What Is Urea Nitrate?

Urea nitrate is a colorless, crystalline compound of carbon, hydrogen, oxygen and nitrogen with the chemical formula (NH₂)₂CO.HNO₃. It is prepared by the reaction of urea with nitric acid: (NH₂)₂CO + HNO₃ -> (NH₂)₂CO.HNO₃. The compound is ionic in nature with the urea — bonded to the hydrogen atom from the nitric acid — forming the cation, and the nitrate (NO3-) group forming the anion. It is soluble in water, but much less so in nitric acid, and so it tends to crystallize out following the above reaction. Urea nitrate decomposes explosively when subjected to shock; however, its sensitivity is fairly low, making it relatively safe to handle under normal circumstances, although it can explode on prolonged heating.

Explosives generally require a fuel, which would be some kind of oxidizable material, and an oxidizer, which in this context is normally a substance which supplies oxygen. In the case of urea nitrate, as with most commercial and military explosives, the fuel and the oxidizer are present in the same molecule, and like most compounds of this type, it is classed as a high explosive. It has a detonation velocity, that is, the speed at which the shock wave travels through the explosive, of around 11,155 feet per second (3,400 meters per second), about half that of trinitrotoluene (TNT). High explosives can be classed as primary or secondary depending on whether they can explode without a detonator; urea nitrate is classed as a secondary explosive, as a small explosive charge is normally required to provide sufficient shock to cause detonation.

Urea nitrate does not normally explode if ignited, but burns easily in air, producing carbon dioxide, water and oxides of nitrogen. It can, however, detonate if subjected to prolonged heating or on contact with some other chemicals. The compound is less likely to explode if it is wet, and in the presence of water, it tends to decompose non-violently into urea and nitric acid.

The explosive properties of this compound have led to some commercial uses, but it has gained notoriety for its use in improvised explosive devices (IEDs). Urea and nitric acid are both manufactured on a huge scale for use in the fertilizer industry — urea is itself a widely used fertilizer — and the availability of these raw materials in bulk quantities combined with the ease of urea nitrate synthesis has led to this explosive often being favored by terrorist groups. Explosive devices which employ it are sometimes described as “fertilizer bombs,” but this term is more frequently applied to bombs based on ammonium nitrate, which is another commonly used fertilizer. It is thought that urea nitrate was the main explosive used in the 1993 car bomb attack on the World Trade Center.

A simple field test can identify urea nitrate by exploiting its reaction with p-dimethylaminocinnamaldehyde (P-DMAC) to form a strongly colored red compound. Urea and other related compounds do not produce this reaction, reducing the chances of a false positive result. Following an explosion, however, it is difficult to verify chemically if this compound was involved as it would then be present only in trace amounts and the decomposition products are similar to those of some other nitrate-based explosives.