About two weeks ago I put some ordinary unleaded petrol into a plastic water bottle, as a temporary measure (long story, don't ask!). It has now formed a reddish liquid at the bottom of the bottle.

The bottle was emptied prior to adding the petrol, but probably still contained a small amount of water when the fuel was poured in. The bottle was stored in ordinary room temperature.

I presume that the fuel has corroded/reacted with the plastic and left this substance, but I have looked to Stack Exchange in the hope of a more scientific answer...

So, what is the red substance and cause of it?

The red substance

Please note that the brownish 'slug-shape' is merely an artifact of the light refraction...

  • $\begingroup$ I assume that the petrol you put in was poured out the same day and what little remained is what turned brown. Is that correct? $\endgroup$
    – LDC3
    May 10, 2014 at 1:18
  • $\begingroup$ No, the bottle was 'clean', then filled to the brim with fuel, sealed, and the red/brown is a liquid that settled on the bottom whilst the bottle is still sealed and full of fuel. The picture above has a yellowish tint: it is still sealed and full of fuel, it is not empty in the picture. Very strange....! $\endgroup$
    – Stacker
    May 10, 2014 at 1:22
  • 1
    $\begingroup$ If there is still petrol in the bottle, then the lower layer must be mostly water. Is the fuel an ethanol blend? $\endgroup$
    – LDC3
    May 10, 2014 at 1:29
  • $\begingroup$ It's super unleaded (so 95 octane) from a UK petrol station.... Sorry, I'm no Chemist so don't know! $\endgroup$
    – Stacker
    May 10, 2014 at 9:30
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    $\begingroup$ 95 unleaded for sure contains good amount of oxygenates and/or alkenes. My bet, traces of water leaded to separation of oxygenates (alcohols) into separate phase and then formed polar phase extracted some colored components from the mixture. However, it is impossible to give exact details without complicated chemical analysis or full listing of fuel composition $\endgroup$
    – permeakra
    Jun 28, 2014 at 20:58

1 Answer 1


Maybe could be some iron oxide or hydroxide from previous storage or ( if you have a iron tap) from it ore maybe directly from the plastic.

Some fuel has red dye in it but I don't think this is the case, because I don't see how it can precipitate.

Maybe it could be a residue of petroleum resin.

  • $\begingroup$ It came from the pump, into a jerry can then into the plastic bottle...the fuel in the jerry can is clean and doesn't have this red substance which makes me think it must be reacting with the plastic. $\endgroup$
    – Stacker
    May 11, 2014 at 10:08
  • $\begingroup$ I wouldn't think it would be iron (III) oxide or iron hydroxide in that kind of quantity. I think it could be petroleum resin though. $\endgroup$ May 30, 2014 at 13:38
  • $\begingroup$ Do you have any sources to support 'petroleum resin' being present in petrol (gasoline)? It seems unlikely to me. $\endgroup$
    – nekomatic
    Aug 11, 2014 at 11:38
  • $\begingroup$ @nekomatic no, is more an hypothesis... $\endgroup$
    – G M
    Aug 11, 2014 at 12:47

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