Countercurrent distribution (CCD, also spelled "counter current" distribution) is an analytical chemistry technique which was developed by Lyman C. Craig in the 1940s. Countercurrent distribution is a separation process that is founded on the principles of

liquid–liquid extraction Liquid–liquid extraction (LLE), also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an org ...

where a chemical compound is distributed (partitioned) between two immiscible liquid phases (oil and water for example) according to its relative solubility in the two phases. The simplest form of liquid-liquid extraction is the partitioning of a mixture of compounds between two immiscible liquid phases in a

separatory funnel A separatory funnel, also known as a separation funnel, separating funnel, or colloquially sep funnel, is a piece of laboratory glassware used in liquid-liquid extractions to separate (''partition'') the components of a mixture into two immisci ...

. This occurs in five steps: 1) preparation of the separatory funnel with the two phase solvent system, 2) introduction of the compound mixture into the separatory funnel, 3) vigorous shaking of the separatory funnel to mix the two layers and allow for

mass transfer Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, ...

of compounds in and out of the phases, 4) The contents of the separatory funnel are allowed to settle back into two distinct phases and 5) the two phases are separated from each other by draining out the bottom phase. If a compound is insoluble in the lower phase it will distribute into the upper phase and stay in the separatory funnel. If a compound is insoluble in the upper phase it will distribute into the lower phase and be removed from the separatory funnel. If the mixture contains one or more compounds that are soluble in the upper phase and one or more compounds that are soluble in the lower phase, then an extraction has occurred. Often, an individual compound is soluble to a certain extent in both phases and the extraction is, therefore, incomplete. The relative solubility of a compound in two phases is known as the

partition coefficient In the physical sciences, a partition coefficient (''P'') or distribution coefficient (''D'') is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. This ratio is therefore a comparison of the solub ...

. While one separatory funnel is useful in separating certain compound mixtures with a carefully formulated biphasic solvent system, a series of separatory funnels may be employed to separate compounds that have different partition coefficients. Countercurrent distribution, therefore, is a method of using a series of vessels (separatory funnels) to separate compounds by a sequence of liquid-liquid extraction operations. Contrary to liquid-liquid extraction, in the CCD instruments the upper phase is decanted from the lower phase once the phases have settled. First, a mixture is introduced to vessel 1 (V₁) charged with both phases and the liquid-liquid extraction process is performed. The upper phase is added to a second vessel (V₂) which already holds fresh lower phase. Fresh upper phase is added to V₁. Both vessels are shaken and allowed to settle. upper phase from V₁ is transferred to V₂ at the same time the upper phase from V₂ is transferred to V₃ which already holds fresh lower phase. Fresh upper phase is added to V₁, all three vessels are shaken and settled and the process continues. Compounds that are more soluble in the upper phase than lower phase faster and farther down the series of vessels (the "train") while those compounds which are more soluble in the lower phase than the upper phase tend to lag behind. A compound insoluble in the upper phase will remain in V₁ while a compound insoluble in the lower phase will stay in the lead vessel.

Historical development

Early work in the development of liquid-liquid separation techniques was undertaken by Cornish et al. with a process called "systematic fractional distribution" as well as Randall and Longtin, however, the central figure is certainly Lyman C. Craig. Lyman Craig's development of countercurrent distribution began with studying the distribution of a pharmaceutical,

mepacrine Mepacrine, also called quinacrine or by the trade name Atabrine, is a medication with several uses. It is related to chloroquine and mefloquine. Although formerly available from compounding pharmacies, as of August 2020 it is unavailable in th ...

(atabrine), between the two layers of an ethylene dichloride, methanol, and aqueous buffer biphasic solvent system. The distribution coefficient (K_c which coincides with

) of atabrine varied by the composition of the solvent system and the pH of the buffer. In the next article, Craig was inspired by the work of Martin and Synge with

partition chromatography Partition chromatography theory and practice was introduced through the work and publications of Archer Martin and Richard Laurence Millington Synge during the 1940s. They would later receive the 1952 Nobel Prize in Chemistry "for their invention of ...

to develop an apparatus that would separate compounds based on their distribution constant (''K'' which coincides with partition coefficient). It was shown that a solvent system composed of benzene, ''n''-hexane, methanol and water would separate mixtures of organic acids. It is remarkable that the mathematical theory developed hand-in-hand with the progression of applications. Craig continued to pursue this method of separation by testing different compounds, formulating biphasic solvent systems, and most importantly developing a commercially viable instrument. The CCD technique was employed in many notable separations such as

penicillin Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from ''Penicillium'' moulds, principally '' P. chrysogenum'' and '' P. rubens''. Most penicillins in clinical use are synthesised by P. chrysogenum using ...

polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the ''INS'' gene. It is considered to be the main anabolic hormone of the body. It regulates the metabolism o ...

bile acids Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver. Bile acids are conjugated with taurine or glycine residues to give anions called bile salts. Primary b ...

ribonucleic acid Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

taxol Paclitaxel (PTX), sold under the brand name Taxol among others, is a chemotherapy medication used to treat a number of types of cancer. This includes ovarian cancer, esophageal cancer, breast cancer, lung cancer, Kaposi's sarcoma, cervical cance ...

, ''Streptomyces'' antibiotics. and many other antibiotics.

References

{{reflist Analytical chemistry