Recrystallization: Gravity Filtration

Gravity Filter the Solution (if necessary)

If you have used the activated charcoal, or if you have observed insoluble impurities in the solution, you will need to filter the solution using the gravity filtration technique. This utilizes a stemless funnel and fluted filter paper to remove the solids. Interestingly, gravity filtration is carried out while heating the entire apparatus, usually with a steambath, to prevent recrystallization of the product on the filterpaper and funnel assembly.

First, Finely Flute (Fold) the Filter Paper.

The filter paper is fluted to keep its surfaces pushed away from the sides of the funnel. As you make the following folds, avoid sharply creasing the paper to avoid forming holes that would ruin the filter paper.

Begin by locating the exact center of the filter paper.

Do this by folding the paper first in half and then in quarters. Return the filter paper to the half-folded state

Next, fold a small section of the filter paper inward with the bottom of the fold located at the center of the filter paper.

Make each of these divisions or flutes as small as possible.

Alternating forward and backward folds, create as many additional flutes as possible.

This procedure is similar to the method used by some children to make toy fans to cool themselves. Note that at the center point many folds come together. Be careful not to pinch this region or a hole will develop.

Gently open the filter paper

Now would be a good time to pre-weigh the filter paper. If you need to make an identifying mark on the paper, use a pencil, not a pen. The ink, soluble in most organic solvents, may later contaminate your sample.

Warm the stemless funnel.

Unwanted recrystallization will occur on the surfaces of a cool funnel. It is therefore necessary to heat the funnel prior to using it to filter the hot solution. Do this by positioning the funnel, upside down on a beaker of hot solvent. Solvent vapors will rise and warm the funnel to the correct temperature.

Quickly assemble the hot gravity filtration apparatus and dump the solution to be filtered.

Working on the hot plate, position the stemless funnel/filter paper assembly over a clean flask. Before the funnel has had time to cool, dump the hot solution containing the dissolved solid/impurities onto the filter paper. Undissolved solids and the decolorizing carbon will be trapped on the filter paper while the solution containing the dissolved product will be collected in the container below. Suggestion: use a folded paper towel to hold the hot flask while pouring the solution.

Use a small amount of hot solvent to rinse the filter paper.

Some product may have recrystallized on the filter paper during the above procedure. To recover this product, use a Pasteur pipette and hot solvent to redissolve and rinse the solid product through the filter paper and into the flask below. Since excess solvent will have to be boiled away from the filtrate before recrystallization occurs, it is to your advantage to minimize the amount of additional solvent you use.