Crystal structure of aqua­(citric acid)(hydrogen citrato)calcium monohydrate, [Ca(HC(6)H(5)O(7))(H(3)C(6)H(5)O(7))(H(2)O)]·H(2)O, from synchrotron X-ray powder data, and DFT-optimized crystal structure of existing calcium hydrogen citrate trihydrate, [Ca(HC(6)H(5)O(7))(H(2)O)(3)]

The crystal structure of ‘aquabis­(di­hydrogen citrato)calcium hydrate’, better formulated as aqua­(citric acid)(hydrogen citrato)calcium monohydrate, (I), has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. The CaO(8) coordination polyhedra are isolated, but occur in layers parallel to the ab plane. Both the Rietveld-refined and DFT-optimized structures indicate that one citrate is doubly ionized and that the other is citric acid. All of the active hydrogen atoms participate in strong (11–16 kcal mol(−1)) hydrogen bonds. Hydrogen atoms were added to the existing calcium hydrogen citrate trihydrate structure [Sheldrick (1974 ▸). Acta Cryst. B30, 2056–2057; CSD refcode: CAHCIT], (II), and a DFT calculation was carried out to assess the hydrogen bonding and compare it to this optimized structure.