“Everything that comes together falls apart.” — John Green
When we think of decomposition, we usually think of it as a slow, inexorable process of breaking down useful things into malodorous glop. Think of that six-month-old head of lettuce in the bottom of the refrigerator. Or the carcass of the dead animal in the woods, being hauled away by ants and other small organisms after being picked over by corvids and other scavengers. When filmmakers make a movie about zombies, the undead are not simply reanimated. They are simultaneously continuing with their gradual decomposition.
Most of us find decomposition disgusting. As a chemical process safety professional, however, it is not slow decomposition that alarms me. It is the rapid, exothermic process of energetic decomposition that is the most worrisome.
Nitrated Explosives
Many of the explosives we know of are nitrated organics: nitroglycerin, nitrocellulose, nitroparaffins, nitrostarch. And of course, the nitrated organic to which all other explosives are compared: 2,4,6-trinitrotoluene – TNT. They explode by decomposing, not slowly, but suddenly and energetically.
The reaction for TNT decomposition is
H3C-C6H3(NO2)3 (s) –> 3 N2 (g) + 5 H2O (g) + 7 CO (g) + 7 C (s)
Breaking all those chemical bonds takes a lot of energy, so the decomposition of TNT is endothermic at first, but the molecular fragments reassemble into small molecules. This releases even more energy than it took to come apart, making the overall reaction exothermic. Worse, the products are mostly gases. One molecule of solids suddenly generates 15 molecules of hot gas, along with soot. That sudden volume expansion is what defines this decomposition as an explosion. The soot gives the TNT explosion its characteristic black cloud.
Three Industrial Chemicals
In the course of my career, I’ve had several opportunities to work with three industrial chemicals that are especially troublesome because of their propensity to explode. They are ammonium nitrate, hydrogen peroxide, and ethylene oxide. Each of them has an endothermic reaction to go through before getting to the exothermic phase that leads to explosion. Also, each of them can be catalyzed, often with something as ubiquitous as rust, which accelerates the decomposition.
Ammonium Nitrate
Primarily used as a fertilizer because it is loaded with nitrogen in a bioavailable form, ammonium nitrate (AN) is also used as a blasting agent, especially when mixed with fuel oil (ANFO). Long considered a stable compound, AN fertilizer revealed its catastrophic explosive nature in an industrial setting on September 21, 1921, in Oppau, Germany when several thousand tons exploded, killing at least 560 people.
The explosion of AN is the result of decomposition. At 170 C (338°F), AN melts. Further heating leads to decomposition of the liquid:
NH4NO3 (l) –> N2O (g) + 2 H2O (g)
One molecule of liquid decomposes into three molecules of hot gas and releases energy. If that wasn’t bad enough, that energy heats the liquid to an even more energetic decomposition:
2 NH4NO3 (l) –> 2 N2 (g) + O2 (g) + 4 H2O (g)
Now we get three-and-a-half molecules of hot gas for every molecule of liquid, an even more powerful explosion. Not as powerful as TNT on a molecular basis, but powerful, nonetheless.
Hydrogen peroxide
Most of us are familiar with hydrogen peroxide in the form of a 3% solution in a brown plastic bottle in the medicine cabinet, although hydrogen peroxide is now largely discredited as a treatment for cuts and scrapes. If it has been in the medicine cabinet for more than a few months, that brown bottle probably just contains water, because hydrogen peroxide decomposes:
2 H2O2 (l) –> 2 H2O (l) + O2 (g)
It’s the oxygen gas that gives hydrogen peroxide its medicinal properties. The concentration is so low, however, that the 97% water can safely absorb the exothermic heat of decomposition.
At higher concentrations, that oxygen makes hydrogen peroxide an excellent bleaching compound for the pulp and paper industries. It also means that there is not enough water to absorb the exothermic heat of decomposition. So much so that OSHA lists hydrogen peroxide at 52% or higher in its List of Highly Hazardous Chemicals, Toxics and Reactives in Appendix A of the PSM Standard.
Ethylene Oxide
Widely used as a sterilizing gas, ethylene oxide (EO) is also useful for building ethoxylated surfactants. OSHA lists EO in Appendix A of the PSM Standard because of its toxicity. Ethylene oxide (EO) is a three-member heterocyclic compound. The bonds of those three atoms – two carbons and an oxygen – are bent so far that it just doesn’t take very much energy to break them. Then the fragments recombine to form hot gases. There are two decomposition reactions, each yielding two moles of hot gas per mole of ethylene oxide vapor:
C2H4O (g) –> CH4 (g) + CO (g)
2 C2H4O (g) –> C2H4 (g) + 2 CO (g) + H2 (g)
Admittedly, the reaction products are all very combustible, yielding an even higher volume of hot gas per molecule of EO. It’s not the combustion that worries me, however. We have good experience with inerting combustible gases, so a flammable mixture doesn’t form. It’s the decomposition that worries me, since excluding oxygen doesn’t stop the decomposition.
For Pranksters
One last decomposition reaction is worth a mention: nitrogen triiodide. This is the first explosive decomposition that I ever experienced. Not as an engineer, but as a high school chemistry student. Nitrogen triiodide is easy enough for interested high school students to synthesize in a high school chemistry lab without any special equipment, and the instructions are widely available on the internet. (No, I’m not giving a link. The stuff is dangerous.)
When the paste is spread on a surface and allowed to dry, it leaves a contact-sensitive explosive solid behind. When touched, even with a feather, it explodes with a percussive clap, sounding much like a gunshot. A favored place for prankster was on toilet seat bumpers. Imagine the shock of the unsuspecting victim who goes to the restroom and sits on the toilet, and the principal who then hears reports of gunshots in the school.
The decomposition is
2 NI3 (s) –> N2 (g) + 3 I2 (g)
Again, notice how one molecule of a solid generates two molecules of hot gas, which is the stuff of explosions.
Applicability of the PSM Standard
According to OSHA’s Explosives and Blasting Agents Standard, TNT (and all explosives and pyrotechnics) is covered under the PSM Standard. And according to 29 CFR 1910.109, there is no threshold quantity of for explosives and pyrotechnics.
Hydrogen peroxide and EO are also covered under the PSM Standard. On the other hand, nitrogen triiodide is not. Perhaps it is because nitrogen triiodide’s instability makes it impractical for use in industrial or military applications. However, the related compound, nitrogen trifluoride, is covered by the PSM Standard.
For reasons that remain mystery to me, despite its destructive history, AN is not covered under the PSM Standard. It seems that it should be.
The Usual Safeguards Are Not Enough
Unlike most decompositions, which are endothermic, there are a few that are sudden and exothermic, resulting in explosions. Be mindful of them and beware that the usual safeguards are not likely to be as useful as they need to be.
