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
(right) 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.
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.
The contents of the top of the
funnel can be transferred to another container (usually, a clean watch glass)
by inverting the funnel and tapping it. If the filter paper continues
to stick to the funnel, use a spatula to separate the filter paper from
the funnel in one spot. This will normally cause all of the contents
to fall onto the watch glass.
- Locate a water tap equipped
with an aspirator.
- 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
- 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
- Connect the side arm of
the filter flask to the side arm of the aspirator
using heavy-walled rubber tubing.
- 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 connection.
- 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
- Place a piece of the correct
sized filter paper, centered and completely flat, against the perforated surface
of the funnel. (In some instances, you may be asked to weigh
the dry filter paper before placing it in the funnel.) and replace
the top part of the funnel in the bottom part.
- 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 with a small amount of water while the aspirator
is running. If the filter paper is wet, you should see images
of the perforations through the paper.
- If a significant amount
of water (more than a few mL) has accumulated in the filter flask, turn
off the aspirator, disassemble the apparatus, discard the water and
reassemble the apparatus. (Some exercises require measuring the volume
of the solution being filtered.)
- Turn on the aspirator.
The aspirator should not be turned off until
you have completed all the steps (through Step 15) described here.
- 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.
- 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.
- 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, or another
liquid. Do not use more wash liquid than is absolutely necessary
or you may begin to dissolve some of the precipitate that you have already
- 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.
- 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.
- Turn off the aspirator.
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.