I was reading up on a wonderful little chemical compound known as chlorine trifluoride ($\ce{ClF3}$). For a primer, check out Dr. Derek Lowe's blog post here: Sand Won't Save You This Time. The title of the post is quite telling; this compound is so reactive with other compounds - any other compound, really - that it will set sand on fire, making this old standby for extinguishing lab fires useless and putting the chemical in very rare company indeed. Obviously, water is a huge no-no for chlorine trifluoride; it will exothermically react with the water to produce clouds of steam containing two nasty acids (hydrochloric and hydrofluoric) and a host of chlorate and fluoride compounds. Somewhere in the middle of all the heat, fluorine and organic-ish compounds, you could probably even get some fluorine oxides, which would just LOVE to rapidly reduce their oxidation state (an event normally accompanied by shrapnel).

I was intrigued; I went and looked up the MSDS for this chemical to see what the instructions were for dealing with industrial spills of this stuff (apparently, there's a decent market for it in the semiconductor industry, and probably also for plastics to produce highly fluorinated polymers like Teflon). "A good pair of running shoes" can't be the only answer to a $\ce{ClF3}$ fire. (well, actually, it pretty much is; get everyone in a one mile radius and two miles downwind the hell away from the area)

Now, here's the question. While this stuff has some serious "synergy" when it comes to being an oxidizing agent (it's a better oxidizer than pure oxygen, loads better than pure chlorine, better even than pure fluorine gas), the NFPA flammability rating of the chemical is 0. This, despite reports from those who originally studied it that the chemical is hypergolic with every known fuel substance, and most things you wouldn't consider fuel (like asbestos, sand, concrete, brick, earth, test engineers). The resulting reaction certainly looks like fire; intense heat, bright light, sparks, smoke, the works. So, if chlorine trifluoride doesn't "burn" by the NFPA's definition, what is the NFPA's definition of "burning"?


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You have found one of biggest flaws in the NFPA "Fire Diamond". The NFPA rates the flammability of the compound as a fuel. ClF3 is not a fuel. It is an oxidant. ClF3 is playing the role that oxygen normally does. It causes fuels (reductants) to burn by oxidizing them rapidly and exothermically. Oxidants are incapable of creating fire in absence of fuel regardless of temperature. Fires started by ClF3 continue to burn after being smothered because oxygen isn't necessary to continue the oxidation process. And since ClF3 is denser than air, it gets trapped under the sand. Many other powerful oxidants, like fluorine and perchloric acid, have low flammability ratings. Oxygen also has a flammability rating of zero. They do not burn; they cause other compounds to burn.

The scale for the flammability rating has to do with flash point, which is the temperature at which a fuel forms an ignitable mixture with air. Since ClF3 doesn't burn (isn't a fuel), and doesn't react with air (N2, O2, CO2, Ar, etc, which are all inert towards oxidation), it does not have a measurable flash point.

The danger posed by ClF3 is summed in the yellow (reactivity or stability) diamond and the white (physical and chemical hazards) diamond. Your msds rates ClF3 as a 3 in reactivity/stability, which is appropriate. Wikipedia's NPFA rating article, even mentions chlorine trifluoride:

3 - Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked (e.g. ammonium nitrate, chlorine trifluoride).

The white triangle will have appropriate hazard information. For ClF3, it has the symbols OXY, which means it is a strong oxidizer, and W, which means it reacts with water.

I have not found the specific reaction that ClF3 undergoes with sand (mostly SiO2). I expect that it involves ${\ce{SiO2 -> SiF}}_6^{2-}$, except that this half reaction is not an oxidation.

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    $\begingroup$ You may want to mention that it's basically the same as oxygen qualitatively. Oxygen also has a fire rating 0, but it supports combustion (while not being combustible itself) $\endgroup$ Jun 27, 2012 at 4:27
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    $\begingroup$ ""The scale for the flammability rating has to do with flash point, which is the temperature at which a fuel forms an ignitable mixture with air."" Aha, but what about solids like wood or coal? What is their flash point? There is a definition in Wikipedia! $\endgroup$
    – Georg
    Jun 27, 2012 at 20:01
  • $\begingroup$ There's a lot of intermediates that could happen here. Garden-variety sand actually contains a lot of hydrated silica, providing water to produce HF. That would in turn react with the silicon dioxide (hydrated or otherwise) to produce a host of silicon fluorides, from silicon tetrafluoride to hexafluorosilicate anion, which would have plenty of hydrogen around to form hexafluorosilicic acid and probably some trace alkali metals to form salts. If the sand was treated as a flame retardant with MgOH or AlOH there's plenty of that to go around too. $\endgroup$
    – KeithS
    Jul 13, 2012 at 20:11
  • $\begingroup$ Then, there's all the oxygen that's displaced when the fluorine barges in to the dioxide, which is a secondary threat only because there's fluorine around; it can reform water (always a good exothermic spectacle) or find something else more fuel-like in the area to oxidize. $\endgroup$
    – KeithS
    Jul 13, 2012 at 20:17
  • $\begingroup$ And don't forget the chlorine; again, reactions with chlorine and any nearby alkali metals are pretty much guaranteed to be spectacular, and with the heat of the other reactions around, and the liberated oxygen, I wouldn't put it past this stuff to form some nice explosive perchlorate byproducts (they wouldn't last long, but...). $\endgroup$
    – KeithS
    Jul 13, 2012 at 20:19

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