What is extraction?

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Extraction is one of the methods used in organic chemistry laboratories to purify and refine compounds. Through extraction, the desired compounds can be obtained from solid or liquid mixtures. Here, common liquid-liquid extraction is introduced. Basic principle: Utilize the difference in solubility or distribution coefficients of a compound in two immiscible (or slightly soluble) solvents to transfer the compound from one solvent to another. After repeated extractions, the vast majority of the compounds were extracted. The distribution law is the fundamental basis of the theory of extraction methods; substances have different solubilities in various solvents. Meanwhile, when a soluble substance is added to two immiscible solvents, it can dissolve in each of these solvents. Experiments have shown that, at a certain temperature, when this compound does not undergo decomposition, electrolysis, association, or solvation in reaction with these two solvents, the ratio of this compound in the two liquid layers remains constant. This is true regardless of the amount of substance added. Express it in terms of formulas. CA/CB=K, where CA and CB represent the molar concentrations of a compound in two immiscible solvents, respectively. K is a constant, known as the “distribution coefficient”. Organic compounds generally have higher solubility in organic solvents than in water. Extracting water-soluble compounds using organic solvents is a typical example of extraction. During extraction, adding a certain amount of electrolyte (such as sodium chloride) to the aqueous solution takes advantage of the \"salting-out effect\" to reduce the solubility of organic substances and the extraction solvent in the aqueous solution, which often improves the efficiency of extraction. To completely extract the desired compound from the solution, one extraction is usually not sufficient; several extractions must be repeated. Using the relationship of the distribution law, it is possible to calculate the remaining amount of the compound after extraction. Let: V be the volume of the original solution; w0 be the total amount of the compound before extraction; w1 be the amount of the compound remaining after one extraction; w2 be the amount remaining after two extractions; w3 be the amount remaining after n extractions; S be the volume of the extraction solution. After one extraction, the concentration of the compound in the original solution is w1/V ； And the concentration of this compound in the extraction solvent is (w0-w1)/S ； The ratio of the two is equal to K, that is: w1/V = K. Therefore, w1 = w0 * KV. Also, (w0 – w1)/S = KV + S. Similarly, after two extractions, we have w2/V = K; thus, (w1 – w2)/S = w2 = w1 * KV = w0 * KV = KV + S. Hence, after n extractions, wn = w0 * (KV)^(n-1) * (KV + S). When a certain amount of solvent is used, it is desirable for as little residue to remain in water as possible. And the expression KV/(KV+S) is always less than 1; therefore, the larger n is, the smaller wn becomes. In other words, it is better to divide the solvent into several portions for multiple extractions rather than using the entire amount of solvent in one extraction. It should be noted, however, that the formula above applies to solvents that are almost immiscible with water, such as benzene and carbon tetrachloride. For solvents such as diethyl ether, which are only slightly miscible with water, the formula above is merely approximate. But it is still possible to qualitatively indicate the expected results