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The Claisen flask or Claisen distillation flask is a special type of distillation flask used in several procedures, like vacuum distillation. It was invented by Rainer Ludwig Claisen.
The Claisen flask is generally used for reduced-pressure distillation. The flask was designed to reduce the probability of having to repeat the distillation, due to bumping of the boiling liquid. It is similar to a Würtz flask, though the distinctive feature of the Claisen flask is a U-shaped neck fused on the top of the flask. The flask itself is often round-bottom or pear-shaped. The U-shape (or bifurcation) is similar to that of a Claisen adapter, hence the name. This design makes it impossible for any splash of distilling liquid projected by bumping to reach the distillate.
Some Claisen flasks have with a short fractionating column on the side arm.
Claisen flasks can be purchased from lab suppliers or online.