3.5B: Heat Source and Bump Prevention

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Crystallizations are generally conducted using a hotplate or steam bath, and the crystallization solvent may affect which heat source is appropriate (Table 3.3). If the crystallization solvent is water, a hotplate can be used as there is no issue with flammability. If the crystallization solvent is highly flammable or volatile (e.g., diethyl ether, low-boiling petroleum ether, or acetone), the vapors produced during boiling could ignite on the hot surface of a hotplate, and it is therefore essential to use a steam bath. Solvents with moderate volatility (e.g., ethanol or ethyl acetate), should be preferably heated with a steam bath, but can be safely heated with a hotplate if extreme care and caution is utilized.

Table 3.3: Solvent and preferred heat source.
Solvent	Water	Diethyl ether, Acetone, Petroleum Ether (low-boiling)	Ethanol, Methanol, Ethyl Acetate, Hexanes
Photo of Substance
Heat source	Use a hotplate.	Use steam bath, never a hotplate	Use steam bath preferably. Can use hotplate with caution.

Boiling stones, boiling sticks, or stir bars must be used for "bump" protection as crystallization involves heating solutions to a boil. Boiling stones are often used for solvents, and may be used with solutions if a hot filtration step is planned (or if the plan is to pick the stones out of the crystallized solid). If no filtration step is planned, it may be easier to use a boiling stick or stir bar for bump protection, as these can be easily removed from the hot solution before crystallization.