1-Carboxymethyl-1′-carboxylatomethyl-3,3′-[p-phenylenebis(oxymethylene)]dipyridinium bromide dihydrate

In the crystal structure of the title salt, C(22)H(21)N(2)O(6) (+)·Br(−)·2H(2)O, pairs of betaine mol­ecules are bridged by protons (the bridging proton is disordered), forming strong and symmetrical O—H⋯O hydrogen bonds, leading to an infinite chain along the b axis. The water mol­ecules are linked to the betaine mol­ecule and the bromide ion through O—H⋯O and O—H⋯Br inter­actions. The central ring, located on an inversion centre, makes dihedral angles of 1.2 (2)° with the outer rings. One of the carboxylic acid groups is deprotonated.