Ammonium Nitrate and Ignorance: A Deadly Combination

Ammonium nitrate, NH4NO3, was used as a fertilizer before people realized that it was potentially explosive. And to this day people who should no better are still unaware of its dangers. Between 1911 and 1921, a German company BASF was producing a mixture of ammonium sulfate and ammonium nitrate in the town of Oppau . The latter easily attracts water, which turns the whole concoction into a hard plaster-like substance. Since the fertilizer was stored in large silos, it was difficult to package the product without first breaking through the hard crust. Someone had the idea of using small amounts of dynamite to convert the ammonium nitrate back into powder form. Because the ammonium sulfate is not explosive, the technique worked without incident for years, but because the Germans were running out of sulfur, they were gradually producing less ammonium sulfate while increasing the concentration of ammonium nitrate. On September 21, 1921, the dynamite not only broke through the crust, it decomposed 450 000 kg of NH4NO3, causing a massive explosion that killed nearly 600 people and injuring 2000. Eighty percent of the homes in Oppau were destroyed, leaving 6500 homeless.

The equation representing the explosion is

NH4NO3(s)→ 2N2(g) + 4 H2O(g) + O2(g)

Two solid molecules produce a total of seven gaseous molecules, causing the sudden increase in pressure responsible for the explosive character.

But upon close inspection, the reaction is bizarre. How could an oxidizing agent like nitrate(NO3) produce an even stronger electron-thief like oxygen? What’s going on?

The equation actually represents an overall reaction, which hides the short-lived sequence of ammonium nitrate’s decomposition.

First NH4NO contains both a reducing agent(NH4+) and an oxidizing agent(NO3). With the proper activation energy, the fertilizer will decompose into dinitrogen monoxide (nitrous oxide or laughing gas) and water.

NH4NO3(s)→N2O(g) + 2 H2O(g)

Although the following is not be the actual mechanism, its electron transfer can be rationalized as follows:

H2O + 2 NH4+ N2O(g) + 8e- + 10H+                 (reduction half reaction)

2 NO3 + 10 H+   + 8e- N2O(g) + 5 H2O      (oxidation half reaction)

(1) Overall : 2 NH4NO3(s) 2 N2O(g) + 4 H2O(g)

 In fact, this is how laughing gas is produced. But when temperature exceeds 240oC, N2O(g) decomposes

(2) 2 N2O(g) 2 N2(g) + O2(g)

If you combine (1) and (2) , the net equation is

2 NH4NO3(s)2 N2(g) + 4 H2O(g)+ O2(g)

texasSince Oppau, there have been more ammonium nitrate disasters including the one in Texas City, the deadliest ever. It occurred aboard a ship carrying the same fertilizer, which was stacked, further increasing pressure for the gaseous products. One of the explosions projected an anchor and a ship’s propeller a mile from the source. Over 500 people were killed and over 5000 injured.

westExplosionThe most recent NH4NO3(s) accident occurred last year(2013) also in Texas, in a town called West. It produced 14 casualties and injured 226 as the blast broke windows 7 miles from the site.
A year later according to the US Chemical Safety Board’s Preliminary Report the fertilizer was not stored in an area with sprinklers, and the volunteer firemen who arrived on the scene did not realize ammonium nitrate is explosive. The West community itself was unaware of the storage area’s potential dangers and had no emergency plan in place.

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