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A Büchner flask (incorrectly, but often spelled Buchner funnel), also known under many other names like vacuum flask, filter flask, suction flask, side-arm flask or Kitasato flask, is a thick-walled Erlenmeyer flask used in combination with a Büchner funnel and a vacuum source in vacuum filtration.
A Büchner flask consists of a with a thick-walled Erlenmeyer flask which has a short glass tube and hose barb protruding about 1-2 cm from its neck. Some Büchner flasks lack the lateral vacuum adapter, and instead have a hole, where a plastic vacuum adapter is added. While this might sound complex, such flasks have a few advantages: are easier to clean and there's less risk of breaking the lateral tube, which would make a typical side-arm flask unusable. Some Büchner flasks have ground glass joint.
Büchner flask can be bought from lab suppliers.
DIY Büchner flask
Although it's more expensive, if you don't have a Büchner flask, you can always use a round-bottom flask with a gas inlet adapter or a Schlenk flask as a makeshift vacuum flask, but this is only if you really don't have one or you broke yours.