Can an identification experiment (using acetonitrile, isopropyl, and cyclohexane) using simple distillation and boiling point be improved by using a digital thermometer?

Can simple distillation be improved in other ways, other than a more accurate thermometer? That's all I've come up with.

  • $\begingroup$ In P-Chem lab I remember using a thermometer that I think was good to 0.01 C. The other odd thing is that you had to increase or decrease the Hg in the bulb by "breaking off" some of the Hg at the top of the thermometer. So the thermometer had a range of maybe 10 C. In the end it would read differences well but it was uncalibrated. So you had to then calibrate it to get absolute temperature readings. Oh the good old dinosaur days... en.wikipedia.org/wiki/Beckmann_thermometer $\endgroup$
    – MaxW
    Feb 15 '17 at 18:15
  • $\begingroup$ There are a zillion ways to tell these compounds apart. What can you use? Wet chemistry, NMR, IR, GC-mass spec, ect ect ect... $\endgroup$
    – MaxW
    Feb 15 '17 at 18:22
  • $\begingroup$ @MaxW I've used those methods before. But I'm trying to improve the simple distillation process itself. Experiment was pure liquid BP identification, miscibility test, then distillation to tell azeotrope and non-azeotrope mixtures apart. What makes simple distillation apparatus/process itself error-prone so that aspect can be fixed? I didn't see much room for instrument error other than calibration $\endgroup$
    – redbull123
    Feb 15 '17 at 18:49
  • $\begingroup$ It is really hard to get "absolutely" pure anything. So 99% may be ACS reagent grade but the BP would be off by a degree or so. $\endgroup$
    – MaxW
    Feb 15 '17 at 18:53
  • $\begingroup$ @MaxW True. Is distillation an issue if mixture components are fairly close in bp? Fractional would be the way to go. $\endgroup$
    – redbull123
    Feb 15 '17 at 19:07

Ok, back to the start on all of this....

If you were looking for pure compounds or azeotropes what I'd want is a setup where a fractional distillation column was used with a very precise thermometer and a balance measuring the weight of the distillate. You could then nicely plot temperature vs weight.

Assuming that you just want to detect the pure compounds and/or azeotropes and not collect them.

Part of the problem here is that it takes some time to get the whole fractional distillation column up to temperature. So the temperature of the first drops is suspect.

If you're using a heating mantle you might be willing to boil the pot dry. But you wouldn't want to do that with a flame. So you might lose a bit on the back end of the distillation too.

The fractional distillation column doesn't have an infinite number of plates so the transitions from one compound/azeotrope to the next will be a bit "fuzzy." So if you were trying to collect "pure" compound/azeotrope fractions you'd avoid collecting the distillate from the transitions in those "pure" compound/azeotrope fractions.

Does this help?


I guess I'd like two thermometers. One over the pot and one at top of the fractional distillation column.

Some sort of computer controlled temperature setting for the pot would be nice too.

  • $\begingroup$ Realized fractional distillation couldn't be used in this case. It won't help with a single pure compound, and isn't useful with the azeotrope. So simple distillation would have to be tweaked in some other way @MaxW $\endgroup$
    – redbull123
    Feb 15 '17 at 21:09
  • $\begingroup$ Correct - it isn't useful if the sample is "pure" azeotrope. $\endgroup$
    – MaxW
    Feb 15 '17 at 21:12
  • $\begingroup$ So for three compounds you could have 3 different pure compounds, 3 different binary azeotropes, and 1 ternary azeotrope. $\endgroup$
    – MaxW
    Feb 15 '17 at 21:15
  • $\begingroup$ Only cyclohexane and isopropanol form an azeotrope @MaxW Any suggestions on distillation tweaks? $\endgroup$
    – redbull123
    Feb 15 '17 at 21:18
  • $\begingroup$ So for the three particular compounds you could have 3 different compounds, and 1 azeotrope as distillates. But could you get a mixture of acetonitrile and cyclohexane (I'd expect them to be miscibile) as your unknown? $\endgroup$
    – MaxW
    Feb 15 '17 at 21:23

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