Differential Extraction

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Aim

To extract a compound that is present in very small amounts in an aqueous solution with the help of an organic liquid in which that compound is highly soluble.

Principle

This method is based on the relative solubility of the compounds in organic solvents versus aqueous medium. The organic liquid in which the compound is more soluble is generally called as 'solvent'. The solvent chosen should satisfy the following conditions:

• The solvent should be immiscible with water.

• The compound initially present in aqueous solution should be highly soluble in the solvent.

Process

The aqueous solution of the compound (to be extracted) is taken in a separating funnel. A small quantity of a suitable organic solvent is added to this. The separating funnel is stoppered and the contents are shaken for some time. The funnel is then allowed to stand undisturbed on a tripod stand for some time. The liquids in the flask separate into two layers.

These two layers are collected separately in two beakers/flasks. The aqueous layer is again transferred to the separating funnel and the process is repeated with the fresh organic solvent.

Fig:16.13 Differential extraction

Organic solvent layers in all extractions are transferred to a distillation flask and the solvent is distilled off. The concentrated solution of the compound in the organic solvent is subjected to crystallization to obtain the crystals of the extracted compound.

Differential extraction becomes more effective if the process is repeated a number of times using a small volume of the organic solvent each time, than by using the whole solvent in one lot, i.e. multiple extraction is more efficient than single extraction.

For example, iodine (I₂) can be extracted from its dilute aqueous solution (solubility of iodine in water in very small) by differential extraction method using carbon tetrachloride (CCl₄). This is because iodine is much more soluble in carbon tetrachloride than in water, and carbon tetrachloride is immiscible with water. Carbon tetrachloride is heavier than water. So, the solution of iodine in CCl₄ forms the lower layer.