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I'm curious how to test purity of isopropyl alcohol? I simply got very curious today. I bought rubbing alcohol and it doesn't say what's the purity anywhere. It could be 70%, but it could also be 90%, or 99% or even 99.9%. I'm a curious man so I started googling but couldn't find anything.

If I were a chemist, how would I test the purity?

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  • $\begingroup$ If you have chemistry equipment ready, then you would use a gas chromatograph (wiki-link). With household items is far more difficult, I cannot think of an accurate way, hence the comment ;) $\endgroup$ – Eljee Mar 10 '15 at 13:41
  • $\begingroup$ I just had an idea myself - since impure isopropyl would contain water, if I took 10 grams of isopropyl and somehow measured how much water there is, then I could tell its purity. For example if there were 3 grams of water, then it would be 70% pure. $\endgroup$ – bodacydo Mar 10 '15 at 13:49
  • $\begingroup$ True, though that only works under the assumption that the only impurity is water. Might it also be ethanol, or methanol? Other hydrocarbons? $\endgroup$ – Eljee Mar 10 '15 at 13:50
  • $\begingroup$ You're right. I didnt think of that. $\endgroup$ – bodacydo Mar 10 '15 at 13:55
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If the isopropanol mixture was bought commercially, the chances that it contains significant amounts of methanol or ethanol are fairly small, I think. (I've never seen commercial isopropanol in the US that contains > 3% of these impurities, at least, but the situation may vary in other countries or markets.)

If you can assume that the only other main component is water, the easiest way to estimate purity is simply by measuring the density. 100% pure isopropanol has a density at 20 °C of 0.786 g per mL. Pure water has a density of 1.00 g/mL. At intermediate concentrations of isopropanol, the density is in between those values. If you can accurately measure both (a) the volume and (b) the weight of a portion of isopropanol, you should be able to measure the density. I know my kitchen scale and kitchen measuring cups would be precise enough for this.

If you can't assume that water is the only other impurity, other methods such as NMR, chromatography, or spectroscopy would probably be required. I think NMR would probably be the most informative (but perhaps the least accessible to the home chemist.)

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  • $\begingroup$ Very good answer. Thanks. I think I can use archimede's law to figure out density at room temperature? $\endgroup$ – bodacydo Mar 10 '15 at 21:18
  • $\begingroup$ Yes, if you use a completely sealed container and are willing to neglect the weight and volume of the container. $\endgroup$ – Curt F. Mar 10 '15 at 21:39
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If I were a chemist I would use the USP monograph for IPA, which involves a GC fitted with a TCD.

No need for a NMR/MS

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For very quick test, you can go for flame test. Pure IPA when poured on a cotton burns upto a certain flame length . when impurities are added , the length of flame decreases. Do not forget to keep the size of cotton same while comparing

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  • $\begingroup$ This doesn't help you to assess the purity unless you have a sample of known purity to compare to. And it doesn't seem very precise anyway. $\endgroup$ – bon May 30 '18 at 9:24

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