TA: HSIEN-PO CHIU Oct 2 & 4, 2007 Exp.3 Recrystallization and Melting-point measurement Principle 1. Recrystallization is based on the fact that the solubility of a solid in a given solvent increases with the temperature of the solvent. 2. In a typical recrystallization procedure, the crude solid is dissolved by heating it in a suitable solvent. The hot solution is then filtered by gravity and the filtrate is allowed to cool to room temperatureor below, whereupon crystals appear in the saturated solution and are collected by vacuum filtration. The crystals are ordinarily much purer than the crude solid because most of the impurities either fail to dissolve in the hot solution, from which they are separated by gravity filtration or transfer, or remain dissolved in the cold solution, from which they are separated by vacuum filtration or centrifugation. 3. Consider the recrystallization of a 7.5 g sample of salicylic acid contaminated by 0.5 g of acetanilide (6.3% contamination). 20
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