Intermediate Topic 902 of 1102 Analytical, Lab & Spectroscopy
Recrystallization
Repeated crystallization to remove impurities from a solid.
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In-depth explanation of Recrystallization
Recrystallization is a repeated crystallization used to remove impurities that a single crystallization did not eliminate, and it is one of the most common purification techniques in organic chemistry. Each cycle increases purity but reduces yield, because some product always remains dissolved in the mother liquor. Purity is checked after each cycle, most often by measuring the melting point, since a sharp melting range near the literature value indicates purity. Crystal growth can be started by scratching the inside of the flask or adding a seed crystal when a solution is supersaturated but no crystals form. Choosing a solvent with a suitable solubility gradient with temperature is the key decision in the procedure.
Going deeper: Recrystallization
Key ideas: After collecting crystals, rinse them with a few drops of ice-cold solvent and dry them fully before taking a melting point. Percent recovery = mass of pure product / mass of crude x 100, with typical recoveries of 60 to 85 percent. Use a mixed solvent pair (a good solvent plus an anti-solvent) when no single solvent works.
Common mistakes and connections: Students think the highest yield means the best purity, but they trade off. Using a damp product gives a low and broad melting point, which is misread as impurity. If crystals do not appear, do not boil off all solvent at once. Related to melting point depression, TLC for purity and crystallization theory.
Visualization of Recrystallization
Benzoic acid dissolves poorly in cold water but well in hot water, the solubility gradient that makes water a good recrystallization solvent. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Benzoic acid recrystallized from hot water gives a product that melts sharply at 122 C.
Frequently asked questions about Recrystallization
What is Recrystallization?
Repeated crystallization to remove impurities from a solid.
Is Recrystallization a beginner, intermediate or advanced topic?
Recrystallization is a intermediate topic in Analytical, Lab & Spectroscopy. It builds on the vocabulary of beginner topics in this area.
What should I learn before Recrystallization?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Fractional Distillation, Steam Distillation, Crystallization.
What comes after Recrystallization?
The next topics in Analytical, Lab & Spectroscopy are Solvent Extraction and Paper Chromatography.
Can I practise Recrystallization with quizzes?
Yes. The interactive lesson includes a self-check question on Recrystallization, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Recrystallization fits in Analytical, Lab & Spectroscopy
Recrystallization is topic 26 of 99 in Analytical, Lab & Spectroscopy.