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 carefully to avoid significant losses of the precipitate. Of greatest concern is the po- tential for solubility losses. Usually the rinsing medium is selected to ensure that 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 determining particle size,
a volatile inert
electrolyte is often
added to the rinse water
to prevent the precipitate from reverting into smaller particles that may not be re- tained by the filtering
device. This process
of reverting to smaller particles
is called peptization. The volatile electrolyte is removed
when drying the
precipitate.
When rinsing a precipitate there
is a trade-off between introducing positive de-
terminate errors due to ionic
impurities from the precipitating solution
and intro- ducing negative determinate errors from
solubility losses. In general, solubility losses are minimized by using several
small portions of the rinse
solution instead of a single large volume. Testing
the used rinse solution for the presence
of impurities is another
way to ensure that the precipitate is not overrinsed. This can be done by testing for the presence of a targeted solution ion and
rinsing until the
ion is no longer detected in a freshly collected sample of the rinse solution. For example, when Cl– is known to be a residual
impurity, its presence
can be tested for by adding
a small amount of AgNO3 to the collected
rinse solution. A white precipitate of AgCl indicates that Cl– is present and additional rinsing is necessary. Additional rinsing is not
needed, however, if adding AgNO3 does not produce
a precipitate.
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