Vacuum Filtration

This procedure is used to separate crystallized product from solvent or to remove crap and solids from a liquid. Figure 8 shows the proper apparatus to use. The collecting flask is called a side arm flask and to that extended nipple (tee heel) is attached a vacuum source. The thing that is shoved through the rubber stopper is called a Buchner funnel and is usually made of white porcelain or, preferably, PP. The Buchner funnel, when viewed from above, can be seen to have lots of pin holes in the bottom surface of its reservoir. Over this surface is layered a single sheet of rounded filter paper or paper towel.

To filter a solution one attaches a vacuum and pours the solution into the Buchner funnel. All the solution will go whoosh! into the flask leaving what is called a filter cake in the funnel. The liquid that has collected in the flask is now called the filtrate. Usually, the filter cake is then washed with a little bit of clean what-

ever-kind-of-solvent-it-was-just-filtered-from. That extra washing is then combined with the other filtrate. The most action this procedure is going to see in a clandestine chemistry lab is at the very end of the drug-making process where the chemist will have crystallized her freebase oil into a final, marketable form and then needs to separate those crystals from the solvent they are in.

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