Most often you will be asked to follow a published crystallization procedure that may be written in one of two varieties:
- Solid A was crystallized using Y solvent, or
- Solid A was crystallized using X mL of Y solvent
When no volume of solvent is specified, the process should be conducted using the minimum volume of boiling solvent to dissolve the solid. If a volume is specified, it probably means the crystallization is tricky and previous trials have shown that a greater than minimal solvent volume produce higher quality crystals, or attempting to dissolve common impurities leads to using too great a quantity of solvent.
The standard crystallization process (Figure 3.38a) is summarized as follows:
- Dissolve the impure solid in the minimum amount of hot solvent.
- Slowly cool the solution to induce crystallization.
- Further cool the solution in an ice bath.
- Collect the solid by suction filtration.
An additional step may be inserted into this general process (between steps 1 and 2) if insoluble impurities are present, or if impurities will be removed with charcoal (Figure 3.38b). After the compound is dissolved in the minimal amount of hot solvent, charcoal can be used, or insoluble impurities removed with hot filtration. The general process can then be continued on to next cool the solution and induce crystallization.