I just finished distilling two liters of IPA because evaporating 5 ml on a watch glass revealed it to contain quite a bit of dissolved impurities. However, it still leaves some residue, even though I used a 200 mm vigreux column.

All glassware was thoroughly cleaned beforehand. I do not get such issues with distilled acetone, for example.

What gives?

  • $\begingroup$ It is correct. A long column will obviously improve separation. As the residue left by the distilled alcohol seems to be "highly non volatile" rather than acting on the distillation, a long column should prevent escape of aerosol. A 20 cm vigreaux is certainly short, especially if nested on flask of the liter scale! $\endgroup$
    – Alchimista
    Apr 15 '18 at 9:15
  • $\begingroup$ How long of a vigreux would you suggest I use? $\endgroup$
    – user62840
    Apr 15 '18 at 9:18

I assume the problem is, like in the watch glass the solutes after IPA evaporates, i.e. higher boiling point solutes.

Making the distillation slower and reduce the amount of aerosol to escape to distillate flask will surely diminish the amount of impurities. More time, less temperature elevation, it should solve the problem.


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