A B?chner flask, also known as a vacuum flask, filter flask, side-arm flask or Kitasato flask, is a thick-walled Erlenmeyer flask with a short glass tube and hose barb protruding about an inch from its neck. The short tube and hose barb effectively acts as an adapter over which the end of a thick-walled flexible hose (tubing) can be fitted to form a connection to the flask. The other end of the hose can be connected to source of vacuum such as an aspirator, vacuum pump, or house vacuum. Preferably this is done through a trap (see below), which is designed to prevent the sucking back of water from the aspirator into the Buchner flask.
The thick wall of the B?chner flask provides it the strength to withstand the pressure difference while holding a vacuum inside. It is primarily used together with a B?chner funnel fitted through a drilled rubber bung or an elastomer adapter (a B?chner ring) at the neck on top of the flask for filtration of samples. The B?chner funnel holds the sample isolated from the suction by a layer of filter paper. During filtration, the filtrate enters and is held by the flask while the residue remains on the filter paper in the funnel.
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