1.0 General Introduction
Extraction is the transfer of a solute from one phase to another. Common reasons to carry out an extraction in chemistry are to isolate or concentrate the desired analyte or to separate it from species that would interfere in the analysis. The most common case is the extraction of an aqueous solution with an organic solvent that are immiscible with and less dense than water; they form a separate phase that floats on top of the aqueous phase1.
Solvent or liquid-liquid extraction is based on the principle that a solute can distribute itself in a certain ratio between two immiscible solvents, one of which is usually water and the other an organic solvent such as benzene, carbon tetrachloride or chloroform. In certain cases the solute can be more or less completely transferred into the organic phase. The technique can be used for purposes of preparation, purification, enrichment, separation and analysis, on all scales of working, from microanalysis to production processes. In chemistry, solvent extraction has come to the forefront in recent years as a popular separation technique because of its elegance, simplicity, speed and applicability to both tracer and macro amounts of metal ions2.
The ability of a solute (inorganic or organic) to distribute itself between an aqueous solution and an immiscible organic solvent has long been applied to separation and purification of solutes either by extraction into the organic phase, leaving undesirable substances in the aqueous phase; or by extraction of the undesirable substances into the organic phase, leaving the desirable solute in the aqueous phase.3
1.1 Background of Study
Although solvent extraction as a method of separation has long been known to the chemists, only in recent years it has achieved recognition among analysts as a powerful separation technique. Liquid-liquid extraction, mostly used in analysis, is a technique in which a solution is brought into contact with a second solvent, essentially immiscible with the first, in order to bring the transfer of one or more solutes into the second solvent4. The separations that can be achieved by this method are simple, convenient and rapid to perform; they are clean as much as the small interfacial area certainly precludes any phenomena analogous to the undesirable co-precipitation encountered in precipitation separations.
Solvent extraction has one of its most important applications in the separation of metal cations. In this technique, the metal ion, through appropriate chemistry, distributes from an aqueous phase into a water-immiscible organic phase. Solvent extraction of metal ions is useful for removing them from an interfering matrix, or for selectivity (with the right chemistry) separating one or a group of metals from others4.
Solvent extraction is one of the most extensively studied and most widely used techniques for the separation and pre-concentration of elements. The technique has become more useful in recent years due to the development of selective chelating agents for trace metal determination5
1.2 Scope of Work