2013: Advent Chemistry: Precipitation

by on December 8, 2013
If you're not part of the solution, you're part of the precipitate

If you’re not part of the solution, you’re part of the precipitate

Precipitation, when it doesn’t mean rain and snow, means roughly the opposite of dissolving. It means taking a solution, and persuading the interesting thing to come back out of solution and be a solid.

We already covered rotary evaporation, which is one way of doing this. Rotary evaporation is a specific example of drying – you take the solvent away and the product has to stop being dissolved. You could do this by spreading the solution out in a tray and just letting it evaporate gradually, but, well, that’s slow, and you end up with your crystals all stuck to the tray.

More common is to boil the mixture, to drive off the solvent as steam. You can generally reduce the quantity by three-quarters reasonably speedily (which is one of several reasons why not using more solvent than you need is a good idea). This has the additional effect that solvent will hold more stuff in solution when it’s hot, so once you’ve reduced your solvent down and you’re getting crystals forming, you can be confident that when you take it off the heat, more will precipitate out as it cools. Ice or freezing mixture may be your friends here if you’re in a hurry or trying to maximise your yield.

A third method is to change the solvent such that your product can’t dissolve in it. One of the quirks of how solutions work is that when, for instance, you add water to an ethanol-based solution, the water mixes with the ethanol to form a solvent that is not quite like either of its components. It’s perfectly possible for something that happily dissolved in mostly-dry ethanol to fall out of solution when the ethanol is mixed half-and-half with water. When this works, it is fantastically quick, but you get poor-quality crystals out of it, and during my degree it was generally frowned upon. It suggested you hadn’t planned ahead and weren’t taking the time to do things properly.

Most of the dramatic examples of precipitation, when you drip something into a test tube and crystals spring out like magic, are not cases of simply removing something from solution. They’re reactions, the product of which isn’t soluble in whatever the raw material was dissolved in. Which is always convenient, when you come across it.

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