The zwitterionic structure of 2-hy­droxy-4-[(2-hy­droxy­benzyl­idene)amino]­benzoic acid

The title compound exists in a zwitterionic form. There is a strong intra­molecular N-H...O hydrogen bond, an intra­molecular O-H...O hydrogen bond, an inter­molecular C-H...O contact and [pi]-[pi] stacking inter­actions, resulting in a three-dimensional network.