Multiple Extractions:

It should be clear in mind that one transfer does not necessarily lead to a clean separation. Under these situations, it is better to combine a number of successive partial separations until the desired level of purity is achieved. While considering how two phases may be brought together repetitively, the following four types of situations may be visualized.

i) It is simple where one shot contact between the two phases is made. The two phases are shaken until equilibrium is attained.

ii) In this, one phase could be brought repeatedly in contact with fresh portions of a second phase. This situation would be applicable where one substance remained quantitatively in one phase while the other one was distributed in the two phases. This would be like the repeated extraction of an aqueous solution with successive portions of an organic solvent. The example of Soxhlet extractor will fall in this category. Another analogy may be provided by reprecipitation in gravimetric analysis.

iii) In this case, one phase moves while in contact with the second phase which remains stationary. Like the various chromatographic techniques the mobile phase moves. This is sometimes designated as countercurrent. But in the real sense, it is not countercurrent as only one phase moves. Therefore, it is actually known as discontinuous countercurrent or pseudo countercurrent.

iv) This case is really countercurrent in which both phases move continuously in contact with each other in opposite directions. A true example of countercurrent is fractional distillation.