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I filled a Zippo lighter with 2-propanol and it lit up easily when the ambient temperature was around 6-7 degrees Celsius.

It was impossible to light it up when the temperature was around 3-4 degrees Celsius. Then I put a drop of lighter fluid (naphtha) on the wick and it lit up first try. After the drop of naphtha was burned the lighter was heated up a bit and then it also sparked easily with the propanol.

The reported flash point of propanol is 12 degrees Celsius while the flash point of naphtha is 30-38 degree Celsius.

How can this be? Shouldn't be the flash point of naphtha lower than propanol recording to my "experiment" ?

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2 Answers 2

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Flash point relates to a open pan or pool of liquid with no wick.

The definition of flash point is the lowest temperture of a combustable liquid at which it will catch fire when a flame is presented above the liquid.

When you have a wick present then it is much more easy to burn a fuel, even if the bulk of the fuel is below the flash point it is possible to burn it at the wick. For example a parafin lamp normally has a wick, you light the wick with a match and then the parafin burns.

So I think that the flash point issue is not so important, I am a bit surprised. Normally the fuel for a zippo is a version of gasoline (US) or petrol (UK) which does not have to have the fuel additives or a given octane rating. Also when you give the flash point of naphtha keep in mind.

"All naphtha is equal but some naphthas are more equal than others".

There are lots of different grades of naphtha, they will vary a lot in their properties (including their flash point). For example if we take the kerosene fraction then the different kerosenes have a wide range of different flash points.

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  • $\begingroup$ Thanks for your answer. It makes sense to me that using a wick can reduce the actual flash point but shouldn't this be true for both substances to the same degree? Or is is possible that the wick supports the flash point of naphtha more than propanol? $\endgroup$
    – flappix
    Feb 26 at 21:18
  • $\begingroup$ The lighter fluid is not from Zippo just a cheap version from a supermarket. Is it possible that it is made of compounds which have a so much lower flash point than the reported 30 degrees? $\endgroup$
    – flappix
    Feb 26 at 21:18
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Interesting because I had a similar issue trying to run a Coleman catalytic heater with kerosene. This lead to some checking of flash points and petroleum distillate definitions.

The distillate "cuts" run from gasoline (flash point -23 C) to Coleman fuel, "white gas", or light naphtha (flash point - 4C) to kerosene (flash point + 38C).

2 propanol flashes at + 12 C. Zippo lighter fluid flashes at "< 6.5 C"$^1$. Hexane flashes at - 22 C. Butane flashes at - 60 C.

So, just as kerosene worked for a warm catalytic heater but not for a cold one, 2 propanol only works when your lighter is "warmed up".

This is because, below the flash temperature, the fluid will not produce enough vapor pressure (as a percentage of air) for the spark to ignite it. This is called the lower flammability (or explosion) limit.

$^1$ Zippo lighter fluid MSDS sheet

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  • $\begingroup$ Do you have a source for the flash point of -4°C for light naphtha? I can't find this anywhere but this would indeed explain it. $\endgroup$
    – flappix
    Feb 27 at 14:08
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    $\begingroup$ @flappix naphtha is an archaic term meaning any distillate fraction above gasoline up to kerosene and diesel. "Light" naphtha is the lower boiling cut, heavy naphtha is kerosene. Coleman fuel is light naphtha. MSDS sheets on anything you buy shoud have flashpoint information. $\endgroup$
    – Robert DiGiovanni
    Feb 27 at 16:16
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    $\begingroup$ @flappix more reading here: petroleum ether, Standards subsection, and here: naphtha $\endgroup$
    – Robert DiGiovanni
    Feb 27 at 16:43

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