The standard method for recrystallization taught is to heat a solvent containing an insoluble sample until it dissolves, then allow it to cool so that crystals from the pure compound can precipitate while small amounts of impurity remain solvated.
Another method I've become aware of is to use a multisolvent system at room temperature, but I don't know the exact details of the technique, and searches seem to land me exclusively by the standard method.
Wikipedia mentions it as a multi-solvent recrystallization but doesn't go much into the details.
I'm assuming this method is more desirable when your compound is unstable under high heat. Does anyone know the specifics of the technique, eg:
- How much of Solvent A to use vs. solvent B
- Must both solvents be miscible with each other?
- Techniques to add the secondary solvent. I surmise dripping it dropwise down the side of the glass is preferable, but is this best done in a round bottom flask, a tube, or something else?
- What is the mechanism for recrystallization exactly? I wanted to do it in a tube, but I thought if the crystals nucleate at the interface between the solvents, they will simply fall down into the more abundant solvent and redissolve. I'm a little confused on that.
- Temperature control: is light heating initially or cold at the end ever advisable?
- How long does the method take? Do you let it sit overnight or wait for observable nucleation then crash out the crystals with cold?
I'm currently looking at a solvent mixture of ether:petroleum ether, where the sample is soluble in ether but not in petroleum ether. However, I am trying to extend the method to another compound I have, but since I am unfamiliar with the technique I'm not entirely sure what properties are ideal for a solvent mixture in terms of volatility and miscibility for optimization. This second sample has impurities soluble in hexane, so I thought maybe something like ethanol and hexane. My plan was to dissolve the sample in ethanol, then pipette hexane dropwise down the sides, let it sit until all the crystals had developed, then pipette out any remaining solvent. I wanted ethanol because I thought the miscibility would be important, and it's not as strong a solvent as DCM so my product yield will be higher. Does this sound like the right thinking?
Sorry for the rambling post, and thanks for any advice!