Rubber Cones For Filtration Flask

Quick Answer: Rubber Cones For Filtration Flask are tapered rubber adapters with a central hole, used to seat a funnel in the neck of a filtering (Buchner) flask. The slope of each cone lets one adapter seal necks of slightly different widths when suction is applied.

How a Filter Cone Seals

In suction filtration, a Buchner funnel sits on a thick-walled side-arm flask connected to a pump. The joint between funnel stem and flask neck has to be airtight, or the pump simply draws air through the gap. A bored rubber stopper can do this, but only for one neck size. A cone-shaped adapter rests in the neck, the stem passes through its hole, and the vacuum pulls the funnel down so that the rubber is squeezed evenly around both neck and stem.

Filter cones are commonly sold in a graded series to cover small and large flasks. Their open shape makes them easier to clean than a bored stopper, and they need no force to fit, which reduces the risk of cracking a flask that will also be stressed by vacuum.

A basic set-up pairs the cones with a filtering flask, a porcelain or plastic Buchner funnel and a water-jet filter pump. For stemless crucibles rather than funnels, the crucible sleeve set is the matching accessory.

Applications

Recrystallisation practicals where crystals are collected by suction filtration
Collecting precipitates quickly in preparative chemistry
Filtering soil or water samples through a membrane or paper in environmental work
Moving one funnel between flasks of different neck sizes without changing stoppers

Specifications

Type	Conical rubber adapter with central bore
Seats	Funnel stems in the neck of a filtering flask
Sealing	Vacuum pulls the funnel down, compressing the rubber
Used with	Buchner and Hirsch funnels, filter flasks, filter pumps
Sizes supplied	Confirm at enquiry

Care & Handling

Moisten the cone before fitting so the funnel seats evenly.
Break the vacuum at the side arm before lifting the funnel, to avoid suck-back and tearing.
Wash cones after each use; dried solutes crack the rubber surface.
Keep them in a closed drawer, away from heat and sunlight.

Why Choose LabEquip

Filter cones are bought by schools and colleges that run suction filtration on several benches with mixed flask sizes. LabEquip lists them in General Lab Products; tell us your flask necks and funnel stems via the contact page.

Frequently Asked Questions

Why use a rubber cone instead of a bored stopper?

A cone fits a range of neck sizes, needs no force to fit and is easy to clean. A bored stopper fits one neck well but has to be pushed in, and different flasks need different stoppers.

Why is air leaking past the cone?

The funnel stem may be too narrow for the hole, the cone may be too small for the neck, or the rubber may be dry or cracked. Try a different size from the series and moisten the rubber before applying suction.

What stops water from the pump entering the flask?

Suck-back happens when the water flow drops while the flask is under vacuum. Fit a trap bottle between pump and flask, and always disconnect the tubing at the side arm before turning the tap off.

Can the cones be used with plastic Buchner funnels?

Yes. The cone seals around any funnel stem of a suitable diameter, whether the funnel is porcelain, glass or polypropylene.

Should the flask be clamped during suction filtration?

Yes. A filter flask with a funnel on top is top-heavy, and the tubing on the side arm pulls sideways. Clamp the neck to a retort stand to stop it tipping.

Are rubber cones affected by solvents?

Organic solvents such as hydrocarbons and chlorinated liquids swell general-purpose rubber. Rinse the cone promptly after filtering solvent mixtures, or use a cone made from a resistant material for regular solvent work.

Last Updated: September 2026