Page 261 - Modern Analytical Chemistry

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244 Modern Analytical Chemistry

funnel is formed by dampening the paper with water and pressing the paper to the
wall of the funnel. When properly prepared, the stem of the funnel will fill with the
solution being filtered, increasing the rate of filtration. Filtration is accomplished by
the force of gravity.
The precipitate is transferred to the filter in several steps (Figure 8.7). The first
supernatant step is to decant the majority of the supernatant through the filter paper without
The solution that remains after a transferring the precipitate. This is done to prevent the filter paper from becoming
precipitate forms. clogged at the beginning of the filtration process. Initial rinsing of the precipitate is
done in the beaker in which the precipitation was performed. These rinsings are
also decanted through the filter paper. Finally, the precipitate is transferred onto the
filter paper using a stream of rinse solution. Any precipitate clinging to the walls of
the beaker is transferred using a rubber policeman (which is simply a flexible rubber
spatula attached to the end of a glass stirring rod).
An alternative method for filtering the precipitate is a filtering crucible (Fig-
ure 8.8). The most common is a fritted glass crucible containing a porous glass
disk filter. Fritted glass crucibles are classified by their porosity: coarse (retaining
particles > 40–60 mm), medium (retaining particles > 10–15 mm), and fine (re-
taining particles > 4–5.5 mm). Another type of filtering crucible is the Gooch cru-
cible, a porcelain crucible with a perforated bottom. A glass fiber mat is placed in
the crucible to retain the precipitate, which is transferred to the crucible in the
same manner described for filter paper. The supernatant is drawn through the
crucible with the assistance of suction from a vacuum aspirator or pump.

Rinsing the Precipitate Filtering removes most of the supernatant solution. Resid-
ual traces of the supernatant, however, must be removed to avoid a source of deter-
minate error. Rinsing the precipitate to remove this residual material must be done
Figure 8.7 carefully to avoid significant losses of the precipitate. Of greatest concern is the po-
Proper procedure for transferring the tential for solubility losses. Usually the rinsing medium is selected to ensure that
supernatant to the filter paper cone. solubility losses are negligible. In many cases this simply involves the use of cold
solvents or rinse solutions containing organic solvents such as ethanol. Precipitates
containing acidic or basic ions may experience solubility losses if the rinse solution’s
pH is not appropriately adjusted. When coagulation plays an important role in de-

Vent

Crucible
to Vacuum

Rubber
adapter

Rubber hose
Trap
Suction
flask

Figure 8.8
Procedure for filtering through a filtering
crucible. The trap is used to prevent water
from a water aspirator from backwashing
into the suction flask.