5-Amino-1H-1,2,4-triazol-4-ium-3-carboxyl-ate hemihydrate.

The asymmetric unit of the title compound, C(3)H(4)N(4)O(2)·0.5H(2)O, comprises two whole mol-ecules of 5-amino-1H-1,2,4-triazole-3-carb-oxy-lic acid in its zwitterionic form (proton transfer occurs from the carb-oxy-lic acid group to the N hetero-atom at position 1), plus one water mol-ecule of crystallization. The organic moieties are disposed into supra-molecular layers linked by N-H⋯O and N-H⋯N hydrogen bonds parallel to the bc plane. Additional O-H⋯O and N-H⋯O hydrogen bonds involving the water mol-ecules and the organic mol-ecules lead to the formation of double-deck supra-molecular arrangements which are inter-connected along the a axis via π-π stacking [centroid-centroid distance = 3.507 (3) Å].