Recrystallization is a fundamental purification technique used to obtain pure solid compounds from an impure mixture. The process relies on the principle that solubility in a given solvent generally increases with temperature. An impure solid is dissolved in a hot solvent, creating a saturated solution. As the solution cools, the desired compound crystallizes out, leaving impurities behind in the remaining liquid, known as the mother liquor. Selecting the correct solvent is the most important factor determining the success of this purification method.
The Ideal Temperature-Dependent Solubility
The primary requirement for a good recrystallization solvent is a significant difference in how much of the compound dissolves at high temperatures versus low temperatures. The solvent should dissolve the target compound completely when heated to its boiling point, which ensures the compound is fully liberated from its original crystal lattice. Using the minimum amount of hot solvent necessary is crucial, as this creates a highly concentrated, saturated solution at the elevated temperature.
Upon cooling, the compound’s solubility must drop sharply, leading to a state of supersaturation. This solubility differential forces the compound to precipitate out, organizing its molecules into a new, pure crystal structure. If the compound remains highly soluble in the cold solvent, little solid will be recovered, resulting in a low yield. Conversely, if the compound is insoluble even when the solvent is hot, purification cannot begin.
How the Solvent Handles Impurities
A successful purification relies on the solvent’s ability to manage the impurities present in the original solid. Ideally, the solvent interacts with impurities in one of two specific ways to ensure they do not contaminate the final crystals.
The first scenario is for impurities to be completely insoluble in the hot solvent. These insoluble contaminants, such as dust or reaction byproducts, remain as solids and can be easily removed from the hot solution via a rapid filtration technique.
The second, and more common, scenario is for the impurities to remain highly soluble in the solvent, even after the solution has cooled significantly. They will not reach the saturation point and will stay dissolved in the cold mother liquor when the purified compound crystallizes. Intermediate solubility for an impurity is detrimental because, upon cooling, the impurity might begin to crystallize alongside the desired compound, or it could be physically trapped within the growing crystal lattice, which lowers the purity of the final product.
Essential Physical and Chemical Properties
A good recrystallization solvent must possess several practical physical and chemical attributes for a safe and effective procedure. The boiling point of the solvent must be lower than the melting point of the compound being purified. If the solvent’s boiling point is higher than the compound’s melting point, the compound will melt and form an oil instead of dissolving, a phenomenon called “oiling out,” which prevents proper crystal formation.
The solvent needs to be chemically inert, meaning it cannot react with the compound or impurities during the heating and cooling process. Reactivity would alter the target compound, defeating the purpose of purification.
Furthermore, the solvent should be volatile enough to be easily removed from the final crystals after filtration. High volatility, typically correlating with a low boiling point, allows the solvent to evaporate quickly when the crystals are dried, leaving behind only the pure solid.
When to Use Mixed Solvent Systems
In many cases, a single solvent that perfectly meets all the solubility criteria is not available. When this occurs, a mixed solvent system, or solvent pair, is employed to artificially create the necessary temperature-dependent solubility profile.
This technique uses two different, but miscible, solvents: a “good” solvent, in which the compound is highly soluble, and a “bad” solvent, in which the compound is nearly insoluble.
The process begins by dissolving the impure compound in the minimal amount of the hot, “good” solvent. The “bad” solvent is then added dropwise to the hot solution until persistent cloudiness or turbidity is observed, indicating that the solution is now saturated with the compound. The addition of the “bad” solvent reduces the overall dissolving power of the mixture, setting up the solution to yield crystals upon cooling. Both solvents must mix completely to form a single homogenous solution, ensuring a uniform crystallization environment.