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Recrystallization

  • Date Submitted: 04/06/2015 01:48 AM
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Recrystallization
When a chemical compound containing impurities is dissolved in a particular solvent, the crystal structure is broken up and the molecules disperse in the solvent, forming a solution.   This often occurs at high temperature, or in the boiling solvent system.   Upon cooling, and if the concentration of dissolved substance is sufficiently great, crystals of pure compound reform and precipitate out.   The impurities usually remain in solution, since the crystal structure regenerates itself, foreign materials are not incorporated into the precise crystal structure.   Crystallization is best when the solution is allowed to stand and cool rather slowly, since this allows for formation of pure crystals without trapping undesirable impurities.

Melting Points
The melting point of a solid material is a criterion of its purity.   In general, pure substances (compounds and elements) melt over a sharp range, usually 1-2 degrees.   Impure solids have imperfect crystalline lattices that break down at lower temperatures then normal, and therefore they usually melt lower then expected and over a wider range.   The melting range is actually the way a melting point should be reported.   It begins with the first appearance of liquid and ends when all of the solid has passed into the liquid phase.   Often, as the temperature rises, crystals will begin to collapse but have not actually melted yet.   This is known as sintering.   The first beads of liquid appear as the melting range is entered.   In order to obtain an accurate melting range determination, heating rate should be very gradual (1-2 deg. Per minute) as the melting range is traversed.   Rapid heating can be done initially, especially if the suspected mp is known, but heating should be slowed to a gradual rate when you are perhaps within 10-15 degrees of the beginning of the melting range.  

PROCEDURE:
Acetanilide is insoluble in cold water but is soluble in hot water.   Therefore water is a good recrystallization...

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