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From: ee5391aa%triton.unm.edu@ariel.unm.edu (Duke McMullan n5gax)
Newsgroups: sci.military
Subject: Re: Fighting Saddam's fires
Message-ID: <1991Feb5.040736.4360@cbnews.att.com>
Date: 5 Feb 91 04:07:36 GMT
References: <1991Jan28.040537.9742@cbnews.att.com>
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From: ee5391aa%triton.unm.edu@ariel.unm.edu (Duke McMullan n5gax)
In article <1991Jan28.040537.9742@cbnews.att.com> JEWELLLW@VM.CC.PURDUE.EDU
(Larry W. Jewell) writes:

>    Remember the "Summer of Fires"?  The planes were dropping potassium
>bicarbonate (Purple K in the Navy) on the fires.  ...
...
>     Just for those who aren't familiar with Purple K, it doesn't work in
>a conventional manner, that is it doesn't remove one leg of the fire
>triangle (spark, fuel, air) but instead it "interrupts the chemical
>reaction of fire" which has always been really clear to me(:-().

The "fire triangle" is passe...they now call it -- get this -- the "fire
tetrahedron".  That can be rough on some of the people in the Fire School:
you'll notice the second word has _more_than_three_syllables!_

OK, enough sarcasm.  The operant idea here is "free radicals."  No, that
doesn't refer to unjailed KKK and PLO types.  A free radical is a transient
chemical particle, normally carrying an electrical charge, that is an essen-
tial part of flame chemistry.

Consider methane gas, CH(4): four hydrogen atoms bonded to a central carbon
atom.  If you have a methane flame, some of the species you might find are:
	CH(4)[+] 	(That's CHsubscript4superscript+)  A complete methane
			molecule, but one that has lost an electron somewhere.

	CH(4)[-]	Same thing, but it grabbed that electron.

	CH(3)		No net electrical charge, but it has an unbonded elec-
			tron in the carbon.  It's not stable.

	CH(3)[+]	This one lost a hydrogen atom, and two electrons.
	
	CH(3)[-]	This one lost a hydrogen atom, but no electrons.

...and what about those hydrogen atoms?

	H		Neutral, but it's atomic hydrogen.  Very unstable.

	H[+]		A free proton.  In all probability, it'll be bonded
			in some way, as H(3)O[+] (hydronium), NH(4)[+] (am-
			monium), or CH(5)[+] (carbonium).  You may note a
			certain inconsistency in those names. You're right.

	H[-]		Hydride.  Unstable.

There are numerous others, of course.  These are only the simplest, and as-
sume the presence of methane (which is present in most fires as a decompo-
sition product, if nothing else.  The potassium bicarbonate is a "free rad-
ical inhibitor," which breaks the free-radical-leg of the tetrahedron, rather
than a leg of the more traditional fire triangle.

I don't know the chemistry of the free radical-KCO(3) interaction, but in
some manner or another, it seems to swallow a lot of free radicals.  That
basically causes the fuel and oxidizer to react much less vigorously, in
spite of the temperature.  With less combustion, the temperature drops. The
fire goes out.

Let's note parenthetically that free radicals have a considerably extended
lifetime at elevated temperatures.  I have no numbers.

						d



--
    "Got to slap these Goddamn Third World nations around, Flynn," he said,
    "until they learn some manners."     -- Gregory MacDonald, from _Flynn_
  Duke McMullan n5gax nss13429r phon505-255-4642 ee5391aa@triton.cirt.unm.edu