Vacuum Filtration

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Vacuum Filtration

Vacuum filtration is a technique for separating a solid product from a solvent or liquid reaction mixture. The mixture of solid and liquid is poured through a filter paper in a Buchner funnel. The solid is trapped by the filter and the liquid is drawn through the funnel into the flask below, by a vacuum.

Setting up and performing a vacuum filtration
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To prepare for a vacuum filtration, gather together a filter flask, Buchner funnel, tubing, filter paper, clean solvent, disposable dropper, and your sample.
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In the Chem 3/5/6 lab, you will use the vacuum outlets in the hoods, which are connected to a central vacuum pump for the entire building. You will find tubing in the hoods, connected to a vacuum trap. This plastic container prevents solvent from being sucked into the central vacuum pump by mistake. The tubing should be connected from the vacuum nozzle to the trap and from the trap to the side arm of your filter flask.

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Turn on the vacuum using the knob on the outside of the hood. Check the vacuum by feeling for suction at the end of your tubing. The vacuum should be strong enough to hold the tubing to your finger without falling off.

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Connect the tubing to the side arm of your filter flask and check the suction at the top of the flask (left). Place the black rubber ring adapter in the top of the flask and then the Buchner funnel. Check again for good suction by placing your gloved hand across the top of the funnel (right). If you do not feel strong suction, there is a poor connection and a leak somewhere in your system.

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Prepare to filter your sample by placing a filter paper in the Buchner funnel and wetting it with clean solvent. You should see the paper being sucked down against the holes in the funnel and the solvent should quickly pass through into the filter flask.

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To filter your sample, slowly pour into the center of the filter paper.

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Use more clean solvent to rinse your beaker, so that all the solid is collected.
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Rinse the solid on the filter paper with more clean solvent. Continue to draw air through the solid, to evaporate any remaining solvent in your sample.

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When you are finished, break the vacuum at the connection between the flask and the trap. Then turn off the vacuum with the knob on the outside of the hood.

Note: Vacuum Pumps are expensive, high maintenance pieces of lab equipment.  For most purposes you can use a much less expensive vacuum aspirator to pump down a vacuum filter flask. The vacuum aspirator screws onto a faucet and uses the jetted flow of the water to pull a vacuum.

Vacu/Trol� Pump (Water Aspirator)
 

The water aspirator will evacuate 10 liters of air to less than 60 mm Hg in less than 6 minutes at 25 psi water pressure.

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