Can I separate paint thinner into acetone and toluene by simply boiling it?

Question

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?

Answer 1

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.

Answer 2

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).

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

Answer 3

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).