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:

1. Surface Adsorption:

   • Impurities adsorb onto the surface of the precipitate particles, leading to contamination.

2. Occlusion and Entrainment:

   • Impurities become trapped within or between the precipitate particles during their formation.

3. 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 (temperature, pH) throughout the analysis to prevent dissolution and re-precipitation.
• Wash the precipitate thoroughly with a suitable solvent to remove soluble impurities.
• Dry the precipitate gently, avoiding high temperatures that may cause decomposition or reactions with impurities.

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