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. :-) $\endgroup$– KarlMar 19, 2019 at 22:14
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2$\begingroup$ 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 CusterMar 19, 2019 at 22:18
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$\begingroup$ A few different concentrations of isopropyl alcohol are sold as rubbing alcohol. The highest I have seen is 90% $\endgroup$– blacksmith37Mar 20, 2019 at 0:20
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2$\begingroup$ 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_blackMar 20, 2019 at 0:26
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$\begingroup$ @matt_black , an entrainer $\endgroup$– TechnetiumJun 23, 2021 at 3:57
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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}$) can be used to remove some water.
That said, there are a few reasons it's unlikely to find isopropanol at much greater strength than 91% at your local pharmacy:
- It is more expensive.
- It offers little advantage in home use over 91%. In fact, 70% ethanol or isopropanol is a more effective antiseptic than 91%. (Reagent grade alcohol is recommended for cleaning precision optics.)
- Water from air would slowly infiltrate most thin-walled plastic bottles, causing swelling (and possibly rupture) of the container.
- After opening, water would be more rapidly absorbed until it approached 91%.
answered Mar 21, 2019 at 3:35
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$\begingroup$ But we do find 99% isopropanol at the pharmacy (in the US) so it is not unlikely. $\endgroup$– 10ppbJun 23, 2021 at 4:56