Extraction Theory



An extraction is an experimental technique that utilizes differences in solubility to purify a product. Two solvents are used that differ in two important ways. First, the solvents must be immiscible. That is they will form two distinct layers when poured together. Second, the solubility of the product must be significantly higher in one solvent than the other.

Initially, the product is dissolved in the solvent associated with the lower solubility. This solution is added to the separatory funnel (making sure the lower valve is closed first). The second solution is then poured through the top of the funnel whereupon two layers are observed.

The separatory funnel is then inverted and gently swished (be sure to hold the stopper). In the inverted position the valve is opened periodically to allow excess pressure to escape. Vigorous shaking is not advised because emulsions may form. An emulsion is a suspension of finely divided droplets of one solvent in another. It can take a very long time for the droplets to coalesce and reform the two layers.

As the solutions are brought into intimate contact with one another, the dissolved product moves from the first (less soluble) solvent into the second (more soluble) solvent. This process, (illustrated in the diagram above) continues until equilibrium is established. At this point, the concentrations of product in the two solvents no longer change. Note that there will always be a fixed, albeit relatively low, concentration of product species in the original solvent. Additional extractions utilizing fresh solvent (more soluble) may performed to remove more of the product from the original solution.