Here’s a explanation:
Co-precipitation
- Happens while the main solid is forming.
- Impurities get trapped inside or on the surface of the solid.
- This makes the precipitate impure.
Example:
When making barium sulfate (BaSO₄), some sodium or nitrate ions may get stuck in it.
How to avoid it:
- Add reagents slowly.
- Use hot and dilute solutions.
- Let the solid stand (digest) before filtering.
- Wash the precipitate well.
Post-precipitation
- Happens after the main solid has already formed.
- A new, unwanted solid forms on the surface of the first one.
- This also makes the result impure.
Example:
After forming barium sulfate (BaSO₄), calcium sulfate (CaSO₄) may later form and stick to it.
How to avoid it:
- Filter soon after the solid forms.
- Don’t let the mixture stand too long.
In short:
- Co-precipitation: Impurities form with the main solid.
- Post-precipitation: Impurities form after the main solid.