3-Carb-oxy-5-(pyridinium-4-yl)benzoate: a redetermination.

The title compound, C(13)H(9)NO(4), crystallizes in a zwitterionic form with the pyridine N atom protonated and the carboxyl OH group deprotonated. The benzene and pyridinium rings are inclined with a dihedral angle of 31.42 (14)° between them. A previous report of this stucture claims, we believe incorrectly, that neither of the carboxyl-ate groups is deprotonated [Zhang et al. (2010 ▶). Acta Cryst. E66, o2928-o2928]. In the crystal, inter-molecular O-H⋯O, N-H⋯O and weak C-H⋯O hydrogen-bonding inter-actions link adjacent mol-ecules into a three-dimensional supra-molecular network.