Quick Answer: A filtering flask is a heavy-walled conical glass flask with a short side arm near the neck, used for suction (vacuum) filtration. A Buchner or Hirsch funnel sits in its neck on a rubber adapter or bung, and the side arm connects by thick tubing to a vacuum source such as a water filter pump.
Built to Withstand a Pressure Difference
A filtering flask, also called a Buchner flask, Bunsen flask or side-arm flask, looks like a conical flask with a hose connection, but its walls are much thicker. The extra glass lets it resist the atmosphere pushing inwards when the air inside is pumped out, which an ordinary conical flask is not designed to do. That is why a standard flask must never be used in its place.
Suction filtration is quicker than gravity filtration and leaves a drier solid. In a typical set-up a Buchner funnel with a disc of filter paper is sealed into the neck, the side arm is connected to a filter pump or vacuum line, and the mixture is poured onto the wetted paper. The pressure difference drives the liquid through, leaving the solid as a compact cake that can be washed with a little cold solvent and sucked dry.
The listing does not give capacity or the form of the side arm, such as a plain barbed tube or a threaded connection. A small trap bottle between flask and pump is worth adding when a water aspirator is used, to stop tap water from backing into the filtrate. LabEquip also lists a complete vacuum filtration apparatus.
Specifications
| Item | Side-arm flask for suction filtration |
| Walls | Heavy-walled, to withstand reduced pressure |
| Connections | Funnel via rubber adapter or bored bung in the neck; vacuum tubing on the side arm |
| Partners | Buchner or Hirsch funnel, filter paper, water filter pump or vacuum line, trap bottle |
| Capacity and side-arm form | Confirm at enquiry |
Applications
- Collecting crystals after recrystallisation, such as purified aspirin in organic preparations
- Separating precipitates made in inorganic practicals quickly, ready for drying
- Filtering fine suspensions that pass too slowly through paper by gravity alone
- Degassing a liquid under reduced pressure, where the flask’s thick walls are needed
Care & Handling
- Never heat a flask of this type; thick glass cracks easily when heated unevenly, and warm solvent under vacuum boils violently.
- Clamp the flask to a stand, since the funnel and tubing make it top-heavy.
- Release the vacuum at the tubing before turning off a water pump, so tap water is not drawn back into the flask.
- Check for scratches and star cracks before each use and retire damaged flasks, as a flaw can cause implosion under vacuum.
Why Choose LabEquip
Organic chemistry teaching labs and research groups use these flasks daily for collecting crystals, and they are often ordered with matching funnels and tubing. The flask is part of LabEquip’s chemistry lab equipment catalogue, and a suitable funnel and pump can be discussed through the contact page.
Frequently Asked Questions
Can I use an ordinary conical flask for vacuum filtration?
No. Ordinary conical flasks have thin walls that are not designed to take the inward push of the atmosphere when the air is removed, and they can implode. A purpose-made suction flask has much thicker walls for this job.
What is the difference between a Buchner funnel and a filtering flask?
The funnel holds the filter paper and the mixture being filtered; the flask underneath collects the filtrate and connects to the vacuum. Both are needed, sealed together with a rubber cone or bung.
Why does water from the tap end up in my filtrate?
If a water aspirator is turned off or its pressure drops while the flask is still under vacuum, tap water can be sucked back through the tubing. Fit a trap bottle between pump and flask, and always break the vacuum first.
How do I stop the filter paper from lifting or tearing?
Choose paper that covers all the holes of the funnel plate without climbing the sides, wet it with the solvent before applying suction, and pour gradually so the paper is pulled flat before most of the mixture arrives.
Can the filtrate be heated in the same flask afterwards?
Transfer it to a beaker or round bottom flask first. Its thick walls heat unevenly and crack easily, so it is not meant for a hot plate or a flame.
Which vacuum source should I use?
A water filter pump fixed to a laboratory tap is enough for most teaching filtrations. A diaphragm vacuum pump gives steadier suction without using water, and it needs a trap to protect it from solvent vapour.
Last Updated: September 2026
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