Why Is Vacuum Filtration Sometimes Preferable to Ordinary Gravity Filtration?

Vacuum filtration is faster than gravity filtration and is typically preferred when removing a single solid from a small amount of solution, usually 10 milliliters or less. The process is most commonly used for capturing crystals from a crystallization process.

The tools required to perform a vacuum filtration operation are: a receiver flask with a sidearm, a Buchner or Hirsch funnel, a paper filtration media, rubber tubing and a vacuum source with an appropriated fluid trap. It's important to inspect the glassware for any cracks or weak spots that could collapse under pressure and make sure the receiver flask and trap unit are fully secured, because the rubber hose may thrash around when vacuum pressure is applied.

The type of funnel used depends on the amount of solution being filtered. The Hirsch funnel is typically for amounts less than 10 milliliters, with the Buchner funnel used for larger amounts. The filtration media should be just large enough to cover all the holes in the funnel, but not big enough to go up the sides, or solids may be lost.

Gravity filtration is generally used when the amount of solution is greater than 10 milliliters. Hot gravity filtration is used when the solution must be heated to separate insoluble impurities. Hot gravity filtration cannot be performed using vacuum filtration.