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My understanding is that an azeotropic mixture of isopropanol and water is 91%. This makes sense as to why there are so many brands of rubbing alcohol sold at 91%. There are also some sold as 99%, which as far as I know is impossible to reach through distillation. Is there another process that the manufacturers are using to reach 99%, or is it just marketing lies?

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  • $\begingroup$ Most brands I find on google sell it at 99.9% even (for cleaning purposes). I checked a few data sheets, inconclusive. The density they give is very close to that on wikipedia (0.786g/ml), whatever that means. Common sense says they sell the azeotrope, but then this stuff is not made via fermentation and distillation. :-) $\endgroup$ – Karl Mar 19 at 22:14
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    $\begingroup$ Any decent chemical supplier certainly has >91% isopropyl available. Not surprisingly, higher purity costs more, but at least one supplier (a common one, no reason to name) has 99.7% pure isopropyl listed. $\endgroup$ – Jon Custer Mar 19 at 22:18
  • $\begingroup$ A few different concentrations of isopropyl alcohol are sold as rubbing alcohol. The highest I have seen is 90% $\endgroup$ – blacksmith37 Mar 20 at 0:20
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    $\begingroup$ There are plenty of ways other than distillation to make pure isopropanol (or other alcohols). And most azeotropes can be broken by adding a suitable third substance. $\endgroup$ – matt_black Mar 20 at 0:26
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You certainly can get high purity isopropanol, e.g. 99.99% from Fisher Scientific. However, that is about US$50/l for high performance liquid chromatography (HPLC) grade alcohol.

The isopropanol can be synthesized without water by hydrogenation of acetone, or water could be removed from the azeotrope using a desiccant. Membrane separation is also efficient. At home, table salt ($\ce{NaCl}$) can be used to remove some water.

That said, there are a few reasons it's unlikely to find isopropanol at much greater strength than 91% at your local pharmacy:

  • It is more expensive.
  • It offers little advantage in home use over 91%. In fact, 70% ethanol or isopropanol is a more effective antiseptic than 91%. (Reagent grade alcohol is recommended for cleaning precision optics.)
  • Water from air would slowly infiltrate most thin-walled plastic bottles, causing swelling (and possibly rupture) of the container.
  • After opening, water would be more rapidly absorbed until it approached 91%.
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