Purity of the precipitate: co-precipitation and post precipitation

• 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.