Flash Points, Autoignition and Fahrenheit 451

book cover face red white abstract

‘That ethanol you spilled has a flash point of 14°C, your clumsiness is going to set the room on fire!’ A highly unlikely scenario actually; we often mistake flash points of substances as the temperature at which they will catch fire. For ethanol this would require its surroundings to be a toasty 365°C, so what’s the big fuss about its flammability? And how does Fahrenheit 451 tie into all this?

Flash Point Vs. Autoignition – What’s the Difference?

A substance is defined as ‘flammable’ if it is ‘easily set on fire’, according to dictionary.com… Fortunately for us and our readers, we can – and will – do a better job on assessing flammability in this post.

Just like most chemical reactions, combustion is an energetically favorable process in that the products contain less energy than the reactants. However this requires overcoming an activation barrier before doing so – yes, entropy is crazy and wants to turn everything to dust. The activation barrier that a substance has to overcome in order to combust is known as its minimum ignition energy (MIE). The MIE is obtained by an ‘ignition source’, such as the spark to start a car’s engine, or the match held to a bunsen burner triggering a roaring flame.

The buildup of static charge without dissipation may cause a discharge, which might deliver more energy than a substance’s MIE, causing it to ignite. A person walking on a carpeted floor may build up enough static energy to deliver a static discharge of 40 mJ (milliJoules), more than enough to overcome the MIE of many fuels in air (0 – 0.1 mJ)1. Therefore grounding of pipes and storage units is important, providing a conductive surface through which static charges can dissipate.

box of explosives TNT dynamite

Despite its use as a  ‘high explosive’ 2,4,6-trinitrotoluene (TNT) has a MIE of 75 mJ, making it very difficult to combust without a proper ignition source.

Flammable substances are taken so seriously by regulatory agencies because even a small ignition source – but containing sufficient energy – can set off a cycle, in which the heat released from the initial combustion is enough to sustain the reaction. The temperature at which this reaction can be sustained is – you guessed it – the flash point! This is why ‘flammable’ chemicals (with flash points close to or below room temperature) must be kept away from ignition sources – because once their MIE is reached, there’s no going back.

Autoignition is the temperature at which the environment provides the MIE, causing the substance to undergo combustion without the need for a specific ignition source. This temperature is usually much higher than its flash point; for example the flash point of diethyl ether is -45°C but requires 180°C for autoignition2.

methyl-THF solvent hazard card

Methy-THF has a flash point of -11°C but an autoignition temperature of 270°C, way above its boiling point of 79°C3.

Fahrenheit 451

What has all this got to do with the Ray Bradbury classic we all know and love? Well the novel’s tagline reads: ‘Fahrenheit 451 – the temperature at which book paper catches fire, and burns…’ With no mention of an ignition source, we can take some liberties in assuming it’s the autoignition temperature that Bradbury is talking about here.

In the book, ‘firemen’ destroy books armed with flamethrowers that – being government supplied – we can assume to be military grade, with napalm as the fuel source. There are different variations of napalm, from the early ones made from long chain fatty acids that burned at around 1000°C (1832°F) to later concoctions of polystyrene and gasoline that could fire up to 2000°C (2632°F)4.

napalm dropped on ecuador

Napalm get is name from its original composition: aluminium salts of naphthenic and palmitic acids.

Anyone notice something funny here? If book paper catches fire when exposed to temperatures of 451 degrees Fahrenheit (233°C), isn’t it overkill for the firemen to wield flamethrowers capable of producing heat exceeding 1000°C? Surely they could’ve made do with heat guns, or simply by tossing the books into a nice warm baking oven?

Surprise! It turns out that ordinary paper – with an autoignition temperature of around 450°C – won’t actually undergo combustion at 233°C5. Okay, but hang on a second, maybe Bradbury was referring to its flash point? Depending on its exact composition, paper’s flash point is estimated to be around 350°C, still off by a hundred degrees or so…

So if neither the flash point nor autoignition temperature of paper is what Ray references in his novel, where then did the number 451 come from?

A possible theory is that Ray Bradbury might have had a little mix-up regarding units, as the autoignition temperature in the region of 450 degrees Celsius could have influenced his decision for the use of 451 in the title. Guess this is what happens when we have two different scales for the same measure – can we all just agree on Kelvin?

Fahrenheit and Celsius

As a little aside, the Fahrenheit scale was invented by Dutch–German–Polish physicist Daniel Gabriel Fahrenheit in 1724. He first defined it using a solution of ammonium chloride in ice (0°F ≈ -17.8°C) and the average human body temperature (96°F ≈ 36°C), as the lower and upper limits, respectively.

The Celsius scale was invented by Swedish astronomer Anders Celsius in 1742 using the boiling and freezing points of water for reference instead. It’s worth noting that the Celsius scale was flipped round a few times over the years because nobody could decide whether 0 should be the freezing or boiling point – bizarre!

Of course, nowadays both systems use absolute zero and the triple point of water as reference points for the sake of accuracy…

Hope you’ve learned something new! The next time you’re accused of being someone who just wants to watch the world burn, you can tell them it’s what the world wants. It’s just entropy, friend. Then proceed to convert them to Kelvins.

Reference

  1. Babrauskas, V. (2003). Ignition handbook (Vol. 318). Fire science publishers: Issaquah, WA.
  2. “Ethyl Ether MSDS”. J.T. Baker. Retrieved 2010-06-24.
  3. Prat, D., Pardigon, O., Flemming, H. W., Letestu, S., Ducandas, V., Isnard, P., … & Hosek, P. (2013). Sanofi’s solvent selection guide: a step toward more sustainable processes. Organic Process Research & Development17(12), 1517-1525.
  4. Neer, R. M. (2013). Napalm. Harvard University Press.
  5. Mark, R. E., Borch, J., Habeger, C., & Lyne, M. B. (Eds.). (2002). Handbook of physical testing of paper (Vol. 1). Crc Press.

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