2013: Advent Chemistry: Hot Filtration

by on December 11, 2013

Filtration, like recrystallisation, is about physically seperating things. It’s not about doing a reaction. It’s just a way of getting the thing you want in a pot on its own.

If this happens, you've got it too hot.

If this happens, you’ve got it too hot.

If what you want is solid, and everything else is liquid, this is quite easy, which is why you try to get all the impurities to dissolve while the product precipitates out. But sometimes, you have solids mixed in with your crystals of product. They’re bits of byproduct, or unused raw materials, or sometimes they’re anti-bumping granules, which we might talk about in a few days. When your product is liquid, or needs extra steps before it will precipitate out, you can just pass it through a filter. But if your product is already crystallising out, you have a challenge.

The difficulty is in keeping the product warm. If you heat up your mixture, and maybe add a little solvent (in teeny bits, you want as little solvent as possible once you reach the recrystallisation step), you can usually persuade your product to dissolve again. If not, you have to find a solvent in which it will dissolve when heated, but not when cold. Finding it can take a while, but without that kind of solvent, you’re going to have considerable difficulty in isolating your product.

This is essentially the opposite of the usual recrystallisation method. You’re going to filter your mixture, such that your dissolved product ends up in the flask and your impurities in the filter. And you’re going to have to keep everything the product comes in contact with warm, or it’ll precipitate out and be wasted.

Hot filtration is not, in concept, any more complex than ordinary filtration. It’s just much more fiddly.

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