The purity of the precipitate is essential in gravimetric analysis, as impurities can significantly affect the accuracy of results.
Two common sources of impurities are co-precipitation and post-precipitation:
1) Co-precipitation:
Co-precipitation occurs when impurities or other ions in the solution precipitate along with the target analyte during the precipitation process.
This can happen due to:
a) Surface adsorption:
Impurities adsorb onto the surface of the precipitate particles, leading to contamination.
b) Occlusion and entrainment:
Impurities become trapped within or between the precipitate particles during their formation.
c) Mixed crystal formation:
Impurities are incorporated into the crystal lattice of the precipitate, resulting in mixed crystals.
Minimizing Co-precipitation:
Optimize precipitation conditions (temperature, pH, and concentration) to favor pure precipitate formation.
Use a selective precipitating agent that reacts only with the target analyte.
Perform digestion by heating or aging the precipitate to improve its purity and particle size.
2) Post-precipitation:
Post-precipitation happens when the initial precipitate partially dissolves, and the analyte re-precipitates with impurities. This often occurs due to changes in the solution's pH, temperature, or concentration during the analysis process.
Minimizing post-precipitation:
Maintain constant conditions, such as temperature and pH, throughout the analysis to prevent the dissolution and re-precipitation of the analyte.
Wash the precipitate thoroughly using a suitable solvent to remove any soluble contaminants or impurities.
Dry the precipitate gently and avoid overheating, as high temperatures can cause the precipitate to decompose or react with the impurities.