I did something today that I haven’t done in several years: a vacuum distillation. That used to be a larger part of every chemist’s life, but advances in chromatography have eaten into a lot of the older techniques for purifying compounds. Recrystallization is another obvious example of a lost art, one that I’ve steadily heard characterized as such for the last twenty years. Well back before my time, people purified their liquids through distillation and their solids by recrystallizing them, and that was that.
Both of those can still be the best way to go, depending on your compounds. When you come across these methods in the older literature, you always have to ask yourself if you should stick with them, or if a chromatography would do the job more easily. Today, though, it was a modern procedure I was following, so distillation it was.
For the non-chemists in the audience, here's how you do it. You rig a glass apparatus onto the top of your round-bottom flask of gunk - there's one at the left. This "still head" has a short neck coming up, a bend that accommodates a thermometer, then a cold-water circulating condenser built in right before a tube to deliver the drops of distilled product. Along that region there's another fitting to hook the vacuum pump up.
Pulling a vacuum on the system lowers the boiling point of the liquids inside it - one of the reasons you have to adjust recipes at high altitude, actually. (If you lower the pressure enough, you can get water to boil at room temperature). Without that lowering, many compounds would have to be heated up so much to distill them that they'd start to decompose. Heating things to that point isn't much fun, in any case. Far better to pump things down and take them over at a more reasonable temperature.
The usual technique is to pump things down first, just to get any bumping and bubbling out of the way as leftover low-boiling solvents and dissolved gases clear out. Then you gradually increase the temperature on the distillation pot until things start to boil. You can see the condensation form on the inside of the still head as things get going, then drops start to condense and drip off the end of the thermometer back into the pot. A bit more heating and things make it over to the condenser, roll down the collection tube, and into the receiver flask.
Of course, you may have more than one thing in that pot. The stuff that's boiling out will eventually all come over, and as you heat things up some more the next higher-boiling component will then start to boil and the process repeats. That's why they make adapters that can fit several receiver flasks - these things will turn to accomodate different fractions, one after the other. The common lab name for these is a "cow" (Germans call them "spiders").
When you're finished, you generally have one or more flasks full of clear liquid on the far end of things, and the distillation pot generally looks just awful. All the high-boiling impurities have concentrated, and the resulting mix has been thoroughly cooked. It's a dramatic illustration of what you've accomplished - dark brown sludge separated out from pure product. Distillation makes you feel as if you've earned your lunch break.