Learn in-depth how to filter in the biological laboratory!

FILTRATION METHODS PRESENTED BY SIMSII, INC.

THE FILTER PAPER First, make sure you use the appropriate size membrane filter. This is most important for vacuum filtration where the paper should smaller in diameter than the base of Buchner or Hirsch funnel (but it must cover all the holes) and should sit flat on the bottom of the funnel with no creases or folds.

Second, folding the filter paper. There are two ways to fold filter papers, the "conventional" method and "fluted".

GRAVITY OR SIMPLE FILTRATION This is the most common method of filtration and is used to remove an insoluble solid material from a solution. The solid could be the required product or impurity or an additive such as a drying agent. A filter paper is folded (conventional or fluted) and placed in a filter funnel which is then placed in the neck of an Erlenmeyer flask or supported in a clamp or ring stand. The solution to be filtered is then slowly and carefully poured into the funnel taking care not to fill the funnel above the edge of the filter paper.

HOT FILTRATION Sometimes during a gravity filtration, crystals can start to grow in the filter funnel and may block the funnel, stopping filtration. This problem can be avoided by using a hot filtration where the whole filtration apparatus is heated in order to prevent the solution from cooling significantly.

The hot filtration process is best carried out using fluted filter paper and a stemless filter funnel. Support the stemless filter funnel on an iron ring clamp if it is at all insecure (see the lower picture). If the pressure builds up in the flask due to the presence of hot vapors, the filtering action will slow down or even stop.

VACUUM FILTRATION In a vacuum filtration, the solution to be filtered is drawn through the filter paper by applying a vacuum to a filter flask with a sidearm adaptor (also known as a Buchner flask). Vacuum filtration is typically a a fast and efficient way of filtering.

The crystals are collected by swirling the mixture of the solid and liquid and then pouring quickly it into the filtration apparatus. This typically comprises a Büchner funnel fitted with the appropriate size filter paper; a clamped filter flask with conical filter adapter, and a vacuum applied to the sidearm of the filter flask.