When it is necessary to separate and save the solid phase that precipitates from a solution, suction filtration is commonly employed.  This technique makes use of the fact that the laboratories are equipped with aspirators (left) -- a device associated with a water tap that permits access to a vacuum while water is flowing through it.  This is a practical application of Bernoulli's principle, which describes the reduced pressure caused by a flowing fluid when it passes through a constriction.

The vacuum filtration apparatus is simple to assemble and use.  Special flasks, called filter flasks (right), are made of thick glass and have a side arm which permits making a connection to a source of reduced pressure (a vacuum).  The top opening of the suction flask accommodates a single hole rubber stopper which in turn supports a Buchner funnel designed especially for suction filtration. 

The Buchner funnels that we use are made in two pieces that can be joined together so as to make an airtight connection.  Filter paper consistent with the size of the funnel must be used.

The procedure for performing a suction filtration is not complicated, but some elements of care must be exercised.

• only heavy-walled rubber tubing must be used in connecting the filter flask to the aspirator.  Thin walled tubing will collapse under reduced pressure which prevents the low pressure from the aspirator to reach the suction flask
• the filter flask must be firmly secured (e.g., with a clamp and ring stand) so that it cannot inadvertently be tipped over.  The heavy-walled tubing is both heavy and relatively inflexible, so it can easily cause the filter flask to tip.
• if the water in the aspirator stops flowing, the reduced pressure inside the aspriator reverts to atmospheric pressure.  As a result, whatever water may be trapped in the aspirator tube will be drawn to any remaining region of low pressure (such as the interior of the filter flask)
• the filter paper must permit no liquid to pass around it edges.  It must lie flat against the perforated face of the Buchner funnel.

A complete assembly for performing a vacuum filtration is shown at the right.

1. Locate a water tap equipped with an aspirator.
2. Rinse a filter flask with distilled water and make sure the flask is clean (but not necessarily completely dry).  Similarly, clean the top part of a Buchner funnel and rinse the bottom part with distilled water
3. Set up a ring stand with a clamp capable of holding the neck of the filter flask and secure the flask at its neck.  The flask should rest on the base of the ring stand.
4. Connect the side arm of the filter flask to the side arm of the aspirator using heavy-walled rubber tubing.
5. Place the rubber stopper which holds the bottom part of the Buchner funnel in the top of the suction flask.  Be sure that the joint between the rubber stopper and the suction flask is air-tight.  Wetting the stopper can help assure such a joint.
6. Join the top part of the Buchner funnel to the bottom part.  It is a good idea to be sure that the system produces the necessary suction.  This can be done by turning on the aspirator and holding ones palm over the open top of the funnel.  A distinct vacuum should form.  If the system is working properly, remove the top part of the funnel and turn off the aspirator.
7. Place a piece of the correct filter paper, centered and completely flat, against the perforated surface of the funnel.  (In some instances, you may need to weigh the dry filter paper before placing it in the funnel.) and replace the top part of the funnel in the bottom part.
8. Turn on the aspirator.  To insure that no paths exist for the liquid to bypass the filter paper, use a wash bottle to wet the entire filter paper while the aspirator is running.  If the filter paper is wet, you should be able to see images of the perforations through the paper.
9. If a significant amount of water (more than a few mL) has accumulated in the filter flask, turn of the aspirator, disassemble the apparatus, discard the water and reassemble the apparatus.
10. Turn on the aspirator. The aspirator should not be turned off until you have completed all the steps (through Step 15) described here.
11. Pour the liquid from the beaker containing the desired precipitate.  It should be poured slowly using a stirring rod to direct the solution and precipitate to the center of the filter paper.  Try to keep the accumulating precipitate in the center of the paper.  This minimizes the chances of the liquid and precipitate passing around the filter paper and into the filter flask.
12. Continue transferring the liquid and precipitate from the beaker into the funnel.  When there is no more liquid in the beaker, use a stirring rod with a rubber policeman to scrape as much of the solid as you can into the funnel.
13. Depending on the solubility of the precipitate, you may wish to wash some of the remaining solid in the beaker using a small amount of water from a wash bottle.  Do not use more water than is absolutely necessary or you may begin to dissolve some of the precipitate that you have already accumulated.
14. Continue to run the aspirator for a while after the last amount of liquid has been transferred to the funnel.  This will draw some air through the precipitate which helps to dry it somewhat.
15. When the precipitate is as dry as desired -- but with the aspirator still flowing -- remove the top part of the Buchner funnel (the part with the precipitate and the filter paper) and allow air to enter the filter flask through the bottom part of the aspirator.
16. Turn off the aspirator.

Examine the filtrate in the filter flask.  If the filtration has been carried out correctly, the filtrate should be clear.  A slight cloudiness indicates that some of the solid was not trapped by the filter paper.  A large amount of solid in the filter flask suggests that there was a significant path for the liquid (and precipitate) to bypass the filter.  In this case, you may wish to repeat the above process with the filtrate, but you should use a second piece of filter paper for this purpose and keep whatever solid you did recover.