2013: Advent Chemistry: Filtration, Faster

by on December 3, 2013

A Buchner funnel and side-arm flask

So, as mentioned, filtration is really slow. Sufficiently so that in reality, it’s hardly ever done that way. Unless there’s a compelling reason not to subject your mixture to anything more aggressive than gravity, you use a related, but more complicated bit of kit, and do vacuum filtration.

Vacuum is produced in the lab by pumps. Using a vacuum pump safely and effectively requires a certain degree of care, but if you’re lucky, you’re working in a nice modern lab, and it has vacuum on tap.

Take a moment to appreciate just how cool having vacuum on tap is.

When you want to filter your mixture, you take a flask with a little pipe coming out of one side, and hook that pipe up to the vacuum tap with a rubber hose. It’s a good idea to have your flask clamped to a stand, to make it more stable and less likely to fly around unpredictably when you turn the vacuum on. On top of the flask you put a stiff rubber ring, and poking through that, a Buchner funnel. A Buchner funnel has a flat, perforated base just above the stem, and the filter paper sits flat on that base.

You put the filter paper in, wet it slightly with whatever solvent your mixture is in (solvents in this sense include water, and you want to match your mixture to make sure you don’t dissolve your product by accident), and turn on the vacuum. The wet filter paper is pulled firmly against the base of the funnel, making a seal around the edges.

Anything put in the funnel now is going to have to go through that paper before it can fall into the flask beneath. So now you can pour your mixture in, and it’ll work just like an ordinary filter – crystals stay in the top, liquids pass through to the bottom.

Only much, much faster.

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