Precipitation Processes

Slow addition of the precipitant in a dilute solution with constant stirring lowers the solute concentration, that is, Q, decreasing the relative supersaturation and increasing the particle size.

In homogeneous precipitation, the precipitant is slowly generated in situ, avoiding local reagent excesses. For example, urea decomposes gradually to release hydroxide ions as the precipitant.

At higher temperatures, solubility increases, and supersaturation decreases, leading to larger crystals.

During precipitate formation, rapidly growing crystals take up ions from the mother liquor, resulting in absorbed or adsorbed coprecipitated impurities.

So, crystalline precipitates are allowed to stand in the hot mother liquor in a process called digestion or Ostwald ripening.

During digestion, smaller crystals dissolve and precipitate onto the surface of larger crystals, while adjacent particles form bridges.

This slow recrystallization increases particle size and decreases the effects of coprecipitation.