Question

How can one prevent oiling out?

Answer 1

Oiling Out
Sometimes during cooling for crystallization, the product separates not as crystals but as a liquid, an "oil". This shows up first as a cloudiness or opalescence, and then as visible droplets. An oil is undesirable because it is often an excellent solvent for impurities. When, or if, the oil finally freezes, the impurities that have dissolved in the oil will remain in the crystals. If the first traces of oil can be caused to solidify by the addition of seed crystals, by vigorous stirring or swirling of the mixture, or by scratching the walls of the flask with a stirring rod, the remainder of the product will usually separate as crystals if the rate of cooling is not too great. If oiling out cannot be prevented-that is, if most of the product separates as an oil before it can be caused to solidify-you can hope that recrystallization of the solidified oil will give a better result, you can try a different solvent, or, probably best, you can purify the product by another method before attempting to recrystallize it.