Triclinic polymorph of sulfasalazine

In this modification of the title compound, 5‐{4‐[(2‐pyridyl­amino)­sulfonyl]­phenyl­diazenyl}salicylic acid, C18H14N4O5S, the mol­ecule is present in the amide tautomeric form. Two azo‐bridged phenyl rings render the bulk of the mol­ecule planar, with the carboxyl­ic acid group at one terminal and the pyridyl­amino residue at the other. The repeating unit in the crystal is a centrosymmetric dimer containing two identical R(8) hydrogen‐bonded ring systems, each involving the carboxyl­ic acid and pyridyl­amino moieties. Additional stabilization is due to an intramolecular hydrogen bond between the 2‐hydroxyl group and the carbonyl O atom of the carboxyl­ic acid group, as well as intermolecular π–π stacking.