Recrystallization: Solid–Solution Equilibria

Recrystallization is a technique used for purifying an impure solid compound using solubility differences between the desired product and the contaminants.

In one approach, the impure solid compound is initially dissolved in just enough hot solvent to form a saturated solution.

The distribution of a substance between the solid and solution phases in a saturated solution is temperature-dependent, typically favoring the solid at lower temperatures.

So, as the solution cools, nucleation begins, in which random aggregation of the concentrated solute leads to the formation of the first crystal, called a 'seed' or 'nucleus'.

More molecules are added to the seed in the particle growth or crystallization phase, creating crystals that can be collected by vacuum filtration.

Cooling rates are crucial in recrystallization. Rapid cooling generates many nucleation sites, forming smaller crystals. Slow cooling induces fewer nucleation sites, producing fewer but larger crystals.