Recrystallization is a fundamental laboratory technique used for purifying solid compounds containing impurities. This process leverages differences in solubility between the desired compound and the contaminants to achieve separation. The primary goal is to obtain a pure, crystalline product by dissolving the impure solid and then inducing the pure compound to solidify again. The technique relies on the principle that most solids are significantly more soluble in a hot solvent than in a cold one.
Selecting the Ideal Solvent
Choosing the correct solvent is the most important step in a successful recrystallization procedure. An ideal solvent must exhibit a high temperature coefficient, dissolving the compound readily at its boiling point but only sparingly at room temperature or below. This difference in solubility drives the purification process, allowing maximum recovery of the pure material upon cooling.
The solvent must not react chemically with the compound being purified, as this would alter the product. Furthermore, the solvent should either dissolve the impurities very well, keeping them in solution even when cold, or not at all, allowing them to be removed by filtration. A solvent with a relatively low boiling point is preferred because it can be easily removed from the final, purified crystals through evaporation.
The Four Stages of Recrystallization
Dissolution
The purification process begins by dissolving the impure solid in the minimum possible amount of hot solvent. The solvent should be heated to near its boiling point before adding it dropwise to the solid while swirling. Using the minimum volume of hot solvent ensures the resulting solution is saturated, which is necessary for effective crystallization upon cooling. If too much solvent is used, the compound will remain dissolved, resulting in a low recovery.
Hot Filtration
If the hot solution contains insoluble impurities, such as dust or undissolved starting material, they must be removed before crystallization begins. This is achieved through hot gravity filtration, where the solution is passed through a filter funnel while still hot. The funnel apparatus must be kept warm to prevent the desired compound from prematurely crystallizing within the filter paper. For colored solutions, a small amount of decolorizing carbon can be added to adsorb colored impurities before filtration.
Cooling and Crystallization
The clear, hot, saturated solution is allowed to cool slowly and without disturbance, first to room temperature and then often in an ice-water bath. Slow cooling promotes the formation of large, well-defined crystals that are inherently purer. Rapid or “shock” cooling can cause the compound to precipitate quickly as an amorphous solid, trapping impurities and defeating the purpose of purification. The gradual temperature decrease reduces the solubility of the desired compound, causing it to form the pure crystal lattice.
Isolation and Drying
Once crystallization is complete, the pure solid crystals are separated from the remaining liquid, called the mother liquor, which contains the soluble impurities. This separation is typically performed using vacuum filtration with a Büchner funnel and a filter flask, drawing the solvent away from the solid. The crystals are then rinsed with a minimal amount of ice-cold solvent to wash away any residual mother liquor. Finally, the crystals are allowed to air-dry completely, often under vacuum, to remove traces of the volatile solvent before the pure product is weighed.
The Underlying Solubility Principle
The effectiveness of recrystallization stems directly from the principle that solubility is a function of temperature for most solids. A solubility curve graphically illustrates this relationship, showing the amount of solute that can be dissolved in a given amount of solvent at various temperatures. The process creates a solution that is saturated at the solvent’s boiling point but highly unsaturated at a cold temperature.
As the hot, saturated solution cools, it enters a state of supersaturation where the solute concentration exceeds its equilibrium solubility limit. This unstable state provides the driving force for the compound to transition back into a solid, forming the crystal lattice. During this crystal growth phase, the highly ordered structure of the forming lattice preferentially excludes foreign molecules, such as impurities.
The impurities, present in much smaller amounts than the desired compound, remain dissolved in the cold solvent because the solution never becomes saturated with respect to them. The act of forming a perfect, repeating crystal structure physically separates the pure compound from the dissolved contaminants. This exclusion mechanism is the core reason recrystallization yields a purer product.
Addressing Common Procedural Problems
A common issue is the failure of crystals to form, often because too much solvent was added initially. If the solution is not sufficiently saturated upon cooling, the compound may remain dissolved. This can be fixed by boiling off excess solvent to concentrate the solution. If a solution becomes supersaturated but resists crystallization, it can be remedied by scratching the inside of the flask with a glass stirring rod to create a rough surface for nucleation.
Another problem is “oiling out,” where the compound separates as a liquid oil instead of a solid crystal upon cooling. This usually happens when the compound’s melting point is lower than the temperature at which it precipitates. To prevent this, more solvent can be added to dilute the solution, followed by re-heating and attempting a much slower cooling process. Using a solvent with a lower boiling point can also minimize the chances of oiling out.