Coprecipitation
Coprecipitation is the phenomenon in which soluble compounds are removed
from solution during precipitate formation.
There are four types of
coprecipitation:
i)
surface adsorption,
ii)
mixed-crystal
formation,
iii)
occlusion,
iv)
mechanical entrapment
Surface adsorption and mixed crystal formation are equilibrium processes,
whereas occlusion and mechanical entrapment arise from the kinetics of crystal
growth.
Surface Adsorption
Adsorption is a common source of coprecipitation that is likely to cause
significant contamination of precipitates with large specific surface areas,
that is coagulated colloids.
Coagulation of a colloid does not significantly decrease the amount of
adsorption because the coagulated solid still contains large internal surface
areas that remain exposed to the solvent. The coprecipitated contaminant on the
coagulated colloid consists of the lattice ion originally adsorbed on the
surface before coagulation and the counter ion of opposite charge held in the
film of solution immediately adjacent to the particle. The net effect of
surface adsorption is therefore the carrying down of an otherwise soluble
compound as a surface contaminant.
Minimizing Adsorbed Impurities on Colloids
The purity of many coagulated colloids is improved by digestion. During
this process, water is expelled from the solid to give a denser mass that has a
smaller specific surface area for adsorption.
Washing a coagulate colloid with a solution containing a volatile
electrolyte may also be helpful because any nonvolatile electrolyte added
earlier to cause coagulation is displace by the volatile species. Washing
generally does not remove much of the primarily adsorbed ions because the
attraction between these ions and the surface of the solid is too strong.
Exchange occurs, however between existing counter ions and ions in the wash
liquid.
Reprecipitation
A drastic but effective way to minimize the effects of adsorption is
reprecipitation, or double precipitation. Here, the filtered solid is
redissolved and reprecipitated. The first precipitate ordinarily carries down
only a fraction of the contaminant present in the original solvent. Thus, the
solution containing the redissolved precipitate has a significantly lower
contaminant concentration than the original, and even less adsorption occurs
during the second precipitation. Reprecipitation adds substantially to the time
required for an analysis.
Mixed-Crystal Formation
In mixed-crystal formation, one of the ions in the crystal lattice of a
solid is replaced by an ion of another element. For this exchange to occur, it
is necessary that the two ions have the same charge and that their sizes differ
by no more than about 5%. Furthermore, the two salts must belong to the same
crystal class. For example, MgKPO4, in MgNH4PO4,
SrSO4 in BaSO4, and MnS in CdS.
The extent of mixed-crystal contamination increases as the ratio of
contaminant to analyte concentration increases. Mixed-crystal formation is
troublesome because little can be done about it. Separation of the interfering
ion may have to be carried out before the final precipitation step.
Alternatively, a different precipitating reagent may be used.
Occlusion and Mechanical Entrapment
When a crystal is growing rapidly during precipitate formation, foreign
ions in the counter-ion layer may become trapped, or occluded, within the
growing crystal.
Mechanical entrapment occurs when crystals lie close together during
growth. Here, several crystals grow together and in so doing trap a portion of
the solution in a tiny pocket.
Both occlusion and mechanical entrapment are at a minimum when the rate
of precipitate formation is low, that is, under conditions of low
supersaturation. Digestion is often remarkably helpful in reducing these types
of copreipitation. The rapid solution and reprecipitation that goes on at the
elevated temperature of digestion opens up the pockets and allows the
impurities to escape into the solution.
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