萃取Liquid–liquid extraction

在一定温度下，溶质在两相中的浓度比为常数，即分配系数 K = c_有机/c_水（能斯特分配定律）。用同样体积的溶剂分多次萃取比一次萃取效率高：经过 n 次萃取后水相剩余量 = m₀·[V_水/(K·V_有机 + V_水)]ⁿ。酸碱萃取利用酸碱反应改变溶解性：羧酸遇 NaHCO₃ 生成盐进入水相，胺遇稀盐酸生成铵盐进入水相，酸化或碱化后又可回到有机相。

At constant temperature the ratio of a solute's concentrations in the two phases is constant, the partition coefficient K = c_org/c_aq (Nernst's distribution law). Several extractions with portions of solvent beat one extraction with the whole volume: after n extractions the amount left in water is m₀·[V_aq/(K·V_org + V_aq)]ⁿ. Acid–base extraction changes solubility chemically: carboxylic acids react with NaHCO₃ and move into water as salts, amines react with dilute HCl and move into water as ammonium salts, and acidifying or basifying returns them to the organic phase.

常用分液漏斗，溶剂常选乙醚、乙酸乙酯、二氯甲烷等。振摇后须及时放气，尤其是用碳酸氢钠洗涤时会产生 CO₂。根据密度判断有机层在上还是在下：二氯甲烷、氯仿比水重在下层，乙醚、乙酸乙酯在上层。出现乳化时可加饱和食盐水破乳（盐析）。下层从活塞放出，上层从上口倒出。

A separatory funnel is used, with solvents such as ether, ethyl acetate or dichloromethane. Vent the funnel promptly after shaking, especially when washing with bicarbonate, which releases CO₂. Density decides which layer is organic: dichloromethane and chloroform are denser than water and sit below, ether and ethyl acetate above. Brine breaks emulsions (salting out). Drain the lower layer through the tap and pour the upper layer out of the top.

萃取用于有机合成后处理、天然产物提取（如从茶叶中提取咖啡因）、湿法冶金（用磷酸三丁酯从核燃料中萃取铀和钚的 PUREX 流程，稀土元素的串级萃取分离）、废水处理和分析化学样品前处理。超临界 CO₂ 萃取用于咖啡脱因和啤酒花提取。徐光宪院士发展的串级萃取理论使中国稀土分离技术居世界领先。

Extraction is used in reaction work-up, isolating natural products (caffeine from tea), hydrometallurgy (the PUREX process, which uses tributyl phosphate to recover uranium and plutonium from spent fuel, and countercurrent separation of rare earths), wastewater treatment and sample preparation. Supercritical CO₂ extraction decaffeinates coffee and extracts hops. Xu Guangxian's theory of countercurrent extraction put China's rare-earth separation technology at the world's forefront.