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A Büchner funnel (incorrectly, but often spelled Buchner funnel) is a piece of laboratory equipment used in filtration, especially in vacuum filtration.
A Büchner funnel is a funnel shaped object, traditionally made of glazed porcelain, though glass and plastic funnels are also available. On top of the funnel-shaped part there is a cylinder with a fritted glass disc/perforated plate separating it from the funnel. Classical Büchner funnels have a long thin glass tube, to drain the filtered liquid, though some models have a ground glass joint or a lateral vacuum inlet tube, or both. Büchner funnels are often used with a side-arm flask (also known as Büchner or Kitasato flask) when doing vacuum filtration, though if vacuum adapters are used, ordinary flasks can also be used.
Jacketed Büchner funnels are a special type that are sometimes encountered in labs, often used when doing cold or hot filtering.
Similar in construction to the Büchner funnel, the Hirsch funnel has an appearance almost identical to that of a classical funnel, with the walls of the funnel angle outward instead of being vertical. The funnel has a small diameter perforated disk near it's "neck". They are sometimes used for filtering smaller amounts of material or dilute suspensions.
Büchner funnels can be purchased from lab suppliers, both the porcelain and glass fritted disk models. Plastic Büchner funnels are also available. Aluminium or stainless steel Büchner funnels can be found in many hardware stores, as spare parts for coffee machines.
Hirsch funnels can also be purchased from lab suppliers and online. Plastic Hirsch funnels can sometimes be found in hardware stores, albeit of a lower quality.
Porcelain Büchner funnels should not be used to filter extremely corrosive reagents, as it might slowly corrode its protective enamel.
Büchner funnels with fritted disks are notorious for being hard to clean. If a base bath is used, they should not be left for more than a day as the base will dissolve the glass bonding between the fritted glass particles. A strong oxidizing solution, like aqua regia or in extreme cases piranha solution can be used to remove even the most persistent stains. These mixtures are among the few chemicals that can destroy graphite.
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