Funnel, Buchner

Quick Answer: A Buchner funnel is a cylindrical filtration funnel with a flat, perforated plate across its base, traditionally made of glazed porcelain, used with a circle of filter paper on a filtering flask under suction to separate solids such as crystals and precipitates from liquids quickly.

How Suction Filtration Works

Gravity filtration through a folded paper cone is slow and leaves the solid wet. In a Buchner funnel the paper lies flat on the perforated plate, and a pump connected to the filtering flask lowers the pressure beneath it. Atmospheric pressure then pushes the liquid through the paper, so filtration takes a fraction of the time and the solid is pulled nearly dry as a flat cake that can be washed in place.

The funnel’s stem passes through a rubber cone for filtration flask or a bored bung into the neck of a filtering flask, whose side arm connects to a water filter pump or a vacuum pump, ideally through a trap bottle. The paper should be a little smaller than the plate, lie flat over all the holes, and be wetted with the solvent before suction is applied, so it seals and no solid slips around the edge.

Funnels of this pattern are made in porcelain, glass and plastic. Porcelain is the traditional choice: rigid, tolerant of hot solutions and resistant to most acids and solvents. The title does not name the material or plate diameter of this funnel, so settle both together with the size of your filtering flask.

Specifications

Good Practice and Care

• Break the vacuum at the flask before switching off the pump, to avoid suck-back.
• Wash the cake with small portions of cold solvent, releasing the suction briefly each time.
• Rinse the plate from below after use so the holes do not block.
• Look for chips on the rim and plate before use; a chipped plate stops the paper sealing.

Applications

• Collecting recrystallised products in organic chemistry practicals.
• Separating precipitates in inorganic preparations, such as double salts or complexes.
• Washing and drying a filter cake before weighing.
• Filtering suspensions that clog gravity filters.

Why Choose LabEquip

Chemistry and pharmacy colleges, research labs and QC units running recrystallisations buy these funnels together with matching flasks and cones. LabEquip lists the funnel in General Lab Products; send the plate size you need through the contact page.

Frequently Asked Questions

Why is a Buchner funnel faster than ordinary filtration?

Suction lowers the pressure under the filter, so atmospheric pressure pushes the liquid through much faster than gravity alone. The solid is also drawn nearly dry.

What size filter paper does it need?

A flat circle slightly smaller than the plate, covering every hole without turning up at the edge. A paper that curls up the wall lets solid get underneath.

What else is needed to use it?

A filtering flask with a side arm, a rubber cone or bored bung to seal the funnel stem into the flask, filter paper, and a water filter pump or vacuum pump, preferably with a trap bottle.

How is suck-back prevented when switching off?

Break the vacuum first by disconnecting the tubing at the flask or opening a valve on the trap, then turn off the pump. This stops water or oil being drawn back into the flask.

What is the difference between a Buchner and a Hirsch funnel?

A Hirsch funnel is conical with a small perforated plate and is used for small amounts of solid. The Buchner funnel has a wide flat plate and suits larger quantities.

How should the plate be cleaned?

Remove the paper and solid, then run water up through the plate from below to clear the holes. Wash with detergent, and use a suitable solvent or dilute acid for stubborn residues.

Last Updated: September 2026