Overview
This lecture covers the step-by-step process of recrystallization, a technique used by chemists to purify crystalline solids, using naphthalene as an example.
Introduction to Recrystallization
- Recrystallization is a common laboratory technique to purify crystalline solids.
- Successful purification depends on selecting and using appropriate solvents and methods efficiently.
Steps of Recrystallization
- There are five main steps: choosing a solvent, dissolving the sample, removing impurities, crystallizing the product, and collecting/drying the crystals.
- The procedure can use one solvent or a mixture of two, depending on compound solubility.
Solvent Selection
- An appropriate solvent is one in which the compound is insoluble at room temperature but soluble at the solvent’s boiling point.
- Perform solubility tests by adding the compound to different solvents at room temperature and after heating.
- Example: Naphthalene is insoluble in water and ethanol at room temperature, but soluble in hot ethanol.
Dissolving the Sample
- Use a minimal amount of hot solvent to dissolve the sample, ensuring high crystal recovery.
- Set aside a small seed crystal before dissolving, which helps in case crystallization does not start spontaneously.
Removal of Impurities
- Decolorizing (removing colored impurities) is done with a small amount of activated charcoal, added to hot but not boiling solution.
- Hot filtration with a filtering agent removes charcoal and any undissolved solids.
Inducing Crystallization
- Cool the filtered solution undisturbed, first to room temperature, then in an ice bath to maximize crystal yield.
- If crystals do not form, scratch the glass or add a seed crystal to induce crystallization.
Collecting and Washing Crystals
- Use vacuum filtration to collect crystals and wash them with cold solvent to remove impurities.
- Crystals are dried using air, a desiccator, oven, or vacuum line, depending on the compound.
Two-Solvent Recrystallization
- If no single ideal solvent exists, use a mixture of two miscible solvents, chosen so that the compound is soluble in one and insoluble in the other.
- Dissolve the sample in minimal hot solvent one, add solvent two dropwise until cloudiness, then add more hot solvent one until clear, and cool to crystallize.
Key Terms & Definitions
- Recrystallization — purification technique for solids based on solubility differences at different temperatures.
- Hot Filtration — method to remove insoluble impurities while keeping the compound dissolved.
- Decolorization — process of removing colored impurities using activated charcoal.
- Seed Crystal — a small, pure crystal used to induce crystallization in a solution.
Action Items / Next Steps
- Read your lab manual and supplementary materials before attempting recrystallization in the lab.
- Prepare necessary solvents and equipment for solubility testing and recrystallization.
- Perform solubility tests to identify suitable solvents for your specific compound.