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A Buchner flask is the heavy-walled filtering flask used in vacuum filtration: a Buchner (or Hirsch) funnel sits on top, a vacuum source draws liquid through filter paper, and the flask collects the filtrate. Who uses it: chemistry and biology laboratory staff and students who need to collect a solid from a liquid faster than gravity filtration allows. The reader should come away able to name each part of the setup, tell the funnel types apart, and follow the vacuum-glassware safety points below.
What is a Buchner flask?
A Buchner flask (also written Büchner flask, and often called a filter flask or filtering flask) is the vessel in a vacuum filtration assembly. Its job is to hold the liquid that passes through the funnel while the vacuum is applied. Because the inside is under reduced pressure, the flask is built to withstand a pressure difference: filtration flasks are described as often having reinforced walls for this reason (Lab Manager, Henderson, “10 Tips to Reduce Vacuum Flask Hazards”). Unlike a general-purpose Erlenmeyer flask, which is made for use at ambient pressure, a filtering flask is meant for vacuum work.
How does the Buchner flask and funnel setup work?
In a typical assembly, the funnel is seated on the flask with a vacuum-tight seal, such as a neoprene adapter, and the flask is connected to a vacuum source. A piece of filter paper lies on the funnel plate and is moistened before use. The mixture is poured in, and the vacuum draws the liquid through the paper into the flask while the solid stays on the paper. This is faster than gravity filtration, which is why the method is common in organic chemistry for collecting recrystallised compounds (Wikipedia, “Büchner funnel”, read 2026-11-07). The vacuum source can be a laboratory pump or a water aspirator.
Two practical points from the same source: limit the amount of liquid so it cannot overflow into the flask neck or the vacuum equipment, and consider a trap between the flask and the vacuum source to reduce the risk to that equipment.
Types and variants
The flask itself is a single type, the filtering flask; the main variants in a vacuum filtration setup are the funnels that sit on it, summarised in the table below.
Buchner funnel vs Hirsch funnel: what is the main difference?
Both funnels sit on a filtering flask and filter under vacuum. The Hirsch funnel is used for smaller quantities, has a much smaller plate, and its walls angle outward; the Buchner funnel is a cylinder above a plate with vertical walls, suited to larger volumes.
| Part | Role in the setup | Key difference or note |
|---|---|---|
| Buchner flask (filtering flask) | Collects the filtrate under reduced pressure | Built for vacuum; often reinforced walls (Lab Manager) |
| Buchner funnel | Holds filter paper and the solid | Cylinder above a fritted disc or perforated plate; traditionally porcelain, also made in glass and plastic (Wikipedia) |
| Hirsch funnel | Same role, for smaller quantities | Much smaller plate; walls angle outward rather than vertical (Wikipedia) |
| Trap (optional) | Sits between flask and vacuum source | Reduces risk to the vacuum equipment (Wikipedia) |
Who invented the Buchner funnel?
The funnel is named for the industrial chemist Ernst Buchner, who described it in 1888 (Chemiker-Zeitung 12(78):1277). He is not the Nobel laureate Eduard Buchner, a frequent mix-up. Reported accounts (Andrea Sella, Chemistry World, 28 October 2009) describe Ernst Wilhelm Buchner as running an ultramarine factory near Darmstadt and modifying Robert Hirsch’s earlier design with vertical sides to handle larger volumes. For the teaching context, see Jensen, J. Chem. Educ. 83(9):1283 (2006), doi:10.1021/ed083p1283.
How do you pronounce “Büchner funnel”?
The name is German, written with an umlaut (Büchner), which is why English texts also print it as Buchner. Pronunciation varies with speaker and dictionary, so check a dictionary entry for the form you prefer; either spelling refers to the same equipment.
What are filtering flasks used for in the laboratory?
They are used wherever a liquid is drawn through a filter under vacuum, most commonly to separate a crystallised or precipitated solid from the liquid it formed in. The solid is collected on the funnel; the filtrate collects in the flask.
Safe use of vacuum glassware
A flask under vacuum can implode, so the safety points come from the flask, not the funnel. From Lab Manager (Henderson):
- Use flasks specifically labelled for vacuum use.
- Thin-walled or round-bottomed flasks larger than 1 litre should never be evacuated; smaller flasks are less likely to fail violently.
- Before each use, check for cracks, scratches, deep scoring or etching. Even minor flaws can compromise structural integrity and raise the implosion risk.
- Wrap the flask in a criss-cross pattern of filament tape, or place a plexiglass barrier around the apparatus, to limit projectiles if it fails.
- Apply the lowest vacuum necessary.
For broader handling and disposal rules, see glassware safety in the laboratory and types of laboratory glassware. Membrane-based alternatives are covered in what is a syringe filter.
Common mistakes
- Using an ordinary conical or round-bottomed flask for vacuum work instead of one labelled for it.
- Skipping the inspection of the flask for scratches or cracks.
- Filling the funnel so far that liquid can overflow into the flask neck or the vacuum equipment.
- Using a poor seal; a vacuum-tight seal between funnel and flask is essential.
- Applying more vacuum than the work needs.
FAQ
What is a Buchner flask?
A heavy-walled filtering flask that collects the filtrate in vacuum filtration, used with a Buchner or Hirsch funnel and a vacuum source.
What is the difference between a Buchner funnel and a Hirsch funnel?
The Buchner funnel has vertical walls above a larger plate and suits larger volumes; the Hirsch funnel has a much smaller plate and outward-angled walls and suits smaller quantities.
Who invented the Buchner funnel?
It is named for the industrial chemist Ernst Buchner, who described it in 1888; he is not the Nobel laureate Eduard Buchner.
What are filtering flasks used for?
Drawing a liquid through a filter under vacuum, most often to separate a crystallised or precipitated solid from the liquid it formed in.
Sources
- Wikipedia, “Büchner funnel”, https://en.wikipedia.org/wiki/B%C3%BCchner_funnel (read 2026-11-07)
- Lab Manager, Trevor J. Henderson, “10 Tips to Reduce Vacuum Flask Hazards”, https://www.labmanager.com/10-tips-to-reduce-vacuum-flask-hazards-19030 (read 2026-11-07)
- Chemistry World, Andrea Sella, “Buchner’s funnel”, 28 October 2009, https://www.chemistryworld.com/opinion/buchners-funnel/3004902.article (read 2026-11-07)
- Jensen, W. B., J. Chem. Educ. 83(9):1283 (2006), doi:10.1021/ed083p1283
