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I have a paint thinner that contains 60% toluene and 40% acetone. I would like to obtain some more-or-less pure toluene. Considering the big difference in boiling points (110°C toluene vs 56°C acetone), and because I only need the toluene and not the acetone, I just tried to pour the paint thinner in a pot and put it on the outside stove.

What I expected to happen, based on my small understanding of chemistry, was that the temperature would rise until 56°C where it would stay for a while and bubble off all the acetone, and then to stop bubbling and the temperature to increase until 110°C where it would start to bubble again.

However what really happened was that both the temperature never stopped rising and the liquid never really stopped bubbling once it had started (although it slowed down to some degree after the first 30 seconds or so). When I only had like 20% of the original amount left, I accepted I have no idea what I'm doing and took the pot off the stove.

...and now I'm here, asking for advice from people who know better than me. Does this whole procedure even make sense? And if yes, how should I approach it? Maybe try to keep the temperature precisely on a certain level?

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    $\begingroup$ Hopefully you had lots of fresh air flowing into the room… $\endgroup$
    – Jon Custer
    Commented Feb 22, 2023 at 2:00
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    $\begingroup$ Why not just to get pure toluene? $\endgroup$
    – Poutnik
    Commented Feb 22, 2023 at 9:20
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    $\begingroup$ This is beyond a little dangerous. Acetone has a very low flash point. Vapors of both are heavier than air. You might be better off as suggested, buying pure toluene. $\endgroup$ Commented Feb 22, 2023 at 13:24
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    $\begingroup$ @Poutnik It doesn't seem to be THAT easy to find, not in stores at least. I found some online but it's more expensive than the paint thinner and I would have to pay for shipping. (Also, I love to try and learn new things.) $\endgroup$
    – MaxD
    Commented Feb 22, 2023 at 20:27
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    $\begingroup$ I assume xylenes may be present too. They would be harder to separate than acetone. $\endgroup$
    – Poutnik
    Commented Feb 23, 2023 at 11:55

3 Answers 3

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Your logic of separating the acetone from toluene by distillation is reasonable, and would work well for separating many substances, such as propane from water (e.g., at a gas well).

However, the primary issue is that those two boiling points are close enough to require fractional distillation. Consider that the starting mixture is more toluene than acetone, and that the kinetic energy of each molecule differs, so that even at 56 °C, there are enough toluene molecules with sufficient energy to leave the mixture (along with the acetone). This is another way of saying the vapor pressure of toluene at that temperature is significant, about 0.2 atm (~20,000 Pa). As the concentration of toluene rises, there is increased probability that a toluene molecule evaporates, rather than an acetone. So one would need a fractionation column, providing a temperature gradient, to get a better separation.

Another possibility is that acetone and toluene might form an azeotropic mixture (though I could not confirm that; see this list, for example) . From Wikipedia: "An azeotrope... is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation... because... the vapour has the same proportions of constituents as the unboiled mixture."

For example, separating water from ethanol is difficult, since at ~95% alcohol/water, the condensate contains the same proportion of water as the starting mix. Other means are needed to remove the remaining moisture, so Absolut® не правда; it's not absolute alcohol.

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The paint thinner may contain some additives other than the two solvents, but a possible separation could be made by adding water.

Acetone is completely miscible with water at all ratios; toluene is hardly soluble in water at all. If you add water to your paint thinner, it will separate into two layers, probably in a nasty-looking way, as the first water-acetone mixture is less dense than toluene (sp.gr toluene = 0.866; sp. gr. acetone = 0.750; sp.gr. H2O = 1) and will float, but will sink as more water is added.

The ternary phase diagram has been published (Ref 1) but the images may be copyrighted. The image can be found on a google search for the terms: "phase diagram toluene water acetone"; the image comes up on google, but not at the actual url of the publication because of a paywall. The phase diagram is very similar to the one of water-ethanol-toluene however (Ref 2).

enter image description here

Imagine that "ethanol" in the diagram = acetone. A separation technique could be as simple as adding enough water to the paint thinner until the mixture separates cleanly with the toluene on top, then siphon off the toluene from the two-phase mixture, leaving behind what will hopefully be a small interfacial foamy layer. (The hashed area in the figure is the two-phase region.) That is the best scenario. But a formulated paint thinner may well contain surfactants or other additives that will complicate the separation by making the interfacial layer quite thick and/or stable; but it might settle down on standing overnight.

Ref 1. https://pubs.acs.org/doi/10.1021/je7000182 Surface Tension of the Ternary System Water + Acetone + Toluene; Sabine Enders, Heike Kahl, and Jochen Winkelmann; J. Chem. Eng. Data 2007, 52, 3, 1072–1079

Ref 2. https://www.researchgate.net/figure/VLLE-mol-diagram-for-the-water-ethanol-toluene-ternary-system-at-1013-kPa_fig1_39441801

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    $\begingroup$ A good idea. Toluene recovered may have a bit of water in it. A follow-up with dryers may be needed. $\endgroup$ Commented Feb 22, 2023 at 17:15
  • $\begingroup$ Thanks, this seems to be the way to go, especially because it doesn't require any fancy equipment I don't have. If I mix water and paint thinner 50:50 in a measuring cup and stir it, the layers shift to like 60:40 with practically no "interfacial foamy layer". The toulene (top layer if I'm not mistaken) however does look very cloudy, so I assume that's where @RobertDiGiovanni s comment comes in? Are "dryers" some other substances I need to buy? Or some kind of device? $\endgroup$
    – MaxD
    Commented Feb 22, 2023 at 20:23
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    $\begingroup$ @MaxD if it's worth your time, wash repeatedly with water. There are "drier" compounds people use to make 100% ethanol. But, for all your trouble, you might just "cut" your thinner with more toluene. Toluene may be less soluble in water at lower temperatures too. Curious, what will be your application with the toluene? $\endgroup$ Commented Feb 22, 2023 at 20:31
  • $\begingroup$ @RobertDiGiovanni I'm experimenting with making lava lamps, and toluene should dissolve the solvent dye I have bought, mix with mineral oil and not mix with water (because the solvent dye doesn't dissolve in mineral oil directly, and I think I need some kind of intermediate substance). $\endgroup$
    – MaxD
    Commented Feb 22, 2023 at 20:51
  • $\begingroup$ @MaxD you might be better off with acetone/toluene than water/toluene. You may try other solvents like alcohol, turpentine, or mineral spirits (or more toluene). Unless there is a specific reason to eliminate acetone, you might just keep it. Toluene is only slightly less flammable. Acetone should dissolve in mineral oil. $\endgroup$ Commented Feb 22, 2023 at 21:29
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As DrMoishe Pippik pointed out in his answer, fractional distillation would separate certain amount of toluene from the mixture (Ref.1) without complicating things like adding water (Water make azeotrope with both acetone and toluene).

$$ \begin{array}{c|ccr} n \ (\text{Frac. #}) & \text{% toluene in the frac.} & \text{mole frac. acetone (calc.)} & \pu{T ^\circ C} \\ \hline 1 & 20 & 0.85 & 57-60 \\ 2 & 30 & 0.77 & 60-68 \\ 3 & 50 & 0.59 & 68-73\\ 4 & 60 & 0.49 & 73-78 \\ 5 & 70 & 0.40 & 78-87 \\ 6 & 80 & 0.27 & 87-97\\ 7 & 90 & 0.14 & 97-103 \\ 8 & 98 & 0.03 & 103-106 \\ 9 & 100 & 0.00 & 106-109\\ \hline \end{array} $$

Note that, in this separation, initial mixture has contained 30% acetone by volume.

References:

  1. H. M. Coleman, "Laboratory demonstration of fractional distillation," J. Chem. Educ. 1967, 44(8), 476-476 (DOI: https://doi-org.srv-proxy2.library.tamu.edu/10.1021/ed044p476).
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    $\begingroup$ If the OP could manage some even primitive column, like a wide inert glass/metal tube, filled by suitable glass/ceramic/inert pieces, not packing themselves too tightly to have enough air around, he could manage distilling out most of acetone before significant amount of toluene evaporates too. But I admit it may be challenging to manage. $\endgroup$
    – Poutnik
    Commented Feb 23, 2023 at 12:11
  • $\begingroup$ The column used in this research is just a air condenser with glass wool in it (I believe). I agree, it'd be challenging and get complicated if decide to add water to azeotrope one or both. $\endgroup$ Commented Feb 23, 2023 at 14:30
  • $\begingroup$ Glass wool IMHO could act as a little strange multiple floor distillation column. It could work for acetone/toluene well. (The link seems to be behind paywall or requires registration and perhaps student/academy status) $\endgroup$
    – Poutnik
    Commented Feb 23, 2023 at 14:53
  • $\begingroup$ I think glass wool would plug up too easily. $\endgroup$ Commented Feb 24, 2023 at 18:11

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