Q1(e). Postprecipitation versus coprecipitation

Answer in brief

Coprecipitation carries impurities down while the main solid forms; postprecipitation is the later deposition of another sparingly soluble substance.

Key points

• Coprecipitation includes surface adsorption, occlusion, inclusion and mixed-crystal formation during formation of the desired precipitate.
• The impurity need not independently exceed its own solubility limit; adsorbed ions or trapped mother liquor contaminate a growing solid.
• Postprecipitation occurs after the main precipitation: a slowly forming second solid deposits during prolonged contact with the mother liquor.
• Example: MgC₂O₄ slowly deposits on CaC₂O₄. Digestion can reduce some coprecipitation but prolonging it may increase postprecipitation.