Allophanic acid is the organic compound with the formula HNC(O)NHCOH.  It is a carbamic acid, the carboxylated derivative of urea.  Biuret can be viewed as the amide of allophanic acid.  The compound can be prepared by treating urea with sodium bicarbonate:
:HNC(O)NH  +  NaHCO  →  HNC(O)NHCOH  +  NaOH
The anionicconjugate base, HNC(O)NHCO, is called allophanate. 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.