In the title type B-2 acid salt, Na(LH)(LH2) . H2O [LH2 = C2H4C(CO2H)(2)] or Na+ . C5H5O4-.C5H6O4 . H2O, the vertices of a distorted octahedron centred on each Na+ cation are defined by six O atoms, one from a water molecule, one from an internally hydrogen-bonded LH- anion and four from three neutral LH2 acid molecules. Chains of edge-sharing O-6 octahedra are stabilized by hydrogen bonds, which interconnect the donor H2O and LH2 molecules and LH. anions. In particular, the LH2 molecule donates H atoms to LH. and H2O and forgoes the internal hydrogen bond which stabilizes the free acid and all of its characterized salts.