If there’s a lab fire, odds are pretty good that one of the solvent stills is burning. The stills, especially those for drying THF, have the dubious honour of being one of the most dangerous pieces of equipment that a graduate student is likely to handle, at least on a daily basis. While Daniel Sejer over at Curly Arrow has written up a great guide for how to use these stills safely, I’d like to go one further and eliminate their use entirely.
Surprisingly, the standard benzophenone/sodium setup is not all that good at removing water, at least not from THF (cite). 3Å molecular sieves are far more effective (6.1ppm water vs. 43.4ppm after 48hr), and this has led to the development of a number of sodium-free solvent purification systems (our department has one, in fact). In each setup the solvent to be dried is passed over prepacked drying columns, filled with either alumina or molecular sieves (cite). The setup is very nice -if you can get one- but for small quantities of solvent it’s much simpler, not to mention cheaper, to do away with the purification system entirely.
Molecular sieves are a modern wonder. Cheap, easy to activate/regenerate and virtually non-toxic, they don’t heat nearly as much as more traditional desiccants when exposed to water, reducing the fire risk substantially. They do require preactivation at 300°C (most use an oven, though I’ve heard the sieves can be activated by heating them for 20min over the flame of a Meker burner). We generally purchase the bead style to dry solvents, crushing them prior to activation if they’ll be used as desiccants in a reaction.
Effective at drying most common laboratory solvents when mixed at 10-20% w/v, 3Å sieves should not be used to dry acetone for prolonged periods of time, due to their slightly basic nature. The resulting aldol reactions will actually increase the water content of acetone, as a nice side effect of contaminating your reaction with dozens of different enones and hydroxyketones. Sieves are by far the easiest way of drying DMSO, as it tends to decompose below its boiling point at atmospheric pressure.
For completeness, a simple procedure for drying organic solvents with 3Å sieves:
2) Pour the sieves slowly into your solvent of interest, or preferably pour the solvent into a bottle containing the sieves. Do not attempt to store acetone over sieves, due to decomposition over 24hr.
3) Seal the bottle with a rubber septum and flush with nitrogen or argon gas (degassing may be required). Allow solvent to dry for at least 6hr before use.