9-Amino-acridin-10-ium 4-amino-benzo-ate dihydrate.

The asymmetric unit of the title hydrated salt, C13H11N2 (+)·C7H6NO2 (-)·2H2O, consists of two independent 9-amino-acridinium cations, two 4-amino-benzoate anions and four water mol-ecules. Both 9-amino-acridinium cations are essentially planar, with maximum deviations of 0.034 (1) and 0.025 (2) Å, and are protonated at the pyridine N atoms. The 4-amino-benzoate anions are approximately planar, with dihedral angles of 9.16 (19) and 5.4 (2)° between the benzene ring and the carboxyl-ate group. In the crystal, the two independent anions are connected by N-H⋯O hydrogen bonds, forming a layer parallel to (100). The layers are connected through the cations by N-H⋯N and N-H⋯O hydrogen bonds. The water mol-ecules, which form O-H⋯O hydrogen-bonded chains along the b-axis direction, connect the anions and the cations by O-H⋯O, N-H⋯O and C-H⋯O hydrogen bonds. The crystal structure also features π-π inter-actions [centroid-centroid distances = 3.6343 (9)-3.8366 (10) Å] and a C-H⋯π inter-action.