Suction filtration (also called vacuum filtration) is a technique used to separate liquids from solids.

In this technique, an aspirator sucks out the air that is contained in the flask where the Büchner funnel with a filter is placed. This causes a difference in pressure, so when the mixture is placed in the funnel, it is forced to go through the filter. After the filtration, the solid stays on the filter, while the liquid goes through and accumulates at the Büchner flask.

Suction filtration focuses on the recovery of the solid since the flow of air created by the aspirator will be much drier than with a simple gravity filtration. By drying the solid much more, we make sure that most of the weight in the filter is indeed due to the solid and not to the solvent.

To do a suction filtration we need to:

1. Tare a filter paper on a glass watch. By doing this before and after the filtration we will be able to estimate the weight of the solid that was in our mixture
2. Assemble the suction filtration equipment and activate the pump to make the air start flowing
3. Place the pre-weighed filter in the Büchner funnel and filter the solution
4. Once the solution is filtered, leave it a bit longer in the funnel to dry out
5. Weight your pre-weighed filter with the solid in the scale that you previously tared to obtain the weight of your crystals

At the end of the process, we will be able to estimate the recovery efficiency by simply calculating the percentage of the original solid that we have recovered.

Recovery efficiency = (Final weight - Initial weight) / Initial weight

Fig 1. Suction filtration protocol.