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Chapter 8 Gravimetric Methods of Analysis 245

termining 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. peptization
When rinsing a precipitate there is a trade-off between introducing positive de- The reverse of coagulation in which a
terminate errors due to ionic impurities from the precipitating solution and intro- coagulated precipitate reverts to smaller
particles.
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 AgNO 3 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 AgNO 3 does not produce a precipitate.
Drying the Precipitate Finally, after separating the precipitate from its super-
natant solution the precipitate is dried to remove any residual traces of rinse solu-
tion and any volatile impurities. The temperature and method of drying depend on
the method of filtration, and the precipitate’s desired chemical form. A temperature
of 110 °C is usually sufficient when removing water and other easily volatilized im-
purities. A conventional laboratory oven is sufficient for this purpose. Higher tem-
peratures require the use of a muffle furnace, or a Bunsen or Meker burner, and are
necessary when the precipitate must be thermally decomposed before weighing or
when using filter paper. To ensure that drying is complete the precipitate is repeat-
edly dried and weighed until a constant weight is obtained.
Filter paper’s ability to absorb moisture makes its removal necessary before
weighing the precipitate. This is accomplished by folding the filter paper over the
precipitate and transferring both the filter paper and the precipitate to a porcelain
or platinum crucible. Gentle heating is used to first dry and then to char the filter
paper. Once the paper begins to char, the temperature is slowly increased. Although
the paper will often show traces of smoke, it is not allowed to catch fire as any pre-
cipitate retained by soot particles will be lost. After the paper is completely charred
the temperature is slowly raised to a higher temperature. At this stage any carbon
left after charring is oxidized to CO 2.
Fritted glass crucibles cannot withstand high temperatures and, therefore,
should only be dried in an oven at temperatures below 200 °C. The glass fiber mats
used in Gooch crucibles can be heated to a maximum temperature of approxi-
mately 500 °C.

Composition of Final Precipitate The quantitative application of precipitation
gravimetry, which is based on a conservation of mass, requires that the final precipi-
tate have a well-defined composition. Precipitates containing volatile ions or sub-
stantial amounts of hydrated water are usually dried at a temperature that is suffi-
cient to completely remove the volatile species. For example, one standard
gravimetric method for the determination of magnesium involves the precipitation
6H
of MgNH 4 PO 4× 2 O. Unfortunately, this precipitate is difficult to dry at lower
temperatures without losing an inconsistent amount of hydrated water and ammo-
nia. Instead, the precipitate is dried at temperatures above 1000 °C, where it decom-
poses to magnesium pyrophosphate, Mg 2 P 2 O 7 .