Allophanic acid is the organic compound with the formula H<sub>2</sub>NC(O)NHCO<sub>2</sub>H. It is a carbamic acid, the carboxylated derivative of urea. Biuret can be viewed as the amide of allophanic acid.
Treating urea with sodium bicarbonate is claimed to give allophanic acid:
Although allophanic acid per se may not have been purified, its conjugate base, H<sub>2</sub>NC(O)NHCO<sub>2</sub><sup>âÂÂ</sup>, allophanate is well known. Salts of this anion have been characterized by X-ray crystallography. The allophanate anion is the substrate for the enzyme allophanate hydrolase.
Allophanate esters arise from the condensation of carbamates.