Urea
Decomposition

Molten urea decomposes into ammonium cyanate at about 152 °C, and into ammonia and isocyanic acid above 160 °C:

Heating above 160 °C yields biuret NH CONHCONH and triuret NH CONHCONHCONH via reaction with isocyanic acid:

At higher temperatures it converts to a range of condensation products, including cyanuric acid (CNOH), guanidine HNC(NH), and melamine.

In aqueous solution, urea slowly equilibrates with ammonium cyanate. This elimination reaction cogenerates isocyanic acid, which can carbamylate proteins, in particular the N-terminal amino group, the side chain amino of lysine, and to a lesser extent the side chains of arginine and cysteine. Each carbamylation event adds 43 daltons to the mass of the protein, which can be observed in protein mass spectrometery. For this reason, pure urea solutions should be freshly prepared and used, as aged solutions may develop a significant concentration of cyanate (20 mM in 8 M urea). Dissolving urea in ultrapure water followed by removing ions (i.e. cyanate) with a mixed-bed ion-exchange resin and storing that solution at 4 °C is a recommended preparation procedure. However, cyanate will build back up to significant levels within a few days. Alternatively, adding 25–50 mM ammonium chloride to a concentrated urea solution decreases formation of cyanate because of the common ion effect.
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