To obtain pure dry crystals, the most common method involves recrystallization, a process of dissolving, purifying, and reforming crystals from a solution. Here's a step-by-step guide:
1. Dissolving the Impure Solid:
- Choose a suitable solvent in which your compound is soluble at high temperatures but relatively insoluble at low temperatures. Water, ethanol, acetone, and ethyl acetate are common choices.
- Heat the solvent to near its boiling point.
- Slowly add the impure solid to the hot solvent, stirring continuously.
- Continue adding the solid until no more dissolves, even with heating. You've now created a saturated solution.
- If insoluble impurities remain, perform a hot filtration (see step 2).
2. Hot Filtration (Optional but Recommended):
- If insoluble impurities are present (e.g., undissolved solid particles), filter the hot solution through fluted filter paper or a filter paper in a heated funnel. This removes the unwanted solid material while the desired compound remains dissolved.
- A heated filtration apparatus can prevent the compound from crystallizing out during the filtration process.
3. Crystallization:
- Allow the hot, saturated solution to cool slowly. Slow cooling promotes the formation of larger, purer crystals. Avoid disturbing the solution during this process.
- Crystal formation can be encouraged by "seeding" the solution with a small, pure crystal of the desired compound.
- If crystallization does not occur readily, try scratching the inside of the flask with a glass rod or cooling the solution further in an ice bath.
4. Isolation of Crystals:
- Once crystallization is complete, collect the crystals by filtration. Use a Buchner funnel and vacuum filtration for efficient separation.
- Wash the crystals with a small amount of cold solvent to remove any remaining impurities. Using cold solvent minimizes the amount of desired product that dissolves during the washing step.
5. Drying the Crystals:
- There are several methods to dry the crystals:
- Air Drying: Spread the crystals on filter paper and allow them to air dry. This is a slow but gentle method.
- Vacuum Desiccator: Place the crystals in a vacuum desiccator containing a desiccant such as anhydrous calcium chloride or silica gel. The desiccant absorbs any remaining solvent. Apply a vacuum to speed up the drying process.
- Oven Drying: Dry the crystals in a low-temperature oven (below the compound's melting point) for a short period. Be careful not to overheat the crystals, which can cause decomposition or melting.
Key Considerations for Purity:
- Solvent Choice: Selecting the right solvent is crucial. It should dissolve the compound well at high temperatures but poorly at low temperatures, and it should not react with the compound.
- Slow Cooling: Slow cooling is vital for forming large, pure crystals. Rapid cooling can lead to the formation of small, impure crystals.
- Washing: Washing the crystals with a small amount of cold solvent removes surface impurities without dissolving a significant amount of the desired product.
Troubleshooting:
- Oiling Out: If the compound separates as an oil instead of crystals, try adding a small amount of a different solvent (a "miscible solvent") to the solution or seeding with a crystal of the desired product.
- No Crystals Forming: Scratch the inside of the flask with a glass rod, seed the solution, or cool the solution further.
By following these steps carefully, you can obtain pure, dry crystals of your desired compound.
