Filtration is a technique used either to remove solid impurities from an organic solution or to isolate an organic solid. The two types of filtration commonly used in organic chemistry laboratories are gravity filtration and vacuum or suction filtration.

Vacuum Filtration

Vacuum filtration is used primarily to collect a desired solid, for instance, the collection of crystals in a recrystallization procedure. Vacuum filtration uses a Buchner funnel and a side-arm flask. Vacuum filtration is faster than gravity filtration, because the solvent or solution and air is forced through the filter paper by the application of reduced pressure. Do not use vacuum filtration to filter a solid from a liquid if it is the liquid that you want, and if the liquid is low boiling. Any solvent which boils at about 125 degrees or lower will boil off under the reduced pressure in the vacuum flask.

Filtration under vacuum using a Buchner funnel is used when you want to isolate the precipitate (the solid) for further work or analysis.

The apparatus needed is:

  • A Buchner funnel
  • A vacuum filtration flask (also known as a Buchner flask)
  • A filter paper (select the grade with the right material and  pore size for your sample and the right size for your Buchner funnel)
  • Adjoining cones (these are used in place of a rubber bung as they are adjustable to fit any flask to any funnel)
  • A retort stand set – used to hold the flask firmly in place
  • Tubing – to join the flask to the vacuum pump, thick walled type for work under vacuum
  • A vacuum pump – either an electrical type or a water stream vacuum pump

All of this apparatus is available together in a basic vacuum filtration kit – this saves you finding parts that will fit together and is a good option for starting out in vacuum filtering.

Gravity Filtration

Gravity filtration is the method of choice to remove solid impurities from an organic liquid. The impurity can be a drying agent or an undesired side product or leftover reactant. Gravity filtration can be used to collect solid product, although generally vacuum filtration is used for this purpose because it is faster.

A filtration procedure called “hot gravity filtration” is used to separate insoluble impurities from a hot solution. Hot filtrations require fluted filter paper and careful attention to the procedure to keep the apparatus warm but covered so that solvent does not evaporate. Hot gravity filtrations are no longer included in the routine procedures for the experiments in the organic chemistry teaching labs.

A basic filtration under gravity is used when you are removing the solid precipitate to isolate the filtrate, the liquid, for further work or testing.

The apparatus needed is:

  • Filter paper – remember to choose the right size paper for the funnel, see our guide here
  • Filter funnel
  • Retort stand set – to hold the funnel
  • Conical flask – to collect the filtrate

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