2013: Advent Chemistry: Distillation

by on December 7, 2013
Delicious distillates

Delicious distillates

Distillation has a lot in common with reflux, except that instead of letting the condensate drip back down into the original mixture, you change the layout a bit and collect it into a second flask.

Distillation is the usual method of making spirits from weaker alcohol, but it can be used to separate, in principle, any combination of compounds with different boiling points. In practice it’s a little bit trickier, because you reach practical limits in how precise your control over temperature can be, and because sometimes mixing things (like ethanol and water, actually) results in a combination boiling point at certain concentrations. That’s why you can’t use heat distillation to purify ethanol much beyond 95% – it takes some water with it when it evaporates.

Fractional distillation is not much more complicated in principle than simple distillation. instead of a single collecting flask, you have several, going up the column of cooling steam. The highest flasks collect the “lightest” components, the ones with the lowest boiling points. The “heavier”, less volatile components condense out further down, and the least volatile stay in the bottom, never evaporating in the first place.

Naturally, to make this work for your particular mixture, you have to choose the right temperature to heat the base mixture to, and the right temperature gradient up the length of your column. “Least volatile” might mean the stuff that hasn’t boiled yet at 40°C.

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