Coprecipitation
Definition and meaning of Coprecipitation in chemistry.
Coprecipitation is the simultaneous removal of a normally soluble substance from solution along with the intended precipitate during a precipitation reaction, so that the isolated solid is contaminated with the impurity.
In more detail
It occurs through several mechanisms: surface adsorption of ions onto the precipitate, occlusion (impurity ions trapped within the growing crystal lattice or in pockets of solvent), isomorphous replacement (an ion of similar size and charge substitutes into the lattice), and mechanical entrapment. Coprecipitation is a major source of systematic error in gravimetric analysis, since the extra mass is weighed as if it were pure analyte. Chemists reduce it by precipitating from dilute, well-stirred solution, digesting (aging) the precipitate to promote recrystallization and expel occluded impurities, and by reprecipitation. Deliberately induced coprecipitation is also used to preconcentrate trace species for later analysis.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Main mechanisms | Adsorption, occlusion, isomorphous replacement, mechanical entrapment |
| Mitigation | Digestion (aging), dilute precipitation, reprecipitation, washing |
| Useful application | Trace-metal preconcentration via a carrier precipitate (e.g., Fe(OH)3) |
When barium sulfate is precipitated for a gravimetric sulfate determination, trace amounts of Pb2+ or Fe3+ present in the sample solution can coprecipitate with the BaSO4, adsorbing onto crystal surfaces or becoming occluded, which inflates the measured precipitate mass and biases the sulfate result high.
Frequently asked questions
How do chemists minimize coprecipitation in gravimetric analysis?
By precipitating slowly from dilute, hot solution, digesting the precipitate to let it recrystallize and release trapped impurities, washing with a suitable electrolyte, and reprecipitating if necessary.
Is coprecipitation always a problem?
Not always. While it introduces error in gravimetric determinations, it is deliberately exploited to concentrate trace ions onto a carrier precipitate before instrumental analysis.