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Chapter 8 Gravimetric Methods of Analysis 239

in a small portion of a suitable solvent at an elevated temperature. The solution is
then cooled to re-form the precipitate. Since the concentration ratio of interferent
to analyte is lower in the new solution than in the original supernatant solution, the
mass percent of included material in the precipitate decreases. This process of re-
precipitation is repeated as needed to completely remove the inclusion. Potential
solubility losses of the analyte, however, cannot be ignored. Thus, reprecipitation
requires a precipitate of low solubility, and a solvent for which there is a significant
difference in the precipitate’s solubility as a function of temperature.
Occlusions, which are a second type of coprecipitated impurity, occur when occlusion
physically adsorbed interfering ions become trapped within the growing precipitate. A coprecipitated impurity trapped
Occlusions form in two ways. The most common mechanism occurs when physically within a precipitate as it forms.
adsorbed ions are surrounded by additional precipitate before they can be desorbed
or displaced (see Figure 8.4a). In this case the precipitate’s mass is always greater than digestion
The process by which a precipitate is
expected. Occlusions also form when rapid precipitation traps a pocket of solution
given time to form larger, purer
within the growing precipitate (Figure 8.4b). Since the trapped solution contains dis- particles.
solved solids, the precipitate’s mass normally increases. The mass of the precipitate
may be less than expected, however, if the occluded material consists primarily of the adsorbate
analyte in a lower-molecular-weight form from that of the precipitate. A coprecipitated impurity that adsorbs
Occlusions are minimized by maintaining the precipitate in equilibrium with to the surface of a precipitate.
its supernatant solution for an extended time. This process is called digestion and
may be carried out at room temperature or at an elevated temperature. During di-
gestion, the dynamic nature of the solubility–precipitation equilibrium, in which CACACACACACACACACACA
the precipitate dissolves and re-forms, ensures that occluded material is eventually ACACACACACACACACACAC
CACACACAMACACACACACA
exposed to the supernatant solution. Since the rate of dissolution and reprecipita-
ACACACACACACAMACACAC
tion are slow, the chance of forming new occlusions is minimal. CACACACACACACACACACA
After precipitation is complete the surface continues to attract ions from solu- AMACACACACACACACACAC
tion (Figure 8.4c). These surface adsorbates, which may be chemically or physically CACACACACACACACACACA
(a)
adsorbed, constitute a third type of coprecipitated impurity. Surface adsorption is
minimized by decreasing the precipitate’s available surface area. One benefit of di-
gestion is that it also increases the average size of precipitate particles. This is not CACACACACACACACACACA
ACACACACACACACACACAC
surprising since the probability that a particle will dissolve is inversely proportional
CACACACACACACACACACA
to its size. During digestion larger particles of precipitate increase in size at the ex- ACACACACACACACACACAC
pense of smaller particles. One consequence of forming fewer particles of larger size CACACACACACACACACACA
is an overall decrease in the precipitate’s surface area. Surface adsorbates also may ACACACACACACACACACAC
CACACACACACACACACACA
be removed by washing the precipitate. Potential solubility losses, however, cannot
(b)
be ignored.
Inclusions, occlusions, and surface adsorbates are called coprecipitates because
C A
they represent soluble species that are brought into solid form along with the de-
A C C
sired precipitate. Another source of impurities occurs when other species in solu- C C C
tion precipitate under the conditions of the analysis. Solution conditions necessary C CACACACAC C
to minimize the solubility of a desired precipitate may lead to the formation of an C ACACACACACACAC
CACACACACACACACACACAC
additional precipitate that interferes in the analysis. For example, the precipitation
(c)
of nickel dimethylgloxime requires a pH that is slightly basic. Under these condi-
tions, however, any Fe 3+ that might be present precipitates as Fe(OH) 3 . Finally, Figure 8.4
since most precipitants are not selective toward a single analyte, there is always a Example of coprecipitation: (a) schematic of
risk that the precipitant will react, sequentially, with more than one species. a chemically adsorbed inclusion or a
physically adsorbed occlusion in a crystal
The formation of these additional precipitates can usually be minimized by lattice, where C and A represent the
carefully controlling solution conditions. Interferents forming precipitates that are cation–anion pair comprising the analyte
M
less soluble than the analyte may be precipitated and removed by filtration, leaving and the precipitant, and is the impurity;
(b) schematic of an occlusion by entrapment
the analyte behind in solution. Alternatively, either the analyte or the interferent of supernatant solution; (c) surface
can be masked using a suitable complexing agent, preventing its precipitation. adsorption of excess C.