EXPERIMENT 2: Recrystallization and Melting Point Recrystallization (or Crystallization) is a technique used to purify solids. This procedure relies on the fact that solubility increases as temperature increases (you can dissolve more sugar in hot water than in cold water). As a hot, saturated solution cools, it becomes supersaturated and the solute precipitates (crystallizes) out. In a recrystallization procedure, an impure (crude) solid is dissolved in a hot solvent. As this solution is cooled, the pure product crystallizes out and the impurities stay dissolved.
General Recrystallization Procedure: 1) Choose an appropriate solvent(s) • product is very soluble in it at high temperatures • product is not soluble in it at low temperatures • impurities are either soluble at all temps or insoluble at all temps (can be filtered off) 2) Dissolve impure solid • weigh out crude solid and record its mass (also take a melting point for reference) • add a boiling chip or boiling stick (otherwise, it may "bump" and spill) • use a minimum amount of hot solvent (Add a bit, heat/swirl. Not dissolved? Add more!) NOTE: IF YOU USE TOO MUCH SOLVENT, YOU WILL GET NO CRYSTALS! 3) ONLY IF NEEDED: Decolorize • most pure compounds are white and give colorless solutions (looks like water) • to remove color (really trace contaminants), add activated charcoal (adsorbs the impurities) 4) ONLY IF NEEDED: Gravity filter to remove insoluble materials (including charcoal!) • use fluted filter paper and a hot, stemless funnel NOTE: IF THE SOLUTION COOLS, PRODUCT WILL CRYSTALLIZE OUT & BE LOST! • use a small amount of hot solvent to rinse flask, filter 5) Crystallize solute (Finally! This is the RECRYSTALLIZATION part!) • cool the solution slowly: hot (boiling) → room temperature → 0 ˚C (put in ice water bath) NOTE: THIS GIVES LARGE, PURE CRYSTALS & LEAVES IMPURITIES IN SOLUTION • may need to scratch glass with a stirring rod or add a seed crystal to start crystallization 6) Collect the pure crystals • quickest method is vacuum filtration (Büchner funnel, water aspirator and trap) • the impurities will stay dissolved in the solvent that is being removed (hopefully) • rinse the pure crystals with a small amount of cold solvent (don't redissolve the crystals!) • OPTIONAL: a second crop of crystals can be obtained from the filtrate (mother liquor) 7) Analyze product • let crystals dry thoroughly (ideally, this means overnight at room temperature or under vacuum) • record mass (how much of your original solid did you recover? % recovery?) • record melting point range (did you succeed in PURIFYING your solid?)
EXPERIMENT 2: Recrystallization and Melting Point
Melting Point is:
•a constant physical property of a solid (the temperature at which it turns to a liquid) •reported in the lab as a melting point range (usually 1–2 ˚C wide), not as a single temperature •used for: 1) characterization of a compound (also IR, NMR, mass spec.) 2) identification of an unknown (compare with known mp's) 3) determination of purity If a sample of a compound contains impurities, its mp is usually depressed (lowered) and the range is broadened. Therefore, a narrow mp range (1–2 ˚) suggests the sample is a pure compound.
How can this fact help us in the lab? 1) take a "mixed melting point" of an unknown (if you mix two samples - typically a known and an unknown - and the mp doesn't change, then the two samples are the same compound) 2) take a mp before and after a purification technique (like recrystallization, this week's lab)
Method for measuring a Melting Point range: • LOAD sample in a capillary (mp) tube (pack the sample down into the CLOSED end of tube) • SLOWLY HEAT the sample and closely WATCH both the crystals and the temperature • at the first sign of melting (crystals look wet), RECORD the temperature (first number of reported range) • when the last crystal melts, RECORD the temperature (second number of reported range) • for best results: use a...
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