At 4/7/08 12:47 PM, pr0de wrote:
recrystallization.. reading from somewhere, its so it can form into a crystal, instead of maybe like a film or something
i know someone who does this with mimosa hostillis, i used a crap method and came out with a messy film to scrape, mimosa is really cheap on the internet, buy some and extract it
Basically, the film you get is your target product and a bunch of other crud that you don't really want, but got caught up in the process anyway. To get rid of it, you use recrystallization.
The key to recrystallization is that the slag is a mixture of different compounds which don't have conflicting solubility when dissolved. Which is to say, there's a small enough amount of each that you could easily dissolve it all in some cold solvent.
On the other hand, you should have a whole bunch of your target compound, much more than you would be able to dissolve in a cold solvent. So what you do is, you take all your crude product, mix it with a suitable solvent, then heat it up so everything is dissolved, but maybe you save a bit of your crude product to use as a seed for later.
Once all your crap is dissolved, you cool it down, then stick it in an ice bath to get even colder. As it cools (assuming you selected a suitable solvent), the solubility of your target compound falls below the amount that's actually in the solution, so it starts to fall out in the form of crystals. It's much easier for the molecules to latch on to a rough surface, and they have an affinity for their own kind, so the crystals tend to build up on themselves to form a solid. If the crystals aren't forming, you either scratch the surface of your vessel, or introduce a seed crystal for them to build on.
The added bonus of this is that the crystal lattice that they form can't easily incorporate impurities into the structure (usually), so you get a very very pure compound this way. Of course, you lose a little bit, but the extra purity is generally worth it. It's really one of the most basic and useful lab techniques around, and anybody who wants to extract anything from anything (not just drugs) should really know how to do it.
It's also often incorrectly called "freeze precipitation" to avoid confusion with evaporation of the solvent, which is often incorrectly called "recrystallization"