Buchner Filtration

Use a Buchner funnel and side-arm vacuum flask to separate a solid from a liquid quickly. The slurry is pulled through wetted filter paper by reduced pressure, leaving the solid on the paper for washing and drying.

1. Assemble apparatus: retort stand and clamp, side-arm vacuum flask, rubber adapter, Buchner funnel, vacuum source (e.g. water aspirator), and appropriate filter paper.
2. Check glassware for cracks; seat the funnel firmly in the adapter; clamp the flask securely; connect vacuum tubing to a trap and then to the vacuum source.
3. Choose filter paper that covers all funnel holes; moisten it with a small volume of the filtrate solvent so it seals flat against the plate.
4. Start gentle vacuum; pour the well-stirred slurry into the funnel in portions, keeping the liquid level below the rim to avoid bypass.
5. Rinse the original vessel with a small amount of cold solvent and pour over the cake to transfer remaining solids.
6. Allow the cake to compact; wash the cake with minimal cold solvent to remove impurities while limiting product loss.
7. Draw air through the cake for several minutes to pre-dry the solid; if needed, press gently with a spatula wrapped in filter paper to improve drainage.
8. Break the vacuum by opening to air before turning off the pump; remove the funnel; lift the paper and transfer the solid to a drying surface or oven.

Buchner Funnel Filtration Technical Guide - Chemical Thinking Laboratory:

Buchner Funnel - University of Bath:

• Use a fritted glass funnel instead of paper when compatible with the solvent and product.
• Precool the solvent and funnel when filtering cold crystallization mixtures to reduce product dissolution.
• Add a thin diatomaceous earth bed for very fine particulates to prevent paper clogging.

• Wear safety glasses, lab coat, and appropriate gloves; work in a fume hood if solvents are volatile.
• Inspect and clamp the vacuum flask; avoid using chipped or cracked glass to reduce implosion risk.
• Always break vacuum before shutting off the pump to prevent backflow and splashback.
• Use a vacuum trap between the flask and the pump to protect the pump from solvent ingress.
• Do not overfill the funnel; keep liquid below the rim to prevent spills and bypassing the paper.
• Verify solvent compatibility with filter paper or frit; avoid hot solvents that can soften adapters.
• Dispose of filtrates and washings following chemical waste guidelines.

• Why wet the filter paper before pouring the slurry? (To seal the paper to the plate and prevent solids from slipping underneath.)
• Why use cold solvent for washing crystals? (To minimize product dissolution while removing impurities.)
• Why break the vacuum before turning off the pump? (To prevent backflow of liquid into the apparatus and sudden splashing.)
• How do you choose filter pore size or paper grade? (Match to particle size; finer grades retain small particles but slow the flow.)
• What does channeling in the cake do to purity and yield? (It allows liquid to bypass parts of the cake, reducing washing efficiency and potentially trapping impurities.)