Recrystallization is a purification technique used to turn an impure solid into cleaner, well-formed crystals. It matters because many chemical products, from aspirin to research samples, must be separated from colored impurities, leftover reactants, or byproducts. The method relies on differences in solubility at high and low temperature.
A good recrystallization can greatly improve purity without needing complicated equipment.
The basic process is to dissolve the impure solid in the smallest amount of hot solvent, remove insoluble material by hot filtration, then cool the solution slowly so the desired compound forms crystals. Ideally, the target compound is very soluble when hot but only slightly soluble when cold. Many soluble impurities remain dissolved in the cold mother liquor, while insoluble impurities are removed earlier by filtration.
The purified crystals are collected by vacuum filtration, washed with cold solvent, and dried before measuring yield and purity.
Key Facts
- Recrystallization works best when the solute has high solubility in hot solvent and low solubility in cold solvent.
- Percent recovery = (mass of purified crystals / mass of impure starting solid) x 100%.
- Use the minimum amount of hot solvent needed to dissolve the solid to avoid losing product in the mother liquor.
- Hot gravity filtration removes insoluble impurities before crystals form.
- Slow cooling usually produces larger, purer crystals because molecules arrange more orderly in the crystal lattice.
- If c_hot is solubility at high temperature and c_cold is solubility at low temperature, the maximum crystallized amount is approximately (c_hot - c_cold) x solvent volume.
Vocabulary
- Recrystallization
- A purification method in which an impure solid is dissolved in hot solvent and then crystallized as the solution cools.
- Solubility
- The maximum amount of a substance that can dissolve in a given amount of solvent at a specific temperature.
- Mother liquor
- The liquid solution left behind after crystals form, often containing dissolved impurities and some dissolved product.
- Hot filtration
- A filtration step performed while the solution is hot to remove insoluble impurities without crystallizing the product too early.
- Nucleation
- The first formation of tiny crystal particles that act as starting points for further crystal growth.
Common Mistakes to Avoid
- Using too much solvent, which is wrong because extra solvent keeps more product dissolved after cooling and lowers percent recovery.
- Cooling the solution too quickly, which is wrong because rapid crystallization can trap impurities inside the crystal lattice.
- Letting the hot solution cool during filtration, which is wrong because product may crystallize in the funnel or filter paper and be lost.
- Washing crystals with warm solvent, which is wrong because warm solvent can dissolve the purified crystals and reduce the final yield.
Practice Questions
- 1 A student starts with 5.00 g of impure acetanilide and obtains 3.80 g of dry crystals after recrystallization. Calculate the percent recovery.
- 2 A compound has a solubility of 18.0 g per 100 mL in hot ethanol and 2.0 g per 100 mL in cold ethanol. If 50.0 mL of ethanol is used, estimate the maximum mass that can crystallize on cooling.
- 3 Explain why a good recrystallization solvent should dissolve the desired compound when hot but not when cold, while impurities either stay dissolved or are removed by filtration.