Dry solvent myths

19 October 2010 - In the laboratory

It is one of these chores in the organic chemistry laboratory: drying wet solvents. Now do not say solvents are supposed to be wet, chemists simply want to dry out the water present in the solvent because it interferes with whatever they intend to do with it. Williams and Lawson Lawton (University of Johannesburg) required some really dry solvents and decided to compare some trusted drying procedures with the aid of advanced calorimetric coulometric Karl Fischer titration (DOI).

THF (107 ppm water) is typically dried (to 43 ppm) using sodium wire and benzophenone (cumbersome and procedure not without danger, see ketyl) but chemists may be pleasantly surprised to learn that storage of THF with 20% by weight molecular sieves for three days is much better (4 ppm). As an alternative silica is disappointing but passing THF over a column of neutral alumina again gets you to 5 ppm and much quicker. Molsieves also beat conventional techniques for drying toluene dichloromethane acetonitrile or methanol. Sounds too much like a molsieve add? The paper was co-funded by industry but at first glance none of them is selling it.

Update: reader P. informs us it should be coulometric not calorimetric titration. So noted!
Update: Author name is not Lawson but Lawton (25 March 2014)