5.19 : Precipitation Processes

The experimental conditions in a gravimetric analysis should be optimized to maximize the particle size and purity of the obtained precipitate. Ideally, the concentration of the precipitating reagent should be low with effective stirring to maintain low relative supersaturation for the growth of large crystals. In homogeneous precipitation, the precipitant is slowly generated by a chemical reaction in the solution to avoid local reagent excesses. For example, urea decomposes gradually to release hydroxide ions in the precipitation of aluminum as its hydroxide. At higher temperatures, solubility increases, and supersaturation decreases, resulting in larger crystals.

The precipitate may take up substances normally soluble in the mother liquor, resulting in coprecipitated impurities that could be adsorbed or absorbed. To minimize these impurities, crystalline precipitates are allowed to stand in the hot mother liquor in a process called digestion (Ostwald ripening). During digestion, small crystals tend to 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.