What is the principle and purpose of extraction?

There are many kinds of extraction, such as solid-liquid extraction and liquid-liquid extraction.

The purpose is to transfer the substance to be extracted from the solvent to the extractant by using the principle that the solubility of the substance to be extracted is different in the solvent and the extractant, while other substances incompatible with the extractant in the solvent remain in the solvent.

Taking separatory funnel extraction as an example, the principle of liquid-liquid extraction is explained.

Liquid-liquid extraction, that is, the solution (water phase) containing the sample and the extractant (oil phase) are all liquids. Oil is lighter than water, so the oil phase is above the water phase and there is an interface between the two phases. The solution and extractant do not react and are immiscible with each other.

Before extraction, the sample and impurities are dissolved in the water phase. Due to the characteristics of the sample, its solubility in the oil phase and the water phase is different, and the solubility of the sample in the extractant is greater, so after fully shaking, the sample will gradually transfer to the extractant. After standing and layering, it can be separated by using a separatory funnel. While the unwanted sample will remain in the water phase.

The following is a video about the operation of the separatory funnel.