Techniques on Gravity Filtration

There are two general methods of filtration: gravity and vacuum. In gravity filtration, the filtrate passes through the filter medium under the force of gravity and the capillary action between the liquid and the funnel stem.

There are several varieties of filter paper. Good filtration depends on the retention of the filter paper and the speed of the filter paper. Usually, fast papers will retain coarse particles. Slow papers will retain fine particles. The optimum choice is a paper, which is as fast as possible, yet retains all visible particles, and thus giving a clear filtrate.

Low-ash or ashless quantitative-grade papers can be ignited without leaving an ash. The residue left by an 11-cm circle of a low-ash paper may be as low as 0.06 mg. An ashless-grade paper typically leaves 0.05 mg or less from an 11-cm circle. This small mass is considered negligible in most analytical procedures.

Decantation is a process used to separate the liquid from the mixture to be filtered. To decant a liquid from a solid, in one hand hold the beaker that has the mixture in it. Hold a glass-stirring rod in the other hand. Touch the lip of the beaker to the glass rod and pour the mixture to be filtered using the glass rod as a guide to pour slowly to ensure that the solid is not carried along. This also prevents the liquid from running back along the outside of the beaker.

Washing of the solid to remove soluble impurities follows the decantation of the supernatant liquid. Use a small amount of wash liquid and mix it thoroughly with the solid. Allow the solid to settle and decant the wash liquid through the filter. Repeat this procedure several times. Several washings with small volumes of liquid are more effective in removing soluble contaminants.

Transfer the washed solid in the filter funnel is the final step. The bulk of the solid is transferred to the funnel by a squirting a stream of wash liquid from a wash bottle. The last traces of the solid are removed from the walls of the beaker by scrubbing with a rubber policeman. Rinse the beaker and rubber policeman and transfer the rinse liquid to the funnel. Repeat this at least two times.

Step to prepare a gravity filtration

1. It is important to use the correct size filter paper. Properly sized filter paper should stop just below the rim of the glass funnel. As a guide, use filter paper whose diameter is about 1 cm less than twice the diameter of the funnel, for example a six-centimeter diameter funnel uses a filter paper of eleven-centimeter diameter. The filter paper should sit a few millimeters from the rim of the funnel (Figure 1).

2. Fold the filter paper by referring to Figure 2.

(a) Fold the filter paper in half.
(b) Fold the filter again to within about 10 deg of a 90 deg fold. The second fold is not exactly at a right angle. Tear off the corner of top fold.
(c) Open the filter paper so that the torn corner is on the outside of the cone. The tear enables the paper to stick better to the funnel.

3. Place the folded filter paper snugly into the funnel by moistening the filter paper with the solvent of the mixture to be filtered. This should ressemble Figure 2 (c).

4. Press the filter paper against the top wall of the funnel to form a seal. Support the funnel with a funnel rack.

5. Set up the gravity filtration apparatus as per Figure 3. Ensure that the funnel rack is positioned so that the funnel stem is inside the beaker. Position the beaker so that the funnel stem is touching the side of the beaker to avoid splashing.

6. Allow the mixture to settle and then decant the liquid from the solid. Wash the solid which remains in the beaker several times. Finally, transfer the washed solid to the funnel.

Techniques on Vacuum filtration

This type of filtration is used with water or high-boiling organic solvents and is much faster than gravity filtration. For the set-up (Figure 4), a filter flask must be clamped in position before attaching the rubber tubing, rubber stopper (adapter) and Büchner funnel (or crucible). This prevents the top-heavy apparatus from toppling over and spilling material. A medium- or slow-speed filter paper is used that is wetted with the solvent before the vacuum is applied with the water aspirator. Use a large beaker under the aspirator to minimize splashing. Check that there is a good seal between the apparatus when vacuum is applied before filtering the sample.

During the filtration, the mixture should be poured at a rate that the bottom of the funnel is covered with some solution. The collected crystals/precipitate can be washed with some chilled solvent. Do not discard the mother liquor (in the filter flask) as more compound can be recovered.

Caution: Running water can be sucked back into the filtration apparatus if the water pressure decreases. Be sure to break the vacuum by disconnecting the tubing at the aspirator before turning off the running water.