Vacuum Filtration:

Vacuum filtration is a fast filtration technique used when gravity filtration is unnecessary, and the particles of solid are of sufficient size to not be forced through the filter paper.

The apparatus can also be used to dry solids once they have been filtered but are still damp or moist.

The image above shows a vacuum filtration about to be performed. The set up consists of a Buchner funnel. This plastic funnel consists of two plastic parts: the funnel, and the round plastic filter. The filter contains large holes, so a piece of filter paper is placed over it to prevent solids from passing through.

There is a round green stopper at the base of the funnel which fits into the filter flask to give the funnel a gas tight seal and create a vacuum.

The filter flask appears from this side to be an Erlenmeyer flask, but behind it is a glass side arm for hooking the hose on. In this picture it is a yellow sidearm which is barely visible.

The hose here is made of Tygon and is attached to an aspirator. The aspirator is an attachment on the faucet which extends the spigot and also has a T-joint allowing the hose to be attached. When the faucet is turned on, water rushing past the hose hook up creates a vacuum. The strength of the vacuum is controlled by the rate of flow and how far the water drops without being impeded. So the water should be turned on with a strong flow. There should be nothing in the sink to obstruct the water dropping to the bottom of the sink.

Note that of you look at the picture carefully, you will see that the paper filter is not flat against the plastic filter. If a mixture were to be poured on here the filter would float on the liquid while a great deal of solid go through the holes of the plastic filter.

To avoid that, it is better to place the paper filter onto the Buchner funnel firmly, then turn on the aspirator at the faucet, and wet the filet paper by pouring a little solvent through it, (usually water), before trying to filter the mixture.

The image shows filter paper being wet before filtration after the vacuum has been applied to insure a tight fit that will insure an efficient filtration.