4-Nitro-anilinium 3-carb-oxy-4-hy-droxy-benzene-sulfonate monohydrate.

In the title hydrated salt, C6H7N2O2 (+)·C7H5O6S(-)·H2O, the benzene ring of the cation makes a dihedral angle of 1.32 (19)° with the attached nitro group. In the anion, an intra-molecular O-H⋯O hydrogen bond with an S(6) ring motif is formed between the carb-oxyl and hy-droxy groups; the dihedral angle between the carb-oxyl group and the benzene ring is 8.76 (8)°. The crystal structure exhibits inter-molecular N-H⋯O, O-H⋯O, C-H⋯O, and π-π [centroid-centroid distances = 3.6634 (9) and 3.7426 (9) Å] inter-actions to form a three-dimensional network.