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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, 2015 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, 2015 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, 2015 at 13:50
  • $\begingroup$ You're right. I didnt think of that. $\endgroup$
    – bodacydo
    Mar 10, 2015 at 13:55
  • $\begingroup$ 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 $\endgroup$
    – Saccades
    Feb 16, 2017 at 17:08

5 Answers 5

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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, 2015 at 21:18
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    $\begingroup$ 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, 2015 at 21:39
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Fractional distillation Column

If you really want to figure what the “purity” Or rather the volume of isopropyl to water by weight aka $70\%$, $91\%$. And you don’t have a gas chromatograph, this is about the closest your going to get to an accurate Volumetric representation.

This is a fractional distillation column same basic principle as distilling drinking alcohol but much different equipment and processes. However using this column you are able to precisely control the temperature of the liquid or in this case the isopropyl alcohol in the heating mantle and allow it to evaporate and be distilled and collect after passing through a water jacket to condense the vapor and as long as you monitor the temperature at the neck located at the very top where the vertical column branches off to the right and have a thermometer right at that joint and maintain a temperature of $\pu{80 ^{\circ}C}$ ($\pu{176 ^{\circ}F}$) and let the isopropyl evaporate. Once the temperature begins to rise above $\pu{80 ^{\circ}C}$ ($\pu{176 ^{\circ}F}$) to $\pu{82 ^{\circ}C}$ ($\pu{180 ^{\circ}F}$) or higher, you stop the heat and remove the distillate container. Once everything has cooled down, you can measure what is remaking in the first Vessel and the distillate vessel. And with a bit of a margin for error using the original volume used, in my case for this rig I used I was using 70% and a starting volume of $\pu{500 ml}$. So my resulting measurements were, distillate - $\pu{347 ml}$, undistilled liquid - $\pu{148 ml}$. Resulting in a total volume of $\pu{495 ml}$ with a distillate percentage of $70.10\%$ (isopropyl) and remaining liquid (water) percentage $29.89\%$.

I hope this helps!!!

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  • $\begingroup$ Other things beside, just from a mechanical perspective, this setup relies too much on the single supporting bar on the left (one clamp only?), behind the column, and the Erlenmeyer flask on the right (high centre of gravity). It does not allow a quick exchange of the receiver flask without demanding a lot to the joints and Keck rings supporting the Liebig condensers, nor quick removal of the heat if needed without compromising the stand. The garden-hose like click connector between the hoses still to clamp is a nice thing, though. And keep the bench below the medical cabinet clean. $\endgroup$
    – Buttonwood
    Jun 26, 2020 at 9:45
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Dissolve non iodine table salt into a known amount and let it sit, measure the amount of water that sits in the bottom compared to the amount of alcohol that is in the top after about 10-15 minutes of sitting. You can then extrapolate what percentage of alcohol you bought. Example would be about half water and half alcohol if you got 50% isopropyl alcohol.

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  • $\begingroup$ I guess you mean that the salt won't disolve in alcohol, right? But how do you distinguish the salty water from alcohol? The salt won't change water color, right? (If so they will be both transparent like water). $\endgroup$
    – JinSnow
    Dec 6, 2020 at 16:28
  • $\begingroup$ I did the test following youtube video, alcohol will be clear layer above murky salt water. But I (almost) didn't see the alcohol layer. I'm using table salt (no iodine smell as I know, but not sure), and sit the mixture for an hour without change. I even drop stamp ink hoping the ink will be dissolved by alcohol nonpolar and not by salt water (the ink just drop directly to salt water layer and keep the ball shape, no ink dissolved on alcohol layer). So that means the alcohol content is very low? $\endgroup$
    – RainerJ
    Dec 18, 2020 at 11:23
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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, 2018 at 9:24
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Simple method is measure 100 ml Isopropyl alcohol water mixture heat at 80 degrees centigrade and after distilling left of liquid,cool and measure the volume. You get water content and the balance quantity is IPA.///

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    $\begingroup$ Even if you were able to separate two compounds with a difference in boiling temperature of 20 degrees Celsius with a column (bp water 100, bp isopropanol 82.6 Celsius) this answer does not account for the azeotropic mixture with 91 vol% of IPA distilling over at about 80 Celsius and atmospheric pressure. So no, there is no sudden stop of the distillation with pure water left in the roundbottom flask. $\endgroup$
    – Buttonwood
    Jun 26, 2020 at 9:21

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