5-Amino-1H-1,2,4-triazol-4-ium-3-carboxyl­ate hemihydrate

The asymmetric unit of the title compound, C(3)H(4)N(4)O(2)·0.5H(2)O, comprises two whole mol­ecules of 5-amino-1H-1,2,4-triazole-3-carb­oxy­lic acid in its zwitterionic form (proton transfer occurs from the carb­oxy­lic acid group to the N hetero­atom at position 1), plus one water mol­ecule of crystallization. The organic moieties are disposed into supra­molecular 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 mol­ecules and the organic mol­ecules lead to the formation of double-deck supra­molecular arrangements which are inter­connected along the a axis via π–π stacking [centroid–centroid distance = 3.507 (3) Å].