|Modified version of an IUCR.org graph|
Controlling the crystallization pressure is essential for both purification by crystallization and for efficient operations on scale. By adjusting solution conditions to decrease the solubility of the product within the metastable zone, the desired molecules can be pressured to come out of solution and crystallize. Gradual cooling without seeding leads to one nucleation event and the formation of relatively small crystals. Slowly applying the crystallization pressure can produce large crystals, depending on the size and number of seeds. If the crystallization pressure is too great, supersaturation occurs and molecules may be forced out of solution too quickly as small crystalline solids, noncrystalline precipitates, oil.There's a very tricky dance in a good crystallization -- you don't want a solution, you don't want a precipitate. You want a long residence in the metastable zone with not too many nucleations (too many nucleations = small crystals.) You want to slowly increase crystallization pressure by lowering the temperature or by adding an anti-solvent. Too fast, and you're going to precipitate and possibly oil out a great deal of product.
Nervous, nervous, nervous.