Laboratory Tutorials - Filtering

Gravity Filtration

Flute filter paper if necessary. Fluting is desirable when a rapid filtration is necessary, as in the filtration of the hot solution during a recrystallization.
Place filter paper in the funnel. Place the funnel either directly on top of the beaker or on a ring stand above the beaker (see Figure 1 below).

Figure 1. A beaker containing a stemless funnel.
Note that the filter paper is fluted; however, that is not necessary for all applications.
Wet the filter paper using a small amount of the liquid that is the solvent of the mixture being filtered.
After the filtrate has been collected, pass a small amount of the wash liquid through the filter paper to wash the residue.

Select a Buchner funnel of appropriate size for the amount of residue you are collecting, not the amount of filtrate. Put a piece of filter paper in the funnel that fits exactly and wet the paper using a small amount of solvent.
Place the funnel in a filter flask and connect the rubber tubing (see Figure 2 below).

Figure 2. A Buchner funnel in a filter flask, with rubber tubing connected to an aspirator.
Using rubber tubing, the filter flask is connected to a trap, which in turn is connected to an aspirator, which provides the vacuum.
After the filtrate has been collected, pass a small amount of pure solvent through the filter to wash the residue.

If the weight of the residue must be obtained, heat a filter crucible in the oven to dry it. Cool in a desiccator and weigh crucible when cool. Repeat until the mass is within 0.2 mg of the previous mass. This ensures that the crucible is completely dry. Store in a desiccator until needed.
To use a filter crucible, set up vacuum filtration by placing the filter crucible in a holder and placing that in a filter flask (see Figure 3 below).

Figure 3. A filter crucible in a crucible holder in a filter flask, with rubber tubing connected to an aspirator.
The rubber tubing is connected to a trap, which in turn is connected to an aspirator, which provides the vacuum.
After the mother liquor has passed completely through the crucible, pass a small amount of solvent through the crucible to wash the residue.
Place the filter crucible in the oven to dry the residue.