Filtration

Filtration is the most common method for separating a solid from liquid. The principle of filtration is that a liquid, containing a solid in suspension, is forced through a barrier which is porous to the liquid, but not to the solid. There are a variety of ways to carry out filtration in the laboratory. Each method has its own advantages and disadvantages and it is nesseasary to consider the desired outcome from the filtration and the time and resources available in order to determine the most appropriate method for a given task.

Filtration choices

In a gravity filtration, the force is simply provided by gravity. Generally, this technique is used when the desired compound is in solution, so the filtrate is required, and the solid is to be discarded. Gravity filtration can be slow, and it is often more convenient to use a vacuum filtration.

Vacuum filtration uses the force provided by the atmospheric air pressure, as the air pressure inside the filtration flask is reduced by vacuum. Historically, this technique was carried out using a Buchner funnel and was best suited for the desired compound being the suspended solid, and the filtrate is to be discarded. However, the development of sinter filters removes this restriction, and vacuum filtration using a sinter can be used when the filtrate, the residue or both are desired.

The use of microscale filtration is desirable where very small amounts of material (typically upto a few millilitres) need to be filtered.

Both vacuum and microscale filtrations can also be adapted to become active filtrations, where the material is filtered through a material in order to 'clean up' the sample in some way. Usually active filtrations are only suitable where the filtrate is the desired material, as the residue will be absorbed onto filtration material, often in an irreversible manner.

The table below may assist with filtration choices.