One pretty reliable recrystallization technique involves a mixed solvent system. For this, you will need two solvents of different strength: one that dissolves the molecules easily (Ethyl Acetate) and another that the molecules are mostly insoluble in (Pentane).
Experiment at a small scale.
A good working sample size is around 20-200 mg, and a 20 mL dram vial with a screwcap, or a small round bottom flask with a stopper are ideal. To the evaporated solid sample in the vial, a few mL of pentane are added and then broken up with a stirring rod or metal spatula, ideally sonicated, to produce a fine suspension of the solid. Then, with stirring and/or sonicating, Ethyl Acetate was added one drop at a time until the cloudiness dissipates to a clear, transparent solution. Let the vial sit at room temperature in an undisturbed ventilated area overnight, loosely capped, allowing a slow evaporation to occur. If crystals form in the next few days, they can be collected and washed with cold pentane.
The Layering Method.
If the evaporation leaves behind more of a residue, you can try a layering method. Re-dissolve the sample in the same way that you did before, but before you cap it, using a pipette, add a few mL of pentane slowly and carefully along the side of the vial to make a distinct layer on top. The crystals will form at this layer. Close the cap tightly this time, and let it sit undisturbed in a freezer for a few days. You can try several different ratios of solvent or temperatures. Once you figure out a method that works, you can try that on a larger scale. If your sample is not pure enough for recrystallization, other isolation methods, such a chromatography, may be necessary beforehand.
If you’re interested in learning more about recrystallization as a method for purifying your extracts, CannaChemist can help. Contact us at firstname.lastname@example.org and let us help you.
This content is written and supported by Orion GMP Solutions, a pharmaceutical engineering firm dedicated to international standardization of GMP Cannabis.