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The apparatus generally used for reduced-pressure distillation, one of the most common physical operations of chemistry, is substantially that known for more than one century as the Claisen flask (1) (Fig. 1). During this time, the relevant changes have been the reduction of the volume of the most used size by two to three orders of magnitude and the use of new connections.
The Claisen flask was designed to reduce the probability of having to repeat the distillation, which in a common flask would become necessary upon bumping ("stossweissem Sieden" [1]), the sudden, violent vaporization of part of the liquid, which projects most or all of the distilland into regions from where any material ends up in the distillate. Bumping is a normal occurrence in lowpressure distillations (2). Its effects are aggravated at microscale size by the bubble size's independence of the flask dimensions, by the slight decay of velocities and accelerations over small distances, and by the rate of heating exceeding the maximum vapor flow rate possible in the apparatus.
The distinctive feature of the Claisen flask (Fig. 1) is a bifurcation of the flask's neck into a neck for the capillary, C, and one for the thermometer, T. Such a geometry makes it impossible for any splash of distilland projected by bumping and following nearly rectilinear trajectories (independent of the vapor flow) to reach the distillate. This applies to big (with a large volume-to-surface ratio) splashes, but not to the droplets of distilland mainly originating from bubble bursts, which are small enough (3) to be entrained by the vapors featuring a high speed due to the low pressure. The situation is worsened by the stream of inert gas from the capillary and by the fact that the...