4-[(2,4-Dihydroxy-benzyl-idene)ammonio]benzene-sulfonate trihydrate

The title Schiff base compound, C(13)H(11)NO(5)S·3H(2)O, formed from sulfanilic acid and 2,4-dihydroxy-benzaldehyde, crystallized out as a zwitterion with the N atom protonated. The asymmetric unit consists of one 4-[(2,4-dihydroxy-benzyl-idene)ammonio]benzene-sulfonate and three water mol-ecules. The zwitterion exists in an E configuration with respect to the central C=N double bond. The two benzene rings of the mol-ecule are oriented at a dihedral angle of 27.33 (8)°. An intra-molecular N-H⋯O hydrogen bond stabilizes the mol-ecular structure. In the crystal, the zwitterions are linked into chains along [101] by inter-molecular O-H⋯O and N-H⋯O hydrogen bonds. The three water mol-ecules link these chains into a three-dimensional framework by additional inter-molecular O-H⋯O hydrogen bonds. A π⋯π inter-action [3.5485 (9) Å] further stabilizes the crystal structure.