Recrystallization is a purification technique used in organic chemistry to separate a desired solid compound from impurities by dissolving it in a hot solvent and then allowing it to slowly cool so that pure crystals form. It works because most solids dissolve more readily in hot solvents than in cold ones. By exploiting that difference in solubility at different temperatures, you can coax your target compound out of solution as clean crystals while the impurities stay dissolved.
The Principle Behind It
The core idea is simple: your compound and its impurities have different solubilities. When you heat a solvent, it can hold much more dissolved solid than when it’s cool. So you dissolve everything (compound plus impurities) in the minimum amount of hot solvent, then let the solution cool. As the temperature drops, solubility drops with it, and the compound you want, present in much larger quantity, crystallizes out first. The impurities, present in smaller amounts, remain dissolved in the cooling solvent because there isn’t enough of them to exceed the solvent’s reduced capacity. You then filter the crystals away from the liquid, leaving the impurities behind.
This only works well when your target compound makes up the majority of the mixture. If impurities are present in very large amounts relative to your compound, they may crystallize out too, defeating the purpose.
Choosing the Right Solvent
Solvent choice makes or breaks a recrystallization. The ideal solvent dissolves your compound well when hot but poorly when cool. If the compound dissolves too easily at room temperature, your crystals won’t form and you’ll lose your product. If it doesn’t dissolve even in boiling solvent, you can’t get it into solution in the first place.
In practice, finding the right solvent involves trial and error. You test small amounts of your compound with different solvents, heating to see if dissolution occurs near the boiling point and then cooling to see if crystals return. Water, ethanol, methanol, ethyl acetate, and acetone are all common choices depending on the polarity of the compound.
Mixed Solvent Systems
Sometimes no single solvent has the right properties. Your compound might be too soluble in one solvent and completely insoluble in another. In that case, a solvent pair can solve the problem. You first dissolve the compound in the solvent where it’s more soluble, keeping the solution hot. Then you slowly add the second solvent (the one where solubility is poor) until the solution turns slightly cloudy, indicating you’re near the crystallization point. The two solvents must be miscible, meaning they mix together completely rather than forming separate layers.
Step-by-Step Procedure
A standard recrystallization follows a predictable sequence:
- Dissolve the impure solid. Add your compound to a flask with a small amount of solvent, then heat. Add solvent in small portions until everything just dissolves. Using the minimum amount of solvent is critical because excess solvent keeps your compound dissolved during cooling, reducing how much you recover.
- Remove insoluble impurities by hot filtration. While the solution is still hot, pour it through a filter (typically fluted filter paper in a stemless funnel) to catch any particles that didn’t dissolve, like dust or degraded material. The hot vapor rising through the funnel keeps the filter paper warm and wet, which prevents your compound from crystallizing inside the funnel and clogging it.
- Remove colored impurities (if needed). If the solution is discolored, a small amount of activated charcoal can be stirred in while the solution is hot. The charcoal has a huge surface area that adsorbs large colored molecules onto its surface. A second hot filtration then removes the charcoal along with the color.
- Cool the solution. Let the filtered solution cool slowly, first to room temperature and then, if needed, in an ice bath. Slow cooling produces larger, more orderly crystals that tend to be purer, because impurities are more easily excluded from a well-organized crystal lattice. Rapid cooling often produces tiny crystals that trap impurities within them.
- Collect the crystals. Once crystallization is complete, vacuum filtration separates the solid crystals from the remaining liquid (called the mother liquor). A quick rinse with a small amount of cold solvent washes away residual impurities clinging to the crystal surfaces.
What to Do When Crystals Won’t Form
Sometimes the solution cools and nothing happens. It becomes supersaturated, holding more dissolved solid than it should at that temperature, but no crystals appear. This is frustrating but fixable.
The easiest method is to scratch the inside of the flask with a glass stirring rod, firmly enough to make an audible scratching sound. The tiny glass particles dislodged from the surface act as nucleation sites where crystals can start to grow. If scratching doesn’t work, adding a seed crystal (a small speck of the crude solid or a bit of the pure compound) gives dissolved molecules a template to organize around. If neither works, you likely have too much solvent. Return the solution to a boil, evaporate off roughly half the solvent, and try cooling again. An ice bath or salt-ice bath (which can reach around negative 10°C) can also push the temperature low enough to trigger crystallization.
Oiling Out
Occasionally, instead of forming crystals, the compound separates as oily droplets that settle at the bottom of the flask. This is called oiling out, and it happens when the compound leaves the solution as a liquid phase rather than organizing into a solid crystal structure. It’s a common headache in pharmaceutical development. The oily material tends to trap impurities rather than exclude them, so it defeats the purpose of recrystallization.
Preventing oiling out usually involves slowing the process down: cooling more gradually, using a different solvent system, or reducing the degree of supersaturation. Sometimes switching to a solvent pair or combining cooling with anti-solvent addition provides enough control to encourage proper crystal formation.
How to Tell If It Worked
Two measurements tell you how successful your recrystallization was: percent recovery and melting point range.
Percent recovery is the mass of purified crystals divided by the mass of crude solid you started with, expressed as a percentage. It will always be less than 100% because some of your compound remains dissolved in the cold solvent and is lost when you filter. For example, if you dissolve 5 grams of a compound in 100 mL of hot water and 0.5 grams stays dissolved after cooling, your recovery is 90%. A higher recovery means less product was sacrificed, but pushing recovery too high (by using very little solvent or cooling extremely aggressively) can compromise purity.
Melting point range is the more important quality check. A pure organic compound melts over a narrow range of 1 to 2°C. A melting range of 5°C or more signals that significant impurities are still present and that a second recrystallization may be needed. If your crystals melt sharply and at the expected temperature for that compound, the purification was successful.
Why Recrystallization Still Matters
Despite the availability of more advanced purification techniques like column chromatography, recrystallization remains a core skill in organic chemistry labs. It uses simple, inexpensive equipment. It scales easily from milligrams in a teaching lab to kilograms in industrial production. And for crystalline solids, it often delivers higher purity than other methods, since the ordered structure of a crystal naturally excludes molecules that don’t fit. Understanding how solubility, temperature, and solvent choice interact gives you a reliable way to clean up almost any solid organic compound.