Active Filtration

Introduction to Active filtration

Problems can arise with vacuum filtration, if the filter paper becomes blocked or clogged by insoluble material. This can sometimes be helped by the use of a filter agent, such as celite. A difficult filtration may proceed more readily when filtered through a pad of celite.

Materials used for active filtration

Celite

Celite is a type of diatomaceous earth (silica): a fine powder with a high surface area, which is an excellent adsorbent. A difficult filtration may proceed more readily when filtered through a pad of celite. The celite pad is formed on the filtering surface by mixing celite with solvent, and filtering the resultant suspension – a depth of 1 cm is generally sufficient. This allows the suspended material to be scraped aside and manipulated if it blocks the funnel, without risk of damaging the filter paper. After filtering the suspension, the celite pad should be thoroughly rinsed with solvent, as celite has a tendency to adsorb all of the components of the suspension – including the desired product.

Silica

Active filtration using a sinter funnel

This technique is on the borderline of filtration, and is usually considered to be a purification technique in its own right. Solutions can be filtered through a silica pad, and very polar compounds are retained by the silica.

Silica filtration through a sinter funnel

Filtration through silica is a rapid purification technique that separates desired compounds from impurities depending upon relative polarities and affinities to silica gel. It is related to flash column chromatography, however it avoids the need to collect multiple fractions using a long column of silica gel. Instead, a short, fat plug of silica is prepared in a sinter funnel which is eluted into a Buchner flask in a single step. It is useful for purifying material where there is only a need to remove baseline impurities which can be observed using Thin Layer Chromatography, for example many inorganic catalysts.

Method

Set up a Buchner flask with rubber ring and sinter funnel, making sure to remember to securely clamp the neck of the flask.
Weigh the appropriate amount of silica into a beaker and add the eluent in order to create a silica slurry
Pour the slurry into the sinter funnel. The beaker can be washed out with additional eluent before allowing the silica to settle in the sinter funnel.
Carefully apply a slight vacuum to the Buchner flask, to move the eluent level down to the drop of the silica pad. Extreme care must be taken to dry out the silica pad.
This can be achieved by not fully opening the vacuum tap and not fully attaching the vacuum tubing to the flask. If the silica pad dries out, the process needs to be started again at step 2.
Empty the eluent from the Buchner flask in preparation for the filtration to begin.
Dissolve the crude compound in the minimum amount of eluent and carefully pipette onto the stop of the silica pad by dripping the solution down the glass walls of the sinter funnel.
Move the solvent layer down to the top of the silica pad by following guidance in step 4.
Wash the flask containing the crude compound with eluent and carefully pipette onto the silica pad to ensure all compound is transferred. Move the solvent layer down to the top of the silica pad following guidance in step 4.
Very carefully add eluent to the top of the silica pad by first pipette solvent round the outside of the sinter funnel and then carefully pouring solvent down the glass. It is very important to not disrupt the surface of the silica pad. This can significantly effect the efficacy of the process.
Repeat step 9 until all of the required eluent has been used up.
Concentrate the filtrate under reduced pressure to yield the purified product.

Video

Filtration through Silica

Active filtration using a Pasteur pipette

Small scale filtration using a Pasteur pipette are often carried out as a form of purification, particularly using silica gel. This is treated as a technique in its own right, being termed a microcolumn.

Related techniques

Column chromatography is often used as a alternative to active filtration.

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