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Science

The chemistry of the Tianjin warehouse blasts

The Tianjin warehouse blasts on August 12 caused a weak surface quake in the Chinese port city, made houses uninhabitable in a 2-km radius, absolutely decimated a parking lot of 8,000 cars in its vicinity, blew a crater below the warehouse, released a very-toxic chemical into the surrounding air and water, and consumed the lives of 114 people and counting. The structure was known to contain hazardous substances, and claims have surfaced that the company managing them – Ruihai International Logistics – was transporting obscene quantities illegally.

But beyond the illegality itself, the explosions had an underlying chemistry whose strength far outstripped the weaknesses of structures in the vicinity being susceptible to shockwaves, a strength that exacerbated the government’s failure in not clamping down on Ruihai earlier, in allowing residential settlements in the neighbourhood of such chemicals and, in the aftermath, not reacting swiftly enough to either allegations of cover-ups as well as informing the media of what had gone up in flames. According to one video (link now dead) on weibo, the Chinese microblogging website, one of the principal explosions occurred after the firefighters arrived, implying that the destruction was drawn out by the already-burning warehouse being sprayed with water by authorities in the know.

For instance, the structure was known to contain ammonium nitrate, potassium nitrate and calcium carbide, as well as sodium cyanide. Of these compounds, ammonium nitrate and calcium carbide are known to be explosive – but for different reasons – while the rest are deadly in their own right.

Ammonium nitrate

Simply put, ammonium nitrate is the Liam Neeson of fertilisers but also the Ra’s al Ghul of explosives. It was used as the explosive material in the 2013 Hyderabad blasts. Each molecule contains two atoms of nitrogen, three of oxygen and four of hydrogen. The way the atoms are bonded, the molecule as a whole is eager to give away an oxygen atom to any other molecule reacting with it because doing so would send it a stable, less energetic state. This eagerness is exemplified by the molecule’s low sensitivity to physical shock and heat: just a little jerk or heating will blow it up, generating a shockwave at 5,270 m/s – more than five times the speed of sound.

What could make such a shockwave even more powerful has to do with another property of ammonium nitrate. If not stored in tightly sealed containers, the compound gradually absorbs moisture from the atmosphere and coalesces into solid lumps. In the Tianjin warehouse, then, large quantities of ammonium nitrate could’ve become moist and formed proximate clumps, and when one section of those clumps got heated, it blew up and generated a shockwave that shot the rest of it to Hell as well.

Calcium carbide

By itself, calcium carbide is mostly harmless. But should you spray it with water, it reacts to produce calcium hydroxide (slaked lime) and acetylene. When acetylene is burnt in the presence of oxygen, it produces a flame of 3,600 K, hot enough to melt a metal as sturdy as tungsten – so its application in welding. And like ammonium nitrate, acetylene is susceptible to shockwaves, especially if the surrounding pressure goes beyond 103,421 pascals (almost equal to the atmospheric pressure at sea level). In such situations, it explodes into its constituent atoms – carbon and hydrogen.

And here it gets worse: hydrogen burns violently with… well, the atmosphere. Remember the reactor-3 explosion during the Fukushima disaster in 2011? That wasn’t the work of any nuclear substance as much as leaked hydrogen.

Potassium nitrate and sodium cyanide

While potassium nitrate is used in the preparation of gunpowder, it’s explosive when reacting with reducing agents – i.e. electron-donators. The incredibly poisonous sodium cyanide on the other hand isn’t considered explosive. However, an explosive derivative presents itself. The compound is hygroscopic, absorbing moisture from the atmosphere to form sodium formate and ammonia, and in sufficiently high quantities, ammonia reacts explosively with air, especially if heated up to 200° C.

Sodium cyanide is also wildly toxic, and already reports have emerged that it has been found in “nearby drains after the blasts“. It is very soluble in water, and if it enters the body in quantities as small as 3 mg/kg, it rapidly knocks out the lungs’ ability to take in oxygen and results in death. The Associated Press claims “several hundred tons” of it were present in the warehouse (according to one estimate, 700 tons, a decidedly unholy quantity within a kilometre’s radius of residential dwellings, and enough to poison 91% of all Asia to death*).

 

*

According to a BBC report, China is the world’s largest consumer of hazardous chemicals, and so can claim expertise in the storage and transport of large quantities of chemicals and no ignorance of how the warehouse came to store over 3,000 tonnes of the chemicals with the nearest houses less than a kilometre away. The chemicals themselves are frequently used in the metals industry, for the manufacture of synthetic substances, and for extracting some precious metals from their ores.

No wonder then that some Ruihai employees have been arrested, as well as a former government officials who’d served in the area called in for questioning. Beyond the difficulty of cleaning up an area made more potent by the addition of water, the bigger challenge facing the Chinese government is the chemistry itself: without a license to handle hazardous substances between October 2014 and June 2015, how did Ruihai amass the ammonium nitrate, calcium carbide, potassium nitrate and sodium cyanide, and why?

*Assuming the average body mass of an Asian adult is 57.7 kg, and with “people” referring only to Asian adults.

The Wire
August 18, 2015

Categories
Science

Why does sodium react so explosively with water?

In January 1947, the American War Assets Administration dumped drums of sodium left over after the end of World War II into Lake Lenore in eastern Washington state. A video of the event – it really was an event – is available from the Internet Archive.

Sodium’s reaction with water – or most other substances in general – is so violent because of the number of electrons in its atoms. Specifically, each sodium atom has one electron more than the atom needs to be in a highly stable state. That one electron keeps the atom highly unstable, and is given away at the first available chemical opportunity. So, when sodium (Na) meets water (HO–H), it rapidly forms sodium hydroxide (Na–OH) and releases hydrogen (H). Simultaneously, the sodium atoms release their extra electrons to form the molecules, and from go being highly unstable to highly stable. As a result, they produce such heat that the hydrogen is ignited, which burns with a bright flame, even as some of the water boils off as steam.

Even if all of this makes sense – and is true – could there be more to this reaction than meets the eye?

That’s the question a bunch of chemists, from the Academy of Sciences of the Czech Republic and the Technical University of Braunschweig, Germany, chose to ask. They figured the explosive nature of the reaction wasn’t solely due to sodium’s eagerness to react with water but also had to do with how its surface changed shape when in contact with water. Using high-speed cameras, they studied how drops of an alloy of sodium and potassium, another explosively reactive metal, responded when they were dropped into water. Watch this closely.


Credit: Mason et al.

Toward the end of the video (as Thunderf00t explains in the 17th minute), you can see how almost as soon as it is dropped, the alloy rapidly develops spike-like protrusions on its surface, on the underside. These spikes form within a few thousandths of a second, and increase the surface area of the metal that is available to react with water. The scientists calculated based on their video footage that the spikes start and finish extending out of the surface at an acceleration of 10,000 ms-2. That’s almost the same acceleration at which you could be shot out of a space gun.

Caption: First experiments of the alkali metal explosion in water performed at the balcony of the Institute in Prague provided important clues for later more rigorous laboratory studies (and lots of fun).
First experiments of the alkali metal explosion in water performed at the balcony of the Institute in Prague provided important clues for later more rigorous laboratory studies (and lots of fun). Credit: Phil Mason

When they performed the same experiment with a drop of liquid aluminium, they couldn’t see any spikes forming on the metal droplet. Apparently, it happened only with the sodium-potassium alloy. Was it because of the explosion that happens? Nope, because sodium-potassium reacts non-explosively with ammonia, but the spikes formed there again. So it definitely happens only with sodium-potassium.

So, to get their answer, the scientists used a computer simulation, which revealed an all too familiar devil in the details.

As soon as the sodium-potassium drop meets with the surface of water, its outermost layer of atoms loses electrons rapidly to the water – so fast that the transfer happens in a few trillionths of a second. The water molecules accept the electrons and subsequently break down into hydroxyl (HO) and hydrogen (H) ions. As a result, at the interface of the drop and the water, there are now four layers: the remaining drop of sodium-potassium atoms, next a layer of sodium-potassium ions (positively charged because they’ve lost electrons), then a layer of hydrogen and hydroxyl ions (positively and negatively charged, respectively), and finally the rest of the water.

ions

As you can see, there is a layer of positively charged sodium and potassium ions, and like charges repel each other. Because the sodium-potassium alloy is in liquid form, the repulsion manifests as highly distended droplets, or spikes. In technical parlance, this phenomenon is called coulomb fission. The resultant increase in surface area prevents the reaction from stalling, which might have happened if the first layer of sodium hydroxide to form was let to act like a blanket protecting the rest of the alloy.

The English physicist John William Strutt (better known as Lord Rayleigh) first predicted this for liquids in 1882. He reasoned that an electrically charged drop could only contain so much charge before its surface tension gave way and let the drop break up into droplets by ejecting jets – called Rayleigh jets – out of their sides. The Czech and German scientists used high-school math to figure that this breakdown happens as soon as the distance between the sodium-potassium ions and the electrons and hydroxyl ions becomes more than 5 angstrom (one angstrom is a ten-billionth of a meter).

So, that’s one high-school chemistry lesson fully unraveled. How about going after the vinegar volcano next?

The scientists’ paper was published in Nature Chemistry on January 26 .