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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.