To precipitate salt {crystallization}|, reduce salt solubility. Slow precipitation makes larger crystals, because they can form and reform. Crystals have fewer impurities, because area is smaller. Maintaining low supersaturation, keeping pH low, holding temperature high, and using the most-dilute precipitating agents promotes slow crystallization. After precipitation, add excess agent, lower temperature, and raise pH to ensure complete precipitation. Precipitated crystals adsorb ions. Timed washing can remove impurities. Water washing can dissolve crystal or make colloid. Washing with volatile reagent or with solution with common ion does not dissolve crystal or make colloid.
Other salts can also precipitate {coprecipitation}, trapping or occluding solvent-molecule impurities. Using optimum conditions for reducing solubility minimizes trapped and occluded molecules. Re-precipitation can remove more impurities.
To prevent colloid formation, solutions do not move {Ostwald ripening} while crystals start to form.
5-Chemistry-Analytical Chemistry
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Date Modified: 2022.0225