When do we use Gravity Filtration?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 filtrationgravity filtration– suction filtrationsuction filtration

• In this course, we will use gravity filtrationgravity filtration primarily when our objective is to further process the solutionsolution.– 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 The ApparatusApparatus

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 PaperPreparing 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 Placing the Filter Paper in the Funnel - 1Funnel - 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 Placing the Filter Paper in the Funnel - 2Funnel - 2

Transferring the liquid into the Transferring the liquid into the funnelfunnel

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 Rinse the original container with a wash bottlebottle

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.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 Wash the precipitate with a small amount of waterwater

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.