Filtration is a technique used either to remove solid impurities from an organic solution or to isolate an organic solid. The two types of filtration commonly used in organic chemistry laboratories are gravity filtration and vacuum or suction filtration.
Gravity filtration is the method of choice to remove solid impurities from an organic liquid. The impurity can be a drying agent or an undesired side product or leftover reactant. Gravity filtration can be used to collect solid product, although generally vacuum filtration is used for this purpose because it is faster.
A filtration procedure called "hot gravity filtration" is used to separate insoluble impurities from a hot solution. Hot filtrations require fluted filter paper and careful attention to the procedure to keep the apparatus warm but covered so that solvent does not evaporate. Hot gravity filtrations are no longer included in the routine procedures for the experiments in the organic chemistry teaching labs. If you need to do such a filtration, read the procedure in the Handbook and consult your TA.
To perform a standard gravity filtration, first select the size of filter paper that, when folded, will be a few millimeters below the rim of your stemmed funnel. Fold the paper into a cone by first folding it in half, and then in half again, as shown.
Next, support the glass funnel in a ring or place it in the neck of an Erlenmeyer flask. Wet the filter paper with a few milliliters of the solvent to be used in the following procedure. Wetting the paper holds it in place against the glass funnel. Pour the mixture to be filtered through the funnel, in portions if necessary.
Fluted filter paper is often better for gravity filtration with organic solvents. A diagram of how to fold fluted filter paper is shown below.
Vacuum filtration is used primarily to collect a desired solid, for instance, the collection of crystals in a recrystallization procedure. Vacuum filtration uses a Buchner funnel and a side-arm flask. Vacuum filtration is faster than gravity filtration, because the solvent or solution and air is forced through the filter paper by the application of reduced pressure. Do not use vacuum filtration to filter a solid from a liquid if it is the liquid that you want, and if the liquid is low boiling. Any solvent which boils at about 125 degrees or lower will boil off under the reduced pressure in the vacuum flask.
|To perform a vacuum filtration, clamp the flask securely to a ring stand.|
|Add a Buchner funnel with a rubber funnel adaptor.|
|Obtain a piece of filter paper in the funnel that is small enough to remain flat but large enough to cover all of the holes in the filter. If necessary, you can cut a larger piece of filter paper down to size.|
|Place the paper in the funnel.|
|Connect the side arm flask to a vacuum source. Always use thick-walled tubing, since Tygon tubing will collapse under reduced pressure.|
|Wet the paper with a small amount of the solvent to be used in the filtration. This causes the paper to adhere to the plate and keeps materials from passing under the paper during filtration. Turn on the vacuum source. Make sure that the paper is secure on the filter, that air is being drawn through the paper, and that all of your apparatus is securely clamped. If you are using a Neoprene filter adaptor, you might need to press on the funnel to engage the seal and thus the vacuum.|
|Pour the mixture to be filtered onto the filter paper. The vacuum should rapidly pull the liquid through the funnel. Watch that particulates do not creep under the edges of the paper. If this happens, start over and carefully pour portions of the solution onto the very center of the paper.|
|Rinse the cake with a small amount of fresh, cold solvent to help remove impurities that were dissolved in the filtrate.|
|Carefully disconnect the rubber tubing.|
|Remove the filter paper and the collected solid that is on it.|
|Usually you will need to set the product on a watch glass and let it air dry for a while.|