Vacuum filtration refers to the process of extracting solid products from a liquid reaction mixture or a solvent. The mixture of the liquid and solid is poured through a Buchner funnel‘s filter paper. The filter traps the solid and finally the liquid flows through the funnel into a flask. Some of the things needed to prepare for vacuum filtration include, filter flask, filter paper, clean solvent, disposable dropper, tubing and your sample.
Vacuum filtration is usually much faster than the simple gravity filtration. However, a number of considerations need to be made before beginning the process. First, because reduced pressure is used in the process, a lot of attention has to be paid to the equipment being used. All the glassware e.g. filter flask, should not have any cracks at the bottom or side. It is also essential to make sure that the filter flask and vacuum trap are properly clamped. This will ensure that the equipment do not move during filtration.
In cases where a proper set-up lacks, the thick walled tubing may act like a whip and slam the filter flask to the hotplate or other metallic objects. This may cause an implosion that could result into the end product landing on the bench or in some instances in a dirty surface. The filter flask and the vacuum trap should be those made of heavy glassware. Thick walled tubing should also be adopted as normal tubes used as water hoses are likely to collapse when vacuum is used.
One should never conduct the procedure without a vacuum trap. The trap must be kept very clean just in case it traps something, because the recovery will be easier as opposed to where a dirty one was used. It is again worth noting that the filtration will only be possible where funnels that have flat surfaces are used e.g. Hirsch or Buchner funnels which are made of porcelain.
All holes in the bottom of the funnel should be adequately covered with the filter paper which should not extend up the sides. It means that only small pieces of filter papers (approximately 1.3 cm) are used and not big circles which hangs over the funnel. They have wrong sides and therefore must be placed properly lest some significant portion of the solid pass through.
Where the filter paper to be used in vacuum filtration is very big, it can be cut down using a pair of scissors. It is also advisable to get a cardboard blank from an existing or used filter paper. The paper is then made moist using the solvent forming the solution. This is to prevent multiple layers forming in the filter flask.
The filter paper must also adhere properly to the bottom before applying the vacuum. The solution can then be poured slowly into the funnel. When the vacuum filtration is finished, the tubing from the flask needs to be removed carefully before the vacuum is switched off. Finally, it is important to note that this filtration method is not appropriate for solvents with low boiling points.