Extractions are among the most widely used methods in analysis to separate mixtures. They are based on the difference in affinity of a solute between two mutually immiscible phases. We give here the principle of the liquid-liquid extraction which makes it possible to transfer a solute from a liquid phase to another liquid phase immiscible with the first.
Liquid–liquid extraction Physico-chemical principle
Liquid-liquid extraction is based on the difference in affinity of a solute between two immiscible liquid phases.
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Rapid operationcal equilibrium
This equilibrium is characterized by a thermodynamic constant Kp called the partition coefficient: Kp=[Aorg]/[Aaq]1. The extraction will be all the more effective as the partition coefficient is large; when possible, an extraction solvent in which the solute is very soluble is chosen. We show below that multiple extractions are more efficient than single extraction for the same volume of solvent.
Liquid–liquid extraction (LLE), although used primarily as a separation technique, is an important enrichment technique. LLE is based on the distribution of an analyte between two essentially immiscible solvents. The distribution ratio of an analyte is defined as the ratio of its total concentration in the organic phase to that in the aqueous phase at equilibrium. When the desired organic compound is extracted, the larger the distribution ratio of the analyte and the smaller that of the matrix, the higher the recovery of the organic compound and enrichment factor. When the matrix is removed by extraction, the reverse is required for successful enrichment. After the extraction, back washing of matrix components selectively from the organic phase into the aqueous phase improves the enrichment factor.
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