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Solid-liquid extraction transfers a solute from a solid phase to a liquid phase, like steeping tea to extract caffeine from leaves. Liquid-liquid extraction transfers a solute between two immiscible liquid phases that do not mix. Both techniques separate components based on solubility differences, but they operate on different starting material states.

Immiscible liquids remain as separate phases and do not form a homogeneous solution, allowing each solute to partition into the phase where it is most soluble. This separation enables efficient isolation of target compounds. If solvents mixed completely, solutes could not be cleanly separated into distinct layers for collection.

The partition coefficient, K, is the ratio of solute concentration in the organic phase to concentration in the aqueous phase. A large partition coefficient indicates the solute strongly prefers the organic solvent, making extraction more efficient. Solutes with small partition coefficients prefer the aqueous phase, requiring different solvent selection or multiple extraction cycles.

Solvents must have different polarities so solutes partition appropriately between phases. Non-polar solutes preferentially dissolve in organic, non-polar solvents, while polar solutes dissolve in aqueous phases. Matching solvent polarity to solute polarity maximizes separation efficiency and ensures target compounds move to the desired phase.

Acid-base extraction transforms acidic or basic compounds into ionic salts by adding acid or base to the mixture. Ionic compounds become water-soluble and transfer to the aqueous phase, while neutral compounds remain in the organic phase. The ionic compound is then deprotonated or reprotonated to recover the original compound.

Solvent density determines which layer settles to the bottom and which floats on top in a separatory funnel. Most organic solvents are less dense than water and rise to the top, except chlorinated organic solvents. Knowing densities allows you to identify and correctly collect each phase through the stopcock.

Choose solvents where the solute is more soluble than in water, ensuring immiscibility with water and no reaction with the solute. The solvent should be volatile for easy removal and have known density. The partition coefficient must favor transfer to your target phase for efficient extraction.