In the 1940s Lyman Creighton Craig invented one of the first instruments, later often referred to as "Craig Apparatus", for counter-current partitioning to have any commercial success; he called this technique Countercurrent Distribution (CCD). Shown here is a apparatus built by Erich Hecker (Tuebingen, Germany) that allowed manual CCD of samples. Craig's Countercurrent Distribution was a breakthrough in separation science and became very popular in the 1940s. CCD machines that occupied whole rooms were capable of handling liters of solvents and consisted of up to several hundred partition elements that automated the manual partitioning of samples in separatory funnels.
How it Works
A Craig apparatus consists of a series of glass tubes (r: 0, 1, 2..) that are designed and arranged such that the lighter liquid phase can be transferred from one distribution tube to the next. The liquid-liquid extractions are taking place simultaneously in all tubes of the apparatus, which is usually driven electromechanically. In the following animated picture of a single glass tube the typical "extraction/transfer" cycle is shown.
The lower (heavier) phase of the two-phase solvent system (e.g. water, blue layer in the picture) is the "stationary phase", whereas the upper (lighter) phase (e.g. hexane, red layer in the picture) is the "mobile phase". In the beginning, tube #0 contains the mixture of substances to be separated in the heavier solvent and all the other tubes contain equal volumes of the lighter solvent phase. Next, the lighter solvent is added to tube #0, extraction (equilibration) takes place, and the phases are allowed to separate. The upper phase of tube #0 is then transferred to tube #1, fresh solvent is added to tube #0, and the phases are equilibrated again. The upper layers of tubes #0 and #1 are simultaneously transferred to tubes #1 and #2 respectively. This cycle is repeated to carry on the process through the other CCD elements.
In this system, substances with a higher distribution ratio move faster compared than those with a lower distribution ratio. In order to understand the whole process, it is helpful to examine the distribution of a substance A in each tube after a given number of equilibration/transfer cycles. Assuming that the volumes of each solvent phase are equal (V), and letting W represent the weight of A in the sample, p and q represent the fraction of A with distribution ratio of D in the upper (organic solvent, o) and lower (water, w) phase, then it is
Since p+q = 1, we have
The fractions of solute in successive tubes after each extraction step are shown in the following figure:
We observe that, after n transfers/equilibration cycles, and since the ratio D=p/q must be maintained for each tube after the equilibration step, the total fraction of A in each tube corresponds to the terms of the binomial expansion (p+q)n. Therefore, the total fraction of a solute in tube r after n transfers is given by (remember that 0!=1).
By combining with the previous expressions of p and q, we finally obtain
A general rule of CCD is that the greater the difference of the distribution ratio of various substances, the better the separation between each other. A much larger number of tubes is required to separate mixtures of substances with almost similar distribution ratios.
Today, the Craig apparatus is only very rarely used (as instruments are hardly available any more), mostly becuase because of the efficiency and and convenient handling of modern chromatographic instruments. However, the principle of countercurrent extraction provides a very useful educational and scientific example, as it introduces the fundamental concepts of equilibration between mobile and stationary phases. In CCD, each tube in which a full equilibration can take place corresponds to one theoretical plate of a chromatographic column. Craig apparatuses with more than about 100 tubes are very difficult to construct and operate. Although they are typically compared with modern chromatographic columns that exhibit efficiencies of hundreds of thousands theoretical plates, this comparison is greatly flawed by the influence of the effective volume of the stationary phase, essentialy voiding the direct comparison of theoretical plates in LC versus CCD/CCC.
The following applet was created and graciously donated by Dr. Constantinos Efstathiou of the National and Kapodistrian University of Athens. This applet demonstrates the principles of countercurrent extraction. A mixture of two substances: A (with green color) and B (with red color) is going to be separated using a Craig apparatus consisting of 20 (0, 1, .. 19) equilibration tubes. The user must enter the values of distribution ratios for the two substances in the corresponding edit boxes. Click the image to start the applet.