Fractional crystallization

Separating Two Solutes by Fractional Crystallization

While ordinary crystallization purifies a single substance of its impurities, fractional crystallization is used to separate two or more soluble substances from one another, provided they have widely different solubilities in the same solvent at room temperature. A mixture of two solutes, say A and B, is dissolved in a suitable hot solvent to make a saturated solution, which is filtered while hot to remove any dust or insoluble particles and then allowed to cool. As the solution cools, the solute that is less soluble crystallizes out first; these crystals are filtered off, washed with solvent, and dried. The remaining mother liquor is then concentrated by evaporating off some of the solvent, causing the second, more soluble solute to cr …