An Experimental

R factor = 0.070; wR factor = 0.158; data-to-parameter ratio = 13.4. In the structure of the phenolate salt of the sulfa drug sulfamethazine with 3,5-dinitrosalicylic acid, C12H15N4O2S C7H3N2O7 , the dihedral angle between the pyrimidine and benzene rings of the cation is 59.70 (17). In the crystal, cation–anion hydrogen-bonding interactions involving pyrim-idine–carboxy N+—H O and amine–carboxy N—H O pairs give a cyclic R2 2(8) motif while secondary N—H O hydrogen bonds between the aniline group and both sulfone and nitro O-atom acceptors give a two-dimensional structure extending in (001). Related literature For background to sulfamethazine and its co-crystals, see