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Suction (Vacuum) Filtration Diagram (Büchner Funnel, Filter Flask, Trap)
Suction filtration separates a solid from a liquid quickly. Use it to collect crystals or precipitates and to remove drying agents and other solids.
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Equipment
- Büchner funnel and filter paper
- Heavy-walled filter flask
- Rubber adapter
- Trap (safety flask)
- Vacuum pump (or aspirator)
Setup tips
- Use filter paper that covers all the holes without climbing the wall. Wet it with the solvent and pull it flat under vacuum before pouring.
- Put a trap between the filter flask and the pump so the filtrate cannot reach the pump, and aspirator water cannot flow back into the filtrate.
- To stop, first let air into the filter flask (disconnect the hose or open a tap), then turn off the pump or aspirator, to prevent suck-back.
- Use a heavy-walled filter flask rated for vacuum, and never a cracked one.
- To wash the crystals, release the vacuum, add a little cold solvent, then pull the vacuum again.
Common mistakes
- Turning off the pump or aspirator while still under vacuum (suck-back).
- Filter paper lifting so solid passes into the filtrate.
- Pulling a vacuum on a thin-walled Erlenmeyer flask.
Always follow your institution's safety rules and your supervisor's instructions.
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