Countercurrent distribution

Countercurrent distribution (CCD, also spelled "co "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 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. 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 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.

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 separation 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.

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