4.6: Recrystallization: Solid–Solution Equilibria
Recrystallization is a purification technique used to separate impurities from solid compounds. In this technique, no chemical reactions occur. Instead, it exploits physical properties only, specifically, the solubility differences between the desired compound and impurities, either at a single temperature or at different temperatures, and under other selected conditions. The solid-solution equilibrium (solubility equilibrium) of each component in the solution represents a binary phase equilibrium, in which the component is distributed between solid and solution phases. At equilibrium, the chemical potential of any single component in its solution phase is equal to that in its solid phase.
During the first step of recrystallization, a compound is dissolved in a minimum amount of hot solvent(s) to obtain a saturated solution. Undissolved impurities are removed from the solution by filtration at this temperature. The solution is then allowed to cool. At this point, the solubility of the desired compound decreases, resulting in the formation of small crystals. The solution is then filtered to retain the purified crystals on the filtration medium.
The solvent plays an essential role in the recrystallization process. For recrystallization based on solubility at different temperatures, the solvent or solvent pairs are chosen so that the desired compound is soluble in the solvent at high temperatures but insoluble in the solvent at low temperatures. The other factor in the choice of solvent(s) is that the impurities should be soluble in the solvent(s) at all chosen temperatures.
Cooling rate also plays a significant role in recrystallization, especially in determining the size and quality of the crystals. Rapid cooling favors the formation of small crystals, whereas slow cooling helps the growth of large and generally purer crystals.
