In a liquid-liquid extraction, how can you experimentally tell which layer is organic and which layer is aqueous? Can you always rely on the relative density of the organic solvent and water?

Liquid-liquid extraction is a very useful method to separate components from a mixture. It consists of separating one or several substances dissolved in a solvent by transferring them to another solvent insoluble or partially insoluble in the first. The transfer of matter is achieved by direct contact between the two liquid phases.

For the extraction process, the solution is placed in a separating funnel, a water-immiscible organic solvent is added (ethyl ether is the most used), the solution with the compound to be separated, the funnel is covered and the funnel is top. Then it shakes. Depending on the solubilities and density, different layers are observed. The denser the compound, the more it will sink.

Since the organic compound is usually much more soluble in ether than in water, most of the organic compound will be dissolved in the ether phase (upper phase) and inorganic salts, which are not soluble in ether, will remain in the aqueous phase (lower phase). Subsequently, by separating the separating funnel the two phases are separated, the organic phase is collected.

Occasionally, after stirring, the two immiscible liquids do not separate sharply, forming an emulsion in the intermediate zone. This is called the colloidal suspension of a liquid in another (system consisting of two or more phases, usually a liquid and another dispersed in the form of generally very fine solid particles). One of the reasons for the formation of an emulsion is when the two phases have similar densities. Then the relative density of the organic solvent and water cannot always be relied upon, although there are methods to facilitate the complete separation of the two phases.