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238 Modern Analytical Chemistry

Cl –
Ag +

Figure 8.3
Schematic model of AgCl showing Bulk silver ion
difference between bulk and surface atoms surrounded by 6 chloride ions
of silver. Silver and chloride ions are not Surface silver ion
shown to scale. surrounded by 4 chloride ions

Avoiding Impurities Precipitation gravimetry is based on a known stoichiometry
between the analyte’s mass and the mass of a precipitate. It follows, therefore, that
the precipitate must be free from impurities. Since precipitation typically occurs in
a solution rich in dissolved solids, the initial precipitate is often impure. Any impu-
rities present in the precipitate’s matrix must be removed before obtaining its
weight.
The greatest source of impurities results from chemical and physical interac-
tions occurring at the precipitate’s surface. A precipitate is generally crystalline,
even if only on a microscopic scale, with a well-defined lattice structure of cations
and anions. Those cations and anions at the surface of the precipitate carry, respec-
tively, a positive or a negative charge as a result of their incomplete coordination
+
spheres. In a precipitate of AgCl, for example, each Ag ion in the bulk of the pre-
–
cipitate is bound to six Cl ions. Silver ions at the surface, however, are bound to no
–
more than five Cl ions, and carry a partial positive charge (Figure 8.3).
Precipitate particles grow in size because of the electrostatic attraction between
charged ions on the surface of the precipitate and oppositely charged ions in solu-
tion. Ions common to the precipitate are chemically adsorbed, extending the crystal
lattice. Other ions may be physically adsorbed and, unless displaced, are incorpo-
rated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions
are less strongly attracted to the surface and can be displaced by chemically ad-
sorbed ions.
inclusion One common type of impurity is an inclusion. Potential interfering ions whose
A coprecipitated impurity in which the size and charge are similar to a lattice ion may substitute into the lattice structure by
interfering ion occupies a lattice site in chemical adsorption, provided that the interferent precipitates with the same crystal
the precipitate.
structure (Figure 8.4a). The probability of forming an inclusion is greatest when the
interfering ion is present at substantially higher concentrations than the dissolved
lattice ion. The presence of an inclusion does not decrease the amount of analyte
that precipitates, provided that the precipitant is added in sufficient excess. Thus,
the precipitate’s mass is always larger than expected.
Inclusions are difficult to remove since the included material is chemically part
of the crystal lattice. The only way to remove included material is through reprecip-
itation. After isolating the precipitate from the supernatant solution, it is dissolved