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3AL Lecture 5 - Purification of Solids Three general...

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Purification of Solids Three general methods: 1. Recrystallization 2. Sublimation/deposition (solid to gas/gas to solid) 3. Chromatography
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Knowledge of intermolecular forces is key! The Process 1. Choose the right equipment for the job. A. The glassware Erlenmeyer flask Test tube (reaction tube) Both of these vessels have a large glass surface area and constricted opening. Richard Erlenmeyer
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1. Choose the right equipment for the job. B. The heating source Sand bath Water bath Oil bath Never use a Bunsen burner or any other open flame to heat a solution in the organic chemistry laboratory!
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2. The Solvent The goal is to find a solvent in which the solid is soluble at a higher temperature and less soluble or insoluble at a lower temperature. Recall: Solubility generally increases with increasing temperature due to the -T Δ S term in Δ G of mixing.
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2. The Solvent, continued Common solvents for recrystallizing organic chemicals include water, methanol, ethanol, acetone, and hexane. The boiling range of typical solvents used for recrystallization is > 50 < 100 o C. Low boiling solvents evaporate easily and don’t allow for much energy input. High boiling solvents make it difficult to remove residual solvent that is adsorbed onto the surface of the crystals. The boiling point of the solvent should, if possible, be lower than the melting point of the solid to avoid melting the compound.
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2. The Solvent, continued Choosing which solvent to use depends on which one of two possible scenarios is applicable. A. The solubility of your solid is known for a variety of solvents and temperatures. B. The solubility of your solid is unknown or you do not know the structure of your solid.
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