Monday, April 15, 2013

How Air Separates Water And Goes Boom

Fertilizer bombs typically use ammonium nitrate, NH4NO3. Bombers use it because the stuff is so ubiquitous and so plentiful.

Wiki draws a pretty picture of ammonium nitrate, which partially reveals its explosive potential:
Don't skip over that structure--it's a work of art: two very different nitrogen atoms separate elements of hydrogen and oxygen. On the left, four hydrogens surround a central nitrogen while on the right, three oxygen atoms surround a central nitrogen. There's an unbalanced symmetry. Those H's and O's would love to get together and quench, making water, leaving the naked nitrogens to couple, making N2 which is essentially air. The chemical structure of ammonium nitrate is a depiction of air separating water.

We never hear about spontaneous explosions of ammonium nitrate. Why is that? An old word from the Greek called stoichiometry answers why. Try and balance the chemical equation of NH4NO3 making N2 and H2O:

             NH4NO3  =>  N2  +  H2O

Look easy? I gave up. The reason is that the hydrogen to oxygen ratio is inherently 4 to 3 on one side and 2 to 1 on the other. No amount of tweaking the coefficients will balance that reaction. The stoichiometry just doesn't work.

This brings up just how fertilizer bombs do work, because they do make air and water from ammonium nitrate. There is always a little admixture of carbon (typically fuel oil) but charcoal would work too. The carbon combines with the "excess" oxygen present in NH4NO3, making CO2. This trick even has a name: cf. detonation vs. deflagration

Here's a question for any munitions experts out there: were ANFO's involved in Boston? I've heard that the differing sounds of detonation versus deflagration are diagnostic.

A second question is technical: is it possible (in theory) to isotopically label the nitrogen in manufactured ammonium nitrate and thus, after isotopic analysis of air samples at the bombing site prior to mixing, identify the source?
[Added]  Well this breaking news should cause a spike in ammo and food prices. The report says that water sprayed on NH4NO3 made it explode. That may actually have been heat from the fire or could have resulted from of what's called heat of hydration when a dry salt suddenly mixes with water. Some salts suddenly mixed with water actually make the water colder.

Ammonium nitrate (AN) is not known to chemically react with water in an explosive way. However, if heated it reacts explosively:

             2NH4NO3  =>  2N2  + O2  + 4H2O

Now that reaction really produces a sky-full: nitrogen, oxygen, & cloud.


  1. I doubt that they'd use ANFO for the sorts of bombs used in Boston because of the typical yield to weight ratio. More likely some sort of plastic explosive such as C-4, PETN or Semtec.

    Since I didn't hear it (high order vs low order), I have no idea what was used, however placement of explosives such as that one is critical to effect. There is also the question of whether they included some sort of fragmenting material. I don't know if there is anything on YouTube about using Bangalore Torpedoes for this sort of anti-personnel mission or not, but in the sense of the modern versions they are not just for wire.

    Based on the reports that came from the scene, I think that it was a high order detonation from multiple devices that had been carried to the location in backpacks. Since there was such extensive wounding, likely C-4 with ball bearings or something along that line (think massive shotgun). There was one device that didn't blow in Boston. It would be interesting to me whether or not that was timed to kill first responders who were arriving to lend assistance to the murdered.

    Two claymore mines (shrapnel propelled by C-4) - not very heavy and designed with a directional fragmentation pattern would have worked well in that scenario and would fit in backpacks. ANFO is usually much heavier to get the yield that you'd want - and with ANFO you need an initiating charge (such as C-4) to start the reaction that you outline above. That's why most ANFO bombs are packed into culverts, into cars or vans and aren't delivered by small backpack (20-30 lbs).

  2. Late breaking news indicates that the murderer/bomber used gunpowder (low order explosive) to propel ball bearings.

  3. Blogger chickelit said...

    After hours of police coaxing, the little man in the boat came quietly.

    Oh no you dih-hint! Damn, you went there!

  4. You could say that was the climax of the investigation.

  5. IIRC, the addition of water to heated ammonium nitrate was what caused the Texas City disaster.

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  7. A second question is technical: is it possible (in theory) to isotopically label the nitrogen in manufactured ammonium nitrate and thus, after isotopic analysis of air samples at the bombing site prior to mixing, identify the source?

    Theoretically it out to be possible.

    But it'd cost so much as to make it utterly pointless - besides, the "source" is almost going to be "a giant chemical company that made a batch of commercial fertilizer".

    It wouldn't be especially meaningful to find out where a very common, essentially un-controlled (and in practical terms un-controllable) chemical came from, would it?

  8. I just figured out why this somewhat old post is getting some action - it was Althouse linked!

    Thank you, Ann Althouse.

    @Sigivald: How many different companies actually make ammonium nitrate? You'd need a Haber-Bosch plant. I agree that the cost of the isotopes would be like a tax and would raise costs, but the actual amount would depend on the level of enrichment-- but do not underestimate the the power of isotopic analytics these days. Mass spectrometric sniffing has come a long ways.

  9. Pollo Raylan - the fewer the number of plants, the worse Sigivald's problem gets. So there's a bomb. Isotopic analysis says - same thing it always says. Oops. Though it would tell you whether the bombers made their *own* ammonium nitrate.