Vacuum filtration is a separation method used to remove a solid from a liquid. Generally the solid is dissolved in a solution. The solution can be unsaturated, saturated, or supersaturated — the difference being the concentration of the solid in the liquid. A filter is used to separate the solid from the liquid, and a vacuum pump is used to force the liquid through the filter.
In most cases, vacuum filtration is used to collect recrystallized solids. These are particles that were dissolved in water or some other solvent and then were recovered, or recrystallized, by heating. The liquid is evaporated in the process.
After most of the liquid has been evaporated, the contents of the beaker or flask are deposited onto a filter paper, which is placed within a Buchner funnel. The funnel is attached to a filter flask, which has a side opening through which air will be sucked out using a rubber tube attached to a vacuum.
The rubber tube is attached to the vacuum source first and tested. If there is suction at the other end of the tube, it is then attached to the side arm of the filter flask. Once secured, the top of the filter flask should have suction.
As the sample is poured onto the filter paper, the vacuum pulls the solvent through the paper and deposits it in the filter flask. The vacuum will remove as much solvent from the solid crystals as possible. The remaining solid on the filter paper is then collected and weighed.
Vacuum filtration is also used to separate solids from liquid solutions. Unsaturated solutions are solutions that can dissolve more solids. It is generally clear and has no suspended particles. Saturated solutions can't dissolve any additional solids, but are generally clear as well. Supersaturated solutions are generally not clear and have suspended or sinking particles.
Solutions with suspended particles can be separated through vacuum filtration. Solutions in which the solid is dissolved, such as unsaturated solutions, generally require some form of chemical treatment or heating before vacuum filtration can be effective. Generally, saturated solutions can't be separated with vacuum filtration, but a few particles may be collected if the filter paper has an adequate pore size.
The amount of solid collected through vacuum filtration generally depends on the pore size of the filter paper. A smaller pore size will yield more material, while a larger pore size will yield less material. Generally, the pore size of a filter paper is less than a micrometer in diameter.