4-(4-Pyrid­yl)pyridinium 3′,4,4′-tricarboxy­biphenyl-3-carboxyl­ate 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 nitro­gen-bound H atom in the 4-(4-pyrid­yl)pyridinium cation and the acidic H atom of the carboxyl­ate 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 mol­ecules are connected by O—H⋯O, C—H⋯O and N—H⋯N hydrogen bonds, forming layers parallel to (2[Image: see text]0). These layer are further connected into a three-dimensional supra­molecular network by O—H⋯O hydrogen bonds involving the water mol­ecules as H-atom donors and by weak π–π stacking inter­actions between neighbouring benzene and pyridine rings, with centroid–centroid distances of 3.756 (5) Å.