A Buchner funnel is used for filtration by performing vacuum (suction) filtration, which separates a solid from a liquid quickly by pulling the liquid through a flat filter paper under reduced pressure. A Buchner funnel is a flat-bottomed funnel with a perforated plate that holds a circle of filter paper. It sits in a rubber adapter on a heavy-wall side-arm flask connected to a vacuum source, so suction draws the liquid (filtrate) into the flask while the solid stays on the paper. Buchner filtration is the standard way to collect crystals and precipitates. Edu Lab China manufactures the lab glassware used for filtration.
| How is a Buchner funnel used for filtration? A Buchner funnel is used for vacuum filtration: a filter-paper circle is laid flat on the funnel’s perforated plate, wetted with solvent and sealed with suction, then the solid-liquid mixture is poured in. A vacuum source connected to the heavy-wall side-arm flask pulls the liquid through, leaving the solid on the paper, which is then washed and dried by continued suction. Always break the vacuum before switching off the pump to prevent suck-back. Buchner filtration is faster than gravity filtration and is used to collect crystals and precipitates. Browse lab glassware and chemistry lab equipment for the apparatus. |
What is a Buchner funnel?
A Buchner funnel is a flat-bottomed laboratory funnel with a perforated plate, used with filter paper and a vacuum source to filter a solid from a liquid by suction. A Buchner funnel is made of porcelain, borosilicate glass or polypropylene, and is named after the chemist Ernst Buchner. It differs from an ordinary conical funnel because its flat perforated plate supports a flat filter paper, which allows vacuum filtration rather than slow gravity filtration. The smaller version with sloping sides, used for small quantities, is called a Hirsch funnel.
What equipment do you need for Buchner funnel filtration?
Buchner funnel filtration needs a Buchner funnel, filter paper, a heavy-wall side-arm filtration flask, a rubber adapter, vacuum tubing and a vacuum source, ideally with a trap bottle. Each component has one job in pulling the liquid through the solid under vacuum, and the side-arm flask must be vacuum-rated rather than an ordinary conical flask. The table lists the equipment, its function, a typical specification and its priority.
| Component | Function | Typical specification | Priority |
| Buchner funnel | Holds the filter paper on a perforated plate | Porcelain/glass/PP; 70-150 mm | Essential |
| Filter paper (circle) | Retains the solid | Sized to cover all holes; e.g. Whatman Grade 1 (11 um) | Essential |
| Side-arm (Buchner) filtration flask | Collects the filtrate under vacuum | Heavy-wall borosilicate; side arm | Essential |
| Rubber adapter / cone | Seals the funnel to the flask | Neoprene or rubber cone | Essential |
| Vacuum tubing | Connects the flask to the vacuum | Heavy-wall vacuum hose | Essential |
| Vacuum source | Provides suction | Water aspirator or vacuum pump | Essential |
| Trap / Woulff bottle | Prevents suck-back into the pump | Secondary heavy-wall flask | Recommended |
| Clamp and stand | Secures the flask | Steel stand and clamp | Essential |
Caption: The equipment needed for Buchner funnel vacuum filtration, with the function and priority of each component.
How is a Buchner funnel used for filtration, step by step?
A Buchner funnel is used for filtration in ten steps: assemble the apparatus, seal the wetted filter paper with suction, pour the slurry, draw the liquid through, wash the solid, dry it under suction, and break the vacuum before switching off the pump. The decision rule below captures the two steps people most often get wrong, and the procedure applies them in order.
The Seal-and-Bleed Rule for Buchner filtration: always (1) seal the filter paper before pouring — wet the paper with the solvent and apply suction so it lies flat over every hole, or solid will bypass it; and (2) break the vacuum before switching off the pump — open the bleed valve or disconnect the tubing at the flask first, so that water or contaminant cannot be sucked back from the pump into the filtrate.
1. Assemble the heavy-wall side-arm flask, Buchner funnel and rubber adapter, and clamp the flask to a stand.
2. Connect the flask side arm to the vacuum source through a trap bottle.
3. Place a filter-paper circle flat on the plate so it covers all the holes without curling up the sides.
4. Wet the filter paper with a little of the same solvent and switch on the vacuum to seal it flat.
5. Swirl and pour the solid-liquid slurry steadily into the centre of the funnel.
6. Let the vacuum pull the liquid through, leaving the solid on the filter paper.
7. Wash the solid with small portions of cold solvent while the suction continues.
8. Keep the vacuum on to pull air through and partly dry the solid.
9. Break the vacuum at the flask, by opening the bleed or disconnecting the tubing, before switching off the pump.
10. Lift out the funnel and scrape the solid off the paper for further drying.
Caption: The ten-step procedure for using a Buchner funnel for vacuum filtration, applying the Seal-and-Bleed Rule.
Which filter paper should you use in a Buchner funnel?
Use a filter paper whose particle retention matches the solid, cut to cover the plate exactly: Whatman Grade 1 (11 micrometres) for general teaching, a coarser fast grade for gelatinous precipitates, and a fine grade for small crystals. Hardened, acid-treated filter papers are particularly suited to Buchner filtration because their tough, high-wet-strength surface withstands suction and scraping. The table compares common filter-paper grades and their particle retention.
| Filter paper (Whatman grade) | Particle retention | Flow rate | Best for |
| Grade 1 | 11 micrometres | Medium | General teaching and qualitative work |
| Grade 4 | ~20-25 micrometres | Very fast | Coarse and gelatinous precipitates |
| Grade 5 | 2.5 micrometres | Slow | Fine precipitates |
| Grade 602h | 2 micrometres | Slow | Finest qualitative filtration |
| Hardened (e.g. Grade 54) | Varies by grade | Medium | Buchner/vacuum filtration (high wet strength) |
| Quantitative ashless (40-44) | 8 down to 2.5 micrometres | Varies | Gravimetric analysis |
Caption: Filter-paper grades for Buchner funnel filtration, with particle retention and best use. Hardened papers are preferred for the suction and scraping of vacuum filtration.
What types and materials of Buchner funnel are there?
Buchner funnels are made of porcelain, borosilicate glass or polypropylene, with sintered-glass (fritted) funnels and the small Hirsch funnel as variants. The material is chosen for chemical resistance, heat tolerance and whether a built-in glass frit is wanted instead of paper. The table compares the types and what each is best for.
| Type / material | Feature | Best for | Note |
| Porcelain Buchner | Robust perforated plate | General use; hot or solvent work | Classic; autoclavable |
| Borosilicate glass Buchner | Transparent; chemically inert | Where visibility is needed | Often paired with a sintered disc |
| Sintered-glass (fritted) funnel | Built-in glass frit; no paper | Fine, defined porosity | Porosity grades per ISO 4793 |
| Polypropylene (PP) Buchner | Unbreakable plastic | Hydrofluoric acid; field use | Not for hot or solvent work |
| Hirsch funnel | Small, with sloping sides | Small quantities (under ~1 g) | Microscale recrystallisation |
Caption: The types and materials of Buchner funnel, with the feature and best use of each.
When should you use Buchner (vacuum) filtration instead of gravity filtration?
Use Buchner (vacuum) filtration when the goal is to collect a solid quickly, and use gravity filtration when the goal is to collect a clear filtrate or to filter a hot solution. Buchner filtration is faster because suction pulls the liquid through, and it partly dries the solid, which makes it the method for collecting recrystallised crystals and precipitates. Gravity filtration, using a fluted paper in a conical funnel, suits hot filtration and removing a drying agent. The table compares the two.
| Factor | Buchner (vacuum) filtration | Gravity filtration |
| Main goal | Collect the solid quickly | Collect the filtrate |
| Speed | Fast (suction-driven) | Slow |
| Funnel | Buchner (flat perforated plate) | Conical funnel with fluted paper |
| Typical use | Recrystallised crystals; precipitates | Hot filtration; removing drying agent |
| Flask | Heavy-wall side-arm flask | Any conical flask or beaker |
| Drying | Air pulled through partly dries the solid | No drying effect |
Caption: Buchner vacuum filtration compared with gravity filtration, by goal, speed and typical use.
What specifications should you check before buying a Buchner funnel?
Before buying a Buchner funnel, check the material, plate diameter, matching filter-paper size, the vacuum-rated flask, and, for glass funnels, the sintered-disc porosity. The most important point is that the filtration flask is heavy-walled and vacuum-rated, because an ordinary conical flask can implode under suction. The table lists the specifications to verify and a typical value or reference.
| Specification | What to check | Typical value / reference |
| Funnel material | Porcelain, borosilicate or PP | Match to chemical and heat |
| Plate diameter | Sized to the solid quantity | 70-150 mm common |
| Filter-paper size | Matches the plate exactly | Covers all holes, no curling |
| Filtration flask | Heavy-wall, side-arm, vacuum-rated | Borosilicate Buchner flask |
| Sintered-disc porosity (glass) | Defined pore grade | ISO 4793 grades (e.g. P16-P160) |
| Adapter fit | Cone or bung seals to the flask | Correct neoprene cone size |
Caption: Specifications to verify before buying a Buchner funnel and filtration flask, with a typical value or reference for each.
What are the safety requirements for Buchner funnel filtration?
The safety requirements for Buchner funnel filtration are a heavy-wall vacuum-rated flask, a clamped apparatus, a trap between flask and pump, breaking the vacuum before switching off, eye protection and crack-free glass. Vacuum filtration puts the glassware under reduced pressure, so an unsuitable or damaged flask can implode. The table states each requirement, why it matters and the action.
| Safety requirement | Why it matters | Action |
| Use a heavy-wall filtration flask | A thin flask can implode under vacuum | Never use an ordinary conical flask |
| Clamp the flask to a stand | Tubing pull can tip the apparatus | Clamp before applying vacuum |
| Fit a trap between flask and pump | Prevents suck-back of water or contaminant | Use a Woulff or trap bottle |
| Break the vacuum before switching off | Prevents suck-back into the flask | Open the bleed or disconnect the tubing first |
| Wear eye protection | Implosion and splash risk | Goggles; safety screen for large work |
| Inspect glass for cracks | Cracked glass fails under vacuum | Reject chipped or cracked flasks |
Caption: The safety requirements for Buchner funnel vacuum filtration, with the reason and action for each.
“The two things that go wrong on a school bench are an ordinary conical flask used under vacuum, which can implode, and the pump switched off before the line is bled, which sucks tap water back into the product. A proper side-arm flask and a trap bottle prevent almost every Buchner filtration accident I have seen.” — Arvind Kumar, Lab Equipment Specialist (12+ years), reviewer of this guide.
How does Buchner filtration match the lab level?
Buchner filtration scales with the laboratory level: a porcelain funnel with a water aspirator for high school, a Buchner or Hirsch funnel with a vacuum pump and trap for college, and sintered-glass funnels of defined porosity for university research. Matching the apparatus to the level keeps a school set-up simple while giving research labs control over fine filtration. Map the apparatus to the practical syllabus in use, such as Gaokao (NCEE), Cambridge/IB or a Ministry of Education curriculum, and confirm the current syllabus edition before citing it in a specification.
| Level | Typical Buchner set-up | Note |
| High school | Porcelain Buchner + side-arm flask + water aspirator | Demonstrate vacuum filtration |
| College / pre-university | Buchner or Hirsch + vacuum pump + trap | Recrystallisation practicals |
| University (undergraduate) | Glass or sintered funnels; defined porosity | Organic synthesis and quantitative work |
| University (research) | Sintered-glass funnels, range of porosities; vacuum line | Critical and fine filtration |
Caption: How Buchner filtration apparatus matches the laboratory level, from school to research.
Vendor evaluation criteria for Buchner funnels and filtration flasks
Evaluate Buchner-funnel and filtration-flask vendors on a weighted scorecard that prioritises a vacuum-rated heavy-wall flask and sound funnel build over price. The weighting below puts the greatest emphasis on the flask and funnel quality, because those determine whether the apparatus is safe under vacuum and filters cleanly.
| Criterion | What to assess | Weighting (%) |
| Vacuum-rated heavy-wall flask | Side-arm flask withstands vacuum | 25% |
| Funnel material and build | Porcelain/borosilicate grade; even plate | 25% |
| Size and filter-paper fit | Plate matches available filter papers | 15% |
| Sintered-disc porosity (glass) | Defined ISO 4793 grade | 15% |
| Adapter and seal quality | Cone or bung seals reliably | 10% |
| Packing and export logistics | Breakage-safe packing; lead time | 10% |
Caption: A weighted vendor evaluation scorecard for Buchner funnels and filtration flasks, totalling 100%.
Common mistakes in Buchner funnel filtration
Mistake 1: Using an ordinary conical flask under vacuum
An ordinary thin-walled conical flask can implode when placed under vacuum, scattering glass. Always use a heavy-wall side-arm filtration flask that is rated for vacuum work.
Mistake 2: Using filter paper that is too large for the plate
Filter paper that curls up the sides of the funnel lets the slurry bypass the paper, so the solid passes into the filtrate. Cut or select a paper circle that lies flat and covers all the holes without turning up the edges.
Mistake 3: Not wetting and sealing the paper before pouring
Pouring the slurry onto dry, unsealed paper lets liquid and solid run under the paper. Wet the paper with the solvent and apply suction first so it seals flat over the plate.
Mistake 4: Switching off the pump before breaking the vacuum
Turning off a water aspirator or pump while the line is still under vacuum sucks water or oil back into the filtrate, contaminating the product. Break the vacuum at the flask first by opening the bleed or disconnecting the tubing.
Mistake 5: Choosing the wrong filter-paper grade
A paper that is too coarse lets fine solid through, while one that is too fine clogs and slows the filtration. Match the filter-paper particle retention to the size of the solid, using a finer grade for small crystals and a faster grade for coarse precipitates.
Mistake 6: Omitting the trap between the flask and the water aspirator
Without a trap, a drop in water pressure at the aspirator pulls tap water straight back into the filtration flask. Fit a trap or Woulff bottle between the filtration flask and the vacuum source.
Related resources and category pages
• Laboratory appliances (vacuum and heating equipment)
• Full educational and scientific lab equipment catalogue
Frequently asked questions
What do you need to set up a Buchner funnel?
To set up a Buchner funnel you need the funnel, a filter-paper circle, a heavy-wall side-arm filtration flask, a rubber adapter or cone, vacuum tubing and a vacuum source, ideally with a trap bottle. The filter paper must cover the perforated plate exactly without curling. The side-arm flask must be vacuum-rated, not an ordinary conical flask, and it should be clamped to a stand. Browse the lab glassware range for Buchner funnels and filtration flasks.
Is Buchner funnel filtration in the school chemistry syllabus?
Buchner funnel vacuum filtration appears in many school and pre-university chemistry syllabuses, usually within recrystallisation and purification practicals. Whether it is required depends on the curriculum in use, such as Gaokao (NCEE), Cambridge/IB or a Ministry of Education syllabus. Confirm the current syllabus edition before citing it in a tender or specification, and check whether a water aspirator or a vacuum pump is expected for the practical.
Why must you use a heavy-wall flask for vacuum filtration?
You must use a heavy-wall side-arm flask for vacuum filtration because the reduced pressure inside can cause a thin-walled flask to implode, scattering glass. A purpose-made Buchner filtration flask has thick borosilicate walls and a side arm for the vacuum connection. An ordinary conical flask is not designed for vacuum and should never be used. Always clamp the flask and wear eye protection during suction filtration.
How much does a Buchner funnel and filtration flask cost?
The cost of a Buchner funnel and filtration flask depends on the material, size and whether a vacuum pump or water aspirator is included, so a fixed figure is not reliable. Porcelain funnels and heavy-wall side-arm flasks are durable, while filter paper is a recurring consumable. Request a current quotation in your local currency (for example Renminbi or USD), include any applicable taxes or import duty, and verify pricing before procurement rather than relying on benchmark figures.
Why does liquid get sucked back during Buchner filtration?
Liquid gets sucked back during Buchner filtration when the pump or water aspirator is switched off while the apparatus is still under vacuum, so the pressure difference pulls water or oil back into the flask. Prevent it by breaking the vacuum at the flask first, opening the bleed valve or disconnecting the tubing before switching off the source. Fitting a trap or Woulff bottle between the flask and the vacuum source gives further protection against suck-back.
What’s the difference between a Buchner funnel and a Hirsch funnel?
A Buchner funnel is a larger flat-bottomed funnel for filtering normal quantities of solid, while a Hirsch funnel is a smaller funnel with sloping sides used for small quantities, typically under about one gram. Both perform vacuum filtration on a perforated plate with filter paper, but the Hirsch funnel suits microscale recrystallisation where little material is involved. Choose the Buchner funnel for routine school and university filtration and the Hirsch funnel for small samples; see the chemistry lab equipment range.
Key takeaways
1. A Buchner funnel is used for vacuum filtration, separating a solid from a liquid quickly by pulling the liquid through a flat filter paper under suction.
2. Whatman Grade 1 filter paper, with 11 micrometre particle retention, is the standard teaching grade, while hardened acid-treated papers are preferred for the suction and scraping of Buchner filtration.
3. Apply the Seal-and-Bleed Rule: seal the wetted filter paper with suction before pouring, and break the vacuum before switching off the pump.
4. Always use a heavy-wall, vacuum-rated side-arm filtration flask, because an ordinary conical flask can implode under vacuum.
5. Use Buchner vacuum filtration to collect a solid quickly and gravity filtration to collect a clear filtrate or filter a hot solution.
6. Follow the ten-step filtration procedure and source the funnel, flask and filter paper from the lab glassware and chemistry lab equipment ranges.
About Edu Lab China
Edu Lab China is a manufacturer and exporter of educational and scientific laboratory equipment headquartered in Zhengzhou City Hi-Tech Development Zone, Henan, China, supplying schools, colleges, universities and government institutions across more than 50 countries worldwide. The company states that its products are manufactured under the guidelines of ISO 9001, ISO 13485 and ISO/IEC 17025, with credentials including CE marking, RoHS, REACH and UL and ETL listing. Its range spans lab glassware, filtration apparatus, chemistry, physics and biology lab equipment, laboratory appliances and lab chemicals used in filtration and separation practicals. For bulk supply, tender documentation and OEM enquiries, contact the Edu Lab China procurement team.