2013: Advent Chemistry: Recrystallisation

by on December 10, 2013
To be honest, it's rarely this pretty.

To be honest, it’s rarely this pretty.

Once you’ve collected your product, in its lovely freshly-grown crystal form, you need to purify it. This first version is full of bits of unspent reactant and solvent and side products and waste products, and everything would be better if it were not. Washing it can only do so much good, as much of the contaminant will be trapped inside the crystals, which were probably grown quite fast and quite untidily.

One of the ways of purifying that product is to recrystallise it. It’s exactly what it sounds like: you dissolve the product all over again, after all your hard work creating and isolating it, and grow the crystals from scratch. You grow them gently, slowly,
and carefully, and the second batch ought to be much purer.

First, dissolve your product in a suitable solvent – ideally, something it’s only just soluble in. Something it can be persuaded to dissolve in when it’s warmed, but will fall back out of when cool. The theory is that the impurities are more soluble than the product and will stay in that cooled solvent, while the product precipitates back out. Warm your solvent and use as little as is necessary to get everything to dissolve. Then let it cool again.

Ideally, let it cool overnight, gentle and slow, so you get nice big clear crystals. If, as is more likely, things didn’t go perfectly to plan and you don’t have time to get started at the end of the day, do the whole thing the next morning and use ice.

Pass it through a vacuum filter, rinse it a couple of times with more cold solvent, and you’re done. Recrystallisation has been achieved.

Leave a Reply