[qw098]

Hi,

I've been doing vacuum filtrations in my labs for the past year or so, but I always seem to have slight water contamination.

What is a good way to improve the efficacy of my vacuum filtrations?

Thanks!

[fledarmus]

Where do you get the water contamination? In the filter cake, or in the mother liquor from using a water aspirator to supply vacuum?

If in the filter cake, this is usually caused either by having hygroscopic materials which absorb water from the air as soon as the solvent is gone, or from very volatile solvents evaporating and lowering the temperature enough that water from the atmosphere condenses on your solids. Either way, you can keep the water in the atmosphere from getting into your solids by doing your filtration under a dry, inert atmosphere. For most of the compounds I work with, I have a nitrogen line with a funnel stuck in the hose by the stem, which I can invert over my vacuum funnel to blanket the system with nitrogen. There is more sophisticated glassware designed to do the same thing.

If it is in the mother liquor from a water aspirator, you can put a water trap in your vacuum line. Usually this is a small glass tower with two hose inlets at the top, one of which has a long glass tube going to the bottom of the tower and the other opens at the top. Any water which gets sucked up the hose will collect in the water trap, leaving your mother liquor dryer. If there is still too much water vapor, you might need a tube with a dessicant in your vacuum line after the water trap.