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FILTRATION TECHNIQUES
Filtration, the technique used to separate solids from liquids, is the act of pouring a mixture onto
a membrane (filter paper) that allows the passage of liquid (the filtrate) and results in the
collection of the solid. Two filtration techniques are generally used in chemical separations in
general chemistry lab: "gravity" filtration and "vacuum" filtration.
Gravity Filtration
Gravity filtration uses a polyethylene or glass funnel with a stem and filter paper. Filter paper
can have pore sizes ranging from small to large to permit slow to fast filtering. The paper is
folded in half (Figure 1), then folded in quarters, and the tip of one corner is torn off to allow for
a snug fit in the funnel cone. (If the paper has been pre-weighed, the torn corner piece must be
saved to add to the post-filter weighing to avoid any errors.) The paper cone is fitted to the
funnel so three thicknesses of the paper line one-half of the cone and one thickness lines the
opposite half (Figure 1). Now place the funnel into a beaker and wet the filter paper completely
Open filter paper
cone and line funnel
Tear off corner.
Save corner if
filter paper has
been weighed.
Fold paper in half^ Fold paper in quarters
Figure 1: How to Fold Filter Paper
with the dominate solvent or solvents in the mixture to be filtered. This step adheres the filter
paper to the funnel walls preventing solid from escaping. Then, support the funnel with a clamp
or ring (if necessary) and place a clean beaker beneath the funnel so the stem rests against the
side of the beaker (this prevents splattering).
Before filtering, allow most of the solid in the mixture to settle. Now pour the supernatant liquid
(the liquid standing over the solid in a mixture) through the filter first. This will allow the initial
part of the filtration to proceed faster and may prevent clogging of the filter by the solid. To
prevent splattering pour the liquid down a glass rod as shown in Figure 2.
Figure 2. Gravity Filtration
Scrape the solid onto the filter with a rubber policeman or spatula. Rinse the spatula, glass rod
and beaker and pour the washings into the filter funnel. If the remaining solid residue is to be
washed, rinse with three small portions (a few milliliters each) of an appropriate solvent. If the
solid is to be saved, remove the filter paper carefully and place it on a watch glass to dry.
Caution: Wet filter paper tears easily.
Supernatant liquid
containing settled
solids. Allow liquid to travel
along a glass stir rod to
prevent splashing &
control flow.
Collection
Beaker
Iron Support
Ring
Line funnel
with filter
paper.
hose is part of the filtration system and should be heavy enough to prevent pinching or collapse
under external atmospheric pressure.
To filter a sample, turn on the aspirator and carry out the filtration in
the same manner described for gravity filtration. (Note: NEVER pry
off the funnel if the system is under vacuum! Water can flash back
into the collection flask or the filter paper can be damaged
resulting in the loss of filtered solid.) Turn off the water aspirator
before carefully removing the wet filter paper without tearing.
Reaction byproducts (either the solid or filtrate) should be placed
into appropriate labeled containers in the hood.
Figure 5.
Vacuum Filtration Assembly
Review Questions:
a) filtrate
b) supernatant liquid
c) water aspirator
d) gravity filtration
e) vacuum filtration
- Why do we wet the filter paper when setting it into the funnel before filtering a solution?
- When using gravity filtration, air bubbles in the liquid in the funnel stem indicates what type
of problem? If this problem is not corrected can solids leak through the funnel into the
filtrate?
- When using gravity filtration, where should the funnel stem be placed? Why?
- For best results, should the solid be washed with a single large washing or with several small
washings?
- What type of funnel should be used in gravity filtration? in vacuum filtration?
- When using vacuum filtration, why should you never pry off the funnel while the system is
under vacuum?
- List some advantages and disadvantages of using:
a) gravity filtration
b) vacuum filtration
