4-(4-Pyrid-yl)pyridinium 3',4,4'-tricarboxy-biphenyl-3-carboxyl-ate dihydrate.

In the title compound, C(10)H(9)N(2) (+)·C(16)H(9)O(8) (-)·2H(2)O, both the cation and anion possess crystallographically imposed centres of symmetry, causing the nitro-gen-bound H atom in the 4-(4-pyrid-yl)pyridinium cation and the acidic H atom of the carboxyl-ate groups at the 3 and 3' positions in the anion to be disordered over two positions with equal occupancies. In the crystal packing, the cations, anions and water mol-ecules are connected by O-H⋯O, C-H⋯O and N-H⋯N hydrogen bonds, forming layers parallel to (20). These layer are further connected into a three-dimensional supra-molecular network by O-H⋯O hydrogen bonds involving the water mol-ecules as H-atom donors and by weak π-π stacking inter-actions between neighbouring benzene and pyridine rings, with centroid-centroid distances of 3.756 (5) Å.