Recrystallization Lab New Techniques Recrystallization method of purifying

Recrystallization Lab

New Techniques • Recrystallization: method of purifying an organic solid • Gravity Filtration: method of removing insoluble impurities from recrystallization solution • Suction Filtration: method of isolating pure solid from liquid (filtrate) using vacuum

In Lab • Recrystallization – Determine best solvent for recrystallization – Weigh crude solid – Recrystallize – Weigh purified product

After Lab • Let recrystallized solid air dry on shelf. • Weigh dry crystals and take a melting point (next week). • Calculations: % recovery: [mass crude/mass recrystallized] x 100 • Conclusions: Discuss effectiveness of recrystallization process in terms of purity and percent recovery. Discuss role/attributes of solvent, sources of error, etc.

Theory of Recrystallization Pure solid: Tight crystal lattice Impurities disrupt the crystal lattice

Steps in Recrystallization 1. Dissolve crude solid in hot solvent (saturated solution) 2. Let solution cool to room temperature, so that crystal lattice reforms. 3. Cool solution in an ice bath. 4. Suction filter the pure solid, leaving impurities in solution. 5. Let solid air dry to remove traces of solvent.

Recrystallization 1. Add hot solvent until solid dissolves: (use minimum amount of hot solvent)

Recrystallization 2. Let solvent cool slowly to room temperature, so crystal lattice can reform. 3. Cool in ice bath to make sure that the maximum amount of solid has crystallized. (Suction filtration gets cold!)

Recrystallization 4. Suction filter solid away from impurities. 5. Let crystals air dry.

An Ideal Recrystallization Solvent Should • should dissolve all of the compound when the solvent is hot (boiling). • should dissolve none of the compound when the solvent is at room temperature. • should have different solubilities for the compound and the impurities. • should have a lower boiling point than the melting point of the compound. • should have a fairly low boiling point • should be cheap, non-toxic, non-reactive, and nonsmelly

Pour a small amount of the hot solvent into the flask containing the solid.

Swirl the flask to dissolve the solid.

Place the flask on the hot plate to keep the solution warm.

If the solid is still not dissolved, add a tiny amount more solvent and swirl again.

When the solid is all in solution, set it on the bench top. Do not disturb it!

Hot Gravity Filtration HOT Gravity filtration is the method of choice to remove solid impurities from an organic liquid. The "impurity" can be a activated carbon or an insoluble impurity. Gravity filtration can be used to collect solid product, although generally vacuum filtration is used for this purpose because it is faster.

After a while, crystals should appear in the flask.

You can now place the flask in an ice bath to finish the crystallization process.

Suction Filtration Apparatus.