Ammonium nitrate: a Jekyll and Hyde chemical

Please donate to the appeal if you can, as hospitals are reporting they are unable to treat further casualties as hundreds of beds immediately filled up following the blast.

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Beirut explosionThe explosion which rocked Beirut’s Port was almost certainly caused by inappropriate storage of 2,750 tons of ammonium nitrate. But what is ammonium nitrate and why is it dangerous and yet used so widely?

Often called the elements of life, carbon, hydrogen, oxygen, sulphur and nitrogen, combine in the most stupendously complex ways, to make proteins, and with the absence of sulphur, DNA, which controls the biochemistry of all species on Earth. Photosynthesis captures elemental carbon, hydrogen and oxygen to make the stuff of plants, which then journey through the animal kingdom from insects to elephants, and even you and me. Nitrogen is a little different because some plants draw it up through their roots and others fix the element from out of the atmosphere. Either way, nitrogen is essential to plant growth, which is why the nitrogen fertiliser market is worth around $55 billion per year. Plant nitrogen sources come in two types, ammonia and nitrate. Ammonia (NH4) is the chemically reduced form of nitrogen, and nitrate is the oxidised form, NO3. Ammonium nitrate NH4NO3, therefore, has both the oxidised and reduced forms of nitrogen in the same molecule, which is why it’s so popular as a fertiliser. But containing both the oxidised and reduced forms also makes it a very odd compound, one which according to some interpretations of its chemistry, should not exist. Put this ghost-like chemical existence together with the fact that nitrogen is omnipresent in explosives and you get some idea of why I’ve called it a Jekyll and Hyde chemical.

Chemical bonds within certain nitrogen based compounds can release large amounts of energy with explosive capacity. Take, nitroglycerine, trinitrotoluene (TNT) and nitroamine in C4 as just three well-known examples. Ammonium nitrate is not in the same explosive category, but those high energy nitrogen bonds are nevertheless present. Sitting on a pivot between its reduced and oxidised forms, ammonium nitrate decomposes to nitrous oxide gas (NO2) and water when it gets hot, then as the temperature increases in turns to nitrogen, oxygen and water. These reactions are exothermic, in that they release heat and under right (or wrong) conditions, a chain reaction runs away releasing the gases with explosive power. The principle gas, nitrogen dioxide, is what caused the brown plume after the Beirut explosion, a characteristic telling chemists ammonium nitrate was involved even before it was announced.

Providing ammonium nitrate is stored and handled correctly, it presents little risk, evidenced by the fact farmers all over the world safely hold stocks as fertiliser. Do it wrong however, and you can get into big trouble. In 1921 in Oppau, Germany, a store of around 4.5 kilotons of ammonium nitrate set hard after it became wet. Workman, tired of using pickaxes to break it up, turned to dynamite. The resulting explosion killed an estimated 700 people and obliterated the town. Other ammonium nitrate explosions across the world have killed several thousand people over the years and in every case, the ultimate cause was poor handling or storage of the chemical.

Chemists can make ammonium nitrate intentionally explosive by mixing it with diesel fuel. It’s used as an industrial explosive known as ANFO (Ammonium Nitrate Fuel Oil) in mining and quarrying, but it also became a readily accessible favourite of the IRA in the 1970s and 1990s. Timothy McVeigh and Terry Nichols used the same mixture in the Oklahoma bombing of 1995.

If you’ve eaten any bread recently, it’s a good bet the nitrogen in the wheat came from ammonium nitrate. But if food production is the Dr Jekyll side of that chemical, its explosive potential is the Mr Hyde, be it deliberate terrorism, or chemical incompetence.

One thought on “Ammonium nitrate: a Jekyll and Hyde chemical

  1. A correction to when I said, The principle gas, nitrogen dioxide, is what caused the brown plume after the Beirut explosion, a characteristic telling chemists ammonium nitrate was involved even before it was announced”

    It is true that ammonium nitrate explosions do leave a brownish hue due to nitrogen dioxide, but the colour of the cloud in Beirut was likely too dense for this to be the only explanation. It might be more likely that iron-rich ground was expelled into the air. In fact the surrounding area in the aftermath seems to be covered with the same dark red material.

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