Recrystallization: Dissolving the Sample

The flowchart (left) summarizes the steps sometimes required by the recrystallization technique. In cases where the product has been contaminated by both soluble colored impurities and insoluble solids, it may be necessary to include all of the outlined steps.

First, choose a recrystallization solvent that behaves as follows:

Cold recrystallization solvent will not dissolve the solid.
Hot recrystallization solvent does dissolve the solid
Cooling the hot solution returns the recrystallize solid product (...note approximately what fraction of the original amount was recovered in this trial experiment. Remember, we want as much of the solid product back as possible).

Selection of an appropriate recrystallization solvent is the key to this purification technique. In the lab you will have many different solvents to choose from. Test each of the solvents with a small amount of your solid in a small test tube. Use a steam bath as a heat source. Above all, be organized! Keep track of your recrystallization solvent tests in a table in your lab notebook.

Dissolve the Solid

Add small amounts of hot solvent to the solid product until nothing more appears to dissolve (..a pasteur pipette works well for this). Remember that insoluble impurities won't dissolve regardless of the amount of hot solvent you add. Only continue to add solvent as long as you observe solid continues to dissolve.

In an actual recrystallization experiment, it is important to use a minimum amount of hot solvent when initially dissolving the solid to be purified. Otherwise, significant amounts of solid may remain dissolved even after the solution has cooled.