3,3'-(Piperazine-1,4-diium-1,4-di-yl)di-propionate dihydrate.

During the recrystallization of 3-[4-(2-carb-oxy-eth-yl)piperazin-1-yl]propionic acid, the carb-oxy-lic acid H atoms were transferred to the piperazine N atoms, forming the title compound, C(10)H(18)N(2)O(4)·2H(2)O, in which the zwitterion lies about an inversion center. In the crystal, bifurcated N-H⋯(O,O) hydrogen bonds connect the zwitterions into a two-dimensional framework parallel to (-102) forming R(4) (4)(30) rings. O-H⋯O hydrogen bonds involving the solvent water mol-ecules connect the two-dimensional framework into a three-dimensional network. In addition, weak C-H⋯O hydrogen bonds are observed.