Chem 233

Lab 1, Part 2: Recrystallization

What is recrystallization?

Purification technique for solids based on controlled solid -> solution -> solid phase transition. Depending on the nature of the impurities, may be used to “clean up” an impure sample of a compound or to separate a compound from a complex mixture. Solubility is the critical physical property.

Solutions and Saturation

• Solubility depends on the solute, the solvent, and the temperature. A given solute has a specific solubility in a given solvent at a given temperature. The solute will dissolve in the solvent until its solubility limit is reached, at which point the solution is “saturated” and no more solute will dissolve. • Solubility usually increases with increasing temperature and decreases with decreasing temperature. • A saturated solution becomes “supersaturated” when cooled, and solute must exit the solution as its solubility decreases.

Recrystallization Basics

1. An impure solid is dissolved in an appropriate hot solvent. 2. Solution is slowly cooled, causing crystals to form and impurities to stay in the solvent.

Recrystallization Concepts

Crystals are formed by cooling supersaturated solution cool Equilibrium: Xsolution Xsolid

heat

Crystal lattice is highly ordered

Impurities are excluded with slow cooling. Cooling too fast traps impurities. Recrystallization ≠ precipitation

Why cooling slowly is better

Slow cooling gives time for crystal lattice to form without impurities Forms larger crystals

Quick cooling causes incorporation of impurities into crystal Forms smaller crystals

Finding the right solvent

A good solvent will: 1. Not react with desired compound 2. When hot, dissolve a large amount of desired compound 3. When cold, dissolve as little of desired compound as possible 4. Be volatile

What kinds of impurities exist?

1. Soluble: Impurities that dissolve in the hot solvent and remain dissolved in the cold solvent 2. Insoluble: Impurities that...