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I am aware that there are better ways to purify acetic acid out of a water-acid mixture than these, but which one out of distillation (using heat) and fractional freezing (using cold) would gain a higher concentration of acetic acid?

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  • $\begingroup$ With the correct equipment and technique distillation of the vinegar will produce glacial acetic acid however things can seem easier in theory than in practice. Freeze precipitation and centrifuge may also produce glacial acetic acid from vinegar but again this might not be so easy without access to the right equipment. $\endgroup$
    – Technetium
    May 9, 2016 at 6:26
  • $\begingroup$ Related: chemistry.stackexchange.com/questions/2506/… $\endgroup$ May 9, 2016 at 8:55
  • $\begingroup$ @Ivan Neretin , the link suggest the acid be approximately 72% for fractional crystallisation . The household vinegar wouldnt be anywhere near this concentration? $\endgroup$
    – Technetium
    May 9, 2016 at 11:32
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    $\begingroup$ That's just an approximation intended to explain how eutectics work. But anyway, typical household vinegar is of course less concentrated than needed for fractional freezing. $\endgroup$ May 9, 2016 at 11:51
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    $\begingroup$ I think that would be an awfully overcomplicated and expensive method (and dangerous too, in case of beryllium). Also, it probably wouldn't work anyway. $\endgroup$ May 9, 2016 at 13:10

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There is no acetic acid/water azeotrope, so distillation can produce glacial (pure) acetic acid.

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    $\begingroup$ Freezing can do the same, once you've got a mixture more enriched with acid than eutectics. $\endgroup$ May 9, 2016 at 8:57
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Some experimental data after a 4 liter jug of 5% distilled white vinegar accidentally froze at -25°C:

  • The first 0.5 liter to melt out had an acetic acid concentration of about 13%, substantially above the starting concentration, and a pungent vinegar smell.
  • The next 1 liter to melt out had an acetic acid concentration of about 7%. The remaining ice was very porous and slushy, slow to drain and probably trapping lots of liquid, and helping explain the non-sharp separation between acetic acid and water.
  • The next 1 liter to melt was about 3% concentration, below the original.
  • The last-to-melt liquid measured at approximately 1% concentration, pH still measures about 3, but has almost no smell.

These concentrations, reported as mass percent, were estimated by measuring the solution density, which for acetic acid changes only slightly (e.g., 20% acetic acid is only 2.5% denser than water). Assuming a 0.1% absolute density measurement error (e.g., thermal compensation difference since the liquids were measured at near 0°C), there aren't a lot of significant figures in the percentages above.

The data above was batch gravity melting, followed by pouring off the liquid fraction. Particularly for the first pours, the melting is mostly on the exterior of the jug. The exact yield curves likely depend on the thermal profiles during both freezing and melting, the vessel shapes, and details like how the liquid is removed from the ice.

Unlike when freezing pure water, the bottom of the frozen vinegar jug wasn't visibly distended or ruptured, and actually showed no signs of significant volume change. At 20C, a 4 liter jug takes over 1 day to fully melt.

Overall, freeze distillation does appear straightforward to concentrate vinegar somewhat over normal household levels. Repeated freezing may improve the concentration further, although there's a freeze eutectic at 30% acetic acid / water (mol/mol), so reaching 100% glacial acetic acid would likely require a different approach.

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