient means (clips or nylon cord, for example). Counterweights are necessary to hold the train in the horizontal resting position (Fig. 1). The cells are joined with plastic tubing softened in hot water. This is satisfactory even when a distribution is to he analyzed photometrically, because any interfering effects from the plastic softeners are soon washed away and no trouble arises. The distributions are carried out as follows: Somewhat more than one liter of the heavy solvent is introduced in the first cells of the train. Turning the apparatus 95' to the right and hack again makes the solvent advance in the cells and after sufficient decantations all the lower halves of the cells are filled with heavy solvent and all the glass is wetted. The mixture to he separatedand theupper phase are introduced in the first cell and one transfer is carried out by (a) rocking backward and forward over an angle of about 30' a t a frequency of one movement per two seconds; (b) waiting in the horizontal position until the phases have separated; and (c) tilting the apparatus slowly to a position 95" to the right, making the upper phases flow into the compartments B and then turning hack to the horizontal position whereby the light solvent flows into the adjoining tubes. During this operation fresh solvent will have run into the first cell. Volume
39, Number 2, February 1962
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97
A simple demonstration experiment consists of the separation of acetic and propionic acid using ether and water as the solvent pair. The partition coefficient of acetic acid in this system is about 0.6 and that of propionic acid is around 2.4. The 0 value or ratio of the two coefficients is therefore approximately 4, and the separation is complete with 50 transfers. An appropriate quantity of the acids is one ml of each; the concentration in the cells a t the end of the distribution is determined by simple titration of an aliquot of the aqueous layer. The interesting point of this distribution is that the result can be made visible in the cells by using water colored with methyl orange. I t is therefore useful for demonstration purposes. The hand operated distribution plus the titrations take about half a day. A typical result obtained by one of our students is shown in Figure 3. This separation shows the main advantages of CCD: large capacity and easy reproducibility.
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Journol of Chemicol Education
The described instrument allows the student to familiarize himself with the technique of CCD and the described separation permits the study of the underlying principles and theory. I t also is a useful research tool.
Figure 3. Separation of acetic acid (A) and propionic acid (P) in ether/ woter. Fihy transfererr.
