Views:12 Author:Site Editor Publish Time: 2014-09-12 Origin:Site
A vacuum filtration is usually faster than a simple gravity filtration using a simple conical funnel. However, there are several points that have to be considered when performing a vacuum filtration. Due to the fact that reduced pressure is used in this procedure, special attention has to be paid to the equipment used in this procedure. The glassware (i.e. filter flask) should not have any cracks on the sides or the bottom. It is also imperative that the filter flask and the vacuum trap are clamped securely, so that they do not move during the filtration. The thick-walled tubing tends to act like a whip sometimes, and slams the filter flask into the hotplate or other metal objects resulting in an implosion (or tips the flask over and the product is found afterwards on the bench or even worse on the dirty floor!).
The clean filter flask and the vacuum trap consist of heavy-walled glassware. Thick-walled tubing should be used as well. Normal tubings that are used as water hoses will collapse when a vacuum is applied!
Never perform a vacuum filtration without a vacuum trap. It is also a good idea to keep the trap clean, just in case something gets sucked in there. It is easier to recover it from a clean trap then from a dirty trap.
Vacuum filtrations can only be performed with funnels that have a flat surface e.g. Hirsch or Büchner funnel, which are both made from procelain! The filter paper has to cover all holes in the bottom of the funnel and does not extend up the sides. This means that the small pieces (~1.3 cm) of filter paper are used for the Hirsch funnel, and not a bigger circle hanging in the funnel. If the filter paper has the wrong size or not properly placed in the funnel, a significant part of the solid will "run through" the filter paper.
If the filter paper is too big for the Büchner funnel, it has to be cut to size with scissors. (From this point of view, it might be a good idea to generate a cardboard blank from an existing filter paper). The filter paper is moistened with the solvent that makes up the solution. This prevents the formation of multi-layer systems in the filter flask. It should adhere well to the bottom before the vacuum is applied. Then the solution is slowly poured slowly into the funnel.