When do we use Gravity Filtration? Filtration is the separation method of choice when the phases to be separated are a solid (precipitate) and a liquid (solution). There are two major filtration techniques –gravity filtration –suction filtration gravity filtration solutionIn this course, we will use gravity filtration primarily when our objective is to further process the solution. –We will normally discard the solid material that is captured in the filter paper Suction filtrationSuction filtration is normally used when the solid phase is the one in which we are interested. A separate presentation deals with suction filtration.* Our solutions will normally be aqueous, i.e., the solvent will normally be water. The same principles apply for other solvents. –In those cases, the wash bottle shown in some of the later steps would contain the appropriate solvent Some analytical techniques use gravity filtration even when the solid is desired. The paper is later burned away. Special paper is used which leaves no ash.

The Apparatus ring stand ring triangle funnel filter paper

Fold the filter paper exactly in half Fold the filter paper a second time, this time with a slight offset. I.e., not exactly in half. Tear a corner from the smaller side of the filter paper. Preparing the Filter Paper

Open filter paper on the larger side so that torn corner is on the side of the cone with three paper surfaces. place funnel in wire triangle supported by ring on ring stand Place filter paper in funnel pressing top edge of cone so that it makes continuous contact with funnel Placing the Filter Paper in the Funnel - 1

Using a wash bottle, wet the filter paper to help make it adhere as much as possible to the funnel. Filtration is most efficient if the stem of the funnel contains a continuous liquid column in the stem of the funnel. Show wash bottle It is not easy to establish a continuous liquid column. Placing the Filter Paper in the Funnel - 2

Transferring the liquid into the funnel Show beaker, stirring rod Use a stirring rod to direct the flow of the liquid to the center of the funnel. Do not let the liquid level rise above the top edge of the filter paper! Do not let the stirring rod touch the filter paper. It may tear – allowing solid to pass through.

Rinse the original container with a wash bottle While we are not interested in recovering the solid, we must be sure to wash the precipitate that remains in the original container to be sure we have captured all of the liquid phase. This is done using several small portions of water from the wash bottle.

Wash the precipitate with a small amount of water Some solution will adhere to the precipitate in the funnel. Washing the precipitate with a wash bottle assures that all of the liquid containing the substance in which we are interested is captured in the receiving vessel. Since the (unwanted) solid may have some solubility in water, use a small amount of water to wash the precipitate.