In films and books, chemists are often portrayed as a doddery, balding, bespectacled, middle-aged¹ gent, losing his grip on reality and his test-tubes. When he makes a mistake (for it is invariably a 'he'), it is a danger only to himself. Real danger requires real malice, and at that point he exits stage-left and the stereotypical 'mad scientist' takes over.

One of the worst ever chemical disasters was not a deliberate act. The disaster that befell the small German town of Oppau in 1921 shows the danger of 'inductive reasoning' - the belief that because a particular outcome has prevailed up to now, it will carry on prevailing - when dealing with matters of human health and safety.

The German Chemical Industry Between the Wars

Open a chemistry textbook and you will find that the reactions are often named after the people who discovered them and that many of those names are German in origin. The list starts at 'A' with the 'Arndt-Eistert Synthesis' and goes all the way up to 'Z' with the 'Ziegler catalyst'. Chemistry might have been born French, with Lavoisier discovering oxygen, but it grew up in Germany as many modern industrial processes were invented there.

The Haber-Bosch Process

Fritz Haber can be regarded as one of the founders of modern German industrial chemistry. The Haber-Bosch process is fundamental to the production of agricultural fertilisers. Haber invented the first method of 'fixing'² nitrogen gas. Carl Bosch, another German, scaled the process up to industrial levels.

Fixed nitrogen is an essential component of chemical fertilisers. The major source of this stuff was either in saltpetre from Chile or ammonia released from coal when coke is made. The Haber-Bosch process is incredibly important as it allows ammonia to be produced in huge quantities³. Hydrogen is reacted with nitrogen in the presence of an iron catalyst under high temperature and pressure:

3H₂ + N₂ → 2NH₃

Ammonia is a gas and gases don't make for very good fertilisers. It is, however, a strong base so it reacts readily with acids to make ammonium salts. One such acid is sulphuric acid, reacting to form ammonium sulphate.

Sulphuric acid itself is made by oxidising sulphur. In the inter-war period, Germany found it hard to get hold of sulphur and so the Ostwald process was employed that oxidised ammonia to nitric acid:

2NH₃ + 4O₂ → 2HNO₃ + 4H₂O

The nitric acid then reacts with more ammonia to make ammonium nitrate, which is in itself an even better fertiliser than ammonium sulphate.

The BASF plant at Oppau was built to produce a mixture of ammonium sulphate and ammonium nitrate, the proportion of the latter increasing as time went on and sulphur grew harder to obtain. The plant had stockpiled 4,500 tonnes of this mixture in a warehouse. During some damp weather the pile had 'caked' into a solid mass, impossible to move or package. On 21 September, 1921, some bright spark had the idea of drilling holes in the mass, dropping in a few sticks of dynamite and detonating them in order to make the mass manageable: it had worked many times for ammonium sulphate, after all.

The Blast

This operation was, by its own objectives, a total success. The resulting blast certainly loosened all the fertiliser. It also created a 90m by 125m crater and demolished 80% of all the homes in Oppau. The pressure wave ripped roofs off houses up to 25km away and was heard in Munich, 300km distant. 6,500 people were made homeless, and 700 killed.

What had happened was that the mass of ammonium nitrate and sulphate had been poorly mixed. Some regions of the mass had more ammonium nitrate than others. The compound is thermally unstable, reacting under heat to form nitrous oxide and water. However, it is also endothermic, meaning that it requires a net input of energy to form it from its elements. When detonated it decomposes to give nitrogen, oxygen and water, all gases:

NH₄NO₃ → N₂ + 2H₂O + 1/2 O₂

It also releases a huge amount of energy in so doing. It was estimated that of the 4,500 tonnes of ammonium nitrate, one tenth of that had actually detonated with a force equivalent to 1.2 kilotonnes of TNT. Ammonium nitrate therefore has roughly about two and a half times the 'bangs per buck' of the other more commonly known explosive.

Eventually, people began to realise that here was an easily made and very powerful explosive. Moreover, it also released lots of oxygen when detonated which meant that, when mixed with combustible substances, one had an even more powerful explosive. Typically, the other material was a hydrocarbon such as kerosene or mineral oil; ANFO (ammonium nitrate - fuel oil) is commonly used in the mining industries for blasting operations.

The disasters, meanwhile, continue down the years. There have been at least 15 significant incidents involving this material since its large-scale manufacture started. For example, the Texas City Explosion in 1947 occurred when a ship blew up in Galveston Bay. A stray cigarette started a fire in the hold of a freighter carrying sulphur and ammonium nitrate. Within minutes, the flames were too hot to dowse with water and the inevitable small crowd gathered to watch the spectacle, including a group of children from the local school. When the ship finally blew it killed hundreds, creating a huge wave and setting off several other secondary explosions, including one in another ship carrying the fertiliser.

One of the most recent disasters was on 21 September, 2001, in Toulouse in France, where, 200 tonnes of the stuff detonated in a warehouse, killing 31 people and injuring over 2,400. Ten thousand homes, five schools and a hospital were seriously damaged. The fire chief in charge of the incident described it as a 'scene from Hell'.

Terrorism

Of course, once Pandora's chemistry set has been opened, anybody - not just a mad scientist - with an ounce of malice and tons of agricultural chemicals can set about causing mayhem. The material became a favoured weapon of the IRA during its heyday in the 1970s. But the most notorious recent terrorist incident making use of it was the Oklahoma City Bombing by Timothy McVeigh and Terry Nichols in 1995. These two malcontents were chagrined at the raid by the FBI upon the headquarters of David Koresh's Branch Davidian religious cult two years earlier; more than 70 people died in a fire that began during the raid. In the warped mindset of the terrorist, this was best avenged by murdering 168 innocent people including 19 children. McVeigh drove a truck containing 2,300kg of home-made ANFO explosive up to the front of the Alfred P Murrah federal building and detonated it. The blast was felt in Bridge Creek over 30 miles away.

McVeigh paid for this act of cold-blooded murder with his life, being subject to one of the first federal executions in years. However, it was not the first such terrorist blast involving home-made ANFO explosive and probably will not be the last. The materials are readily available, given the overwhelming reliance the farming industry has upon artificial fertilisers, and a few bags here or there out of a yearly consumption of millions are unlikely to be missed. Given the ease with which this particular chemical ploughshare is beaten back into a sword, the prospects of curtailing the murderous activities of a few 'modern-day Guy Fawkeses' are bleak indeed.