Principle of Solvent Extraction:

Solvent extraction is based on the principle that when a solute is brought in contact with two immiscible solvents, one of which is invariably water and the other organic, the solute distributes itself in a fixed ratio in the two solvents. In certain cases, at some optimum conditions, the solute can be more or less completely transferred from one phase to another. Thus, the technique finds application in separation, purification and enrichment. There is another important aspect of solvent extraction in the form of extractive spectrophotometry.  The absorbance of coloured metal complexes, particularly metal chelates, extracted in the organic phase is measured. In favourable circumstances, it may be possible to concentrate the metal, separate it from interferences and develop the absorbing system in a single step. Thus, the extractive spectrophotometry invariably scores over normal spectrophotometry in terms of selectivity and sensitivity.

It may be important to point out here that the general principles, terminology, and methodology used for the separation of inorganic and organics are the same. The scope of applications of the technique for the two types of materials is by no means different. But in order to illustrate the principles more vividly and the variety of situations that are encountered, our focus will be mainly on the extraction of metal ions.

The next unit on solvent extraction will deal mainly with different types of extraction equilibria, factors influencing extraction, different approaches for metal ion separations and the criteria for the choice of organic phase.

Objectives

After studying this unit, you should be able

• Explain general principles and terminology used to express partitioning of solute and efficiency of separations,
• Classify extractants used for metal ions, and
• The role of diluents and modifiers in the extraction of metal ions.