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:
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.