Filtration and Centrifugation

 Filtration

Filtration is a process which can be used to separate a mixture of a solid and a liquid. The mixture is poured into the filter funnel and the liquid substance passes through it while the solid substance is trapped in the filter paper. The residue from the suspension is the solid which is left on the filter paper and the liquid is called the filtrate as it is filtered through the funnel(see diagram). This form of separation is used for many suspensions such as chalk and water and sand and water.

Centrifugation.

This does the same job, but it can do much more besides.

In this method the suspension is spin around very fast. This generates a force that pushes the suspended solid to the bottom of a test tube. It pushes it so hard that the solid sticks to the bottom of the tube and the liquid can be decanted off.

If the suspension has several different solids in it, the heaviest solid goes to the bottm quickest, followed by the next, etc. Layers are then produced. In filtration such solids will be all mixed together on the filter paper.

Centrifugation is used a lot in biochemistry. For example it is possible to make a suspension of cells. After breaking up the cells in some fashion, the different bits (nuclei, chloroplasts etc.,) can be separated from each other by spinning the suspension at different rates - relatively slow at first to get the heaviest bits (the nuclei) - then speeding up a bit to get the next heaviest, etc

By Michelle Cassley, Chemistry Department, Loreto College, Coleraine.