Overview
This lecture covers the procedure and techniques for recrystallizing caffeine, including practical tips, safety, maximizing yield, and calculating percent recovery.
Recrystallization Procedure
- Begin with a known mass of crude caffeine (508 mg or 0.508 g).
- Dissolve the caffeine in the minimal amount of hot solvent, here ethanol (start with ~15 mL).
- Add the solvent to the solid before heating to prevent "oiling out" (undesired melting).
- Stir constantly during heating; solid should dissolve as solvent boils.
- If some solid remains, add solvent dropwise using a pipette.
- Once dissolved, remove from heat and let cool undisturbed at room temperature for best crystal formation.
- If crystals do not form, place the solution in an ice bath or scratch the glass with a rod to create a nucleation site.
Filtration and Isolation
- Set up a vacuum filtration apparatus using a Buchner funnel, vacuum flask, vacuum hose, and filter paper.
- Wet filter paper with ethanol to adhere it to the funnel, preventing loss of solid through the sides.
- Pour or decant the cooled mixture onto the filter while vacuum is on to isolate crystals.
- Optionally, rinse crystals with cold ethanol to recover more product with minimal loss.
- Spread crystals on filter paper under vacuum for 5–10 minutes to dry.
Yield Calculation and Maximizing Recovery
- Weigh the dried crystals (final mass example: 373 mg or 0.373 g).
- Calculate percent recovery: (mass after recrystallization / initial mass) × 100.
- Some material loss is expected during transfer, filtering, or incomplete crystallization.
- Recrystallize remaining material from all vessels to maximize yield if desired.
At-Home Application
- Try recrystallization at home with sugar or salt using minimal hot water and cooling to form crystals.
- Be cautious with hot liquids; use stove or microwave as available.
- Observe differences in crystal shape between sugar (organic) and salt (inorganic).
Key Terms & Definitions
- Recrystallization — purification method based on differential solubility of a solid in hot versus cold solvent.
- Percent Recovery — percentage of pure product obtained after purification relative to the starting amount.
- Oiling Out — phenomenon where the substance melts instead of dissolving, ruining crystal formation.
- Nucleation Site — a point (scratch or seed crystal) where crystal growth initiates.
- Decanting — pouring off liquid while leaving solid behind.
- Vacuum Filtration — technique using reduced pressure to separate solids from liquids efficiently.
- Buchner Funnel — a cylindrical, flat-bottomed funnel used for vacuum filtration.
Action Items / Next Steps
- Calculate percent recovery for the caffeine experiment: (final mass / 0.508 g) × 100.
- Attempt recrystallization at home with sugar and salt, observing and comparing the resulting crystals.
- Practice using minimal solvent and proper cooling for optimal crystal formation.