We have now 10% ethanol mixed petrol. Seller says that if this petrol comes in contact even 10 ml of water, all ethanol which is mixed in the petrol will be extracted by water, is it true?

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    – Jan
    Sep 28, 2015 at 17:21
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    $\begingroup$ There is an element of truth to it. You might want to search for the partition coefficient of ethanol in fuel/water systems. Or just check it yourself on a small amount. Chemistry is an experimental science, they say. $\endgroup$ Sep 28, 2015 at 17:46

1 Answer 1


Water and ethanol are highly "miscible"; a science term that means virtually any amount of water is fully soluble in ethanol, and vice-versa. The next thing to consider is that ethanol and gasoline are quite miscible. Think of E-10 (10% ethanol) all the way to the E-85 some petrol engines are designed to function well on (with some loss in power output). However, water is very poorly soluble in pure gasoline.
So adding when water to E-10 the water mixes well with the ethanol, but the H2O/ethanol mix looses solubility rapidly as more water is added.
In theory, if you set up a separation flask, basically a clear glass vessel with a petcock at the very bottom, filled it at room temp with a gallon of E-10 and added ~3 tablespoons of water, shook it all up then let it settle to separate, you could drain away the bottom (denser) layer of liquid, which would be the water/ethanol mix, an leave you with ~0.9 gallons of relatively alcohol-free petrol. My final remark would be to ask why anyone would wish to do this as I can think of no reason in the world why someone would bother.

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    $\begingroup$ Definitely not gallon vs 10ml, maybe 100ml vs 10ml $\endgroup$
    – Mithoron
    Sep 28, 2015 at 19:29
  • $\begingroup$ Maybe the concern is that if you get a small amount of water in your gas tank (e,g, by condensation) it will "extract" the ethanol from your gas tank and create a sizable "aqueous" phase - that could cause problems in your engine or gas line. But I think those "dry-gas" products like STP (mostly methanol) may help to dissolve the water that collected in the bottom of the tank into the gasoline so it will not longer clog your gas line. $\endgroup$
    – iad22agp
    Sep 28, 2015 at 20:15
  • $\begingroup$ A reason to consider doing this is with smaller engines like a lawnmower . Ethanol impurities in the fuel can cause a viscous residue in the carby and lines that can build up and cause problems. Because the ethanol is so hygroscopic it attracts moisture from the air which can end up contaminating the fuel also. $\endgroup$
    – Technetium
    Oct 29, 2015 at 0:04
  • $\begingroup$ I'm a chemist and my Mom's car had this problem. I can agree that it was caused by water in the fuel, but the exact amount that would cause a problem seems fuzzy. There are a couple of factors. One is temperature and the other the presence of various stabilizers which may, or may not, be added to the gas. But it does seem to be caused by a small amount of water. This paper has a ballpark of about 0.5% at 60 degrees F. www3.epa.gov/otaq/regs/fuels/rfg/waterphs.pdf $\endgroup$
    – MaxW
    Oct 29, 2015 at 5:48

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