Suction filtration

Ordinary filtration can be very slow, particularly if the solid is very fine and clogs up the pores of the filter paper. Suction (or vacuum) filtration uses a side arm conical flask called a Buchner flask and a porcelain, flat bottomed perforated funnel. Larger funnels are called Buchner funnels, and smaller ones are called Hirsch funnels. By applying suction to the side arm the rate of filtration is speeded up considerably.

Diagram showing suction filtration apparatus



Buchner flask	Buchner funnel	Hirsch funnel

It is usually sensible to clamp your Buchner flask in place. They are often pulled over by the particularly heavy vacuum tubing, and you will lose your product. Place your mouse over the image to see the effect of poor security.

Heavy vacuum tubing can pull the flask over

A Buchner funnel with correctly fitting, dampened filter paper

A flat filter paper of the appropriate size is placed in the funnel. The appropriate size is just smaller than the funnel so that it covers all of the perforations. Students often try to squash larger filter papers into the available space - don't! The filter paper is then slightly dampened so that it sticks to the base of the funnel. If this is not done then some of the solid will escape behind the paper and through the base.

A vacuum tap

The funnel is then placed in the side arm flask and vacuum is applied. We use a vacuum line connected to a series of grey taps around the lab, but you may also be using a water pump attached to a lab water tap. You can check for suction by putting your finger over the end of the vacuum tubing - it should suck! Water pumps can be very unreliable, and without suction the process will be very slow.

A water pump

The mixture to be filtered is then added to the funnel. Towards the end the mixture must be swirled and added rapidly to the funnel. Otherwise the solid will simply stick to the sides of the original container. However, if you are too enthusiastic you will slosh the mixture over the sides of the funnel. Wash any remaining solid from the original container using two or three small lots of cold solvent. Leave the suction applied for about five minutes as this will draw air through the solid and help to dry it. Always break the vacuum before turning the suction off. This can be done by removing the vacuum pipe or separating the funnel from the flask. If this is not done with a water pump, water is often sucked back into the side arm flask to contaminate the filtrate. Decide whether you need to keep the filtrate. If it is not needed it can be discarded. Finally, use a spatula to carefully separate your solid from the filter paper. Avoid scraping off bits of filter paper that will contaminate your product.

Video - suction filtration

Common student errors

Apparatus topples over. It is often pulled over by the weight of the vacuum tubing. It is best to clamp the flask in place.

Wrong size of filter paper used. If the filter paper is too big it will not fit properly and solid will get past the edges.

Solid remains in the glassware if it is poured off too slowly

Addition too slow. When the liquid has gone the wet solid sticks to the sides of the glassware. Pour off excess liquid, swirl the remaining solid and liquid and add swiftly to the Buchner funnel.

Addition too fast. The mixture goes over the side of the Buchner funnel and product is lost. Pour off excess liquid, swirl the remaining solid and liquid and add swiftly to the Buchner funnel.

Too rapid addition leads to spillage

Water can be sucked back from a water pump when the water is turned off

Water pump turned off before vacuum is broken. Water may be sucked back into the Buchner flask to contaminate the filtrate. Remove the vacuum tubing from the Buchner flask before turning off the water pump.