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Updated: Mar 19, 2026
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X-ray Powder Diffraction in Conservation Science: Towards Routine Crystal Structure Determination of Corrosion Products on Heritage Art Objects
Published on: June 8, 2016
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A second polymorph of sodium di-hydrogen citrate, NaH2C6H5O7: structure solution from powder diffraction data and DFT
Alagappa Rammohan1, James A Kaduk2
1Atlantic International University, Honolulu, HI, USA.
Summary
A new crystal structure for sodium di-hydrogen citrate was identified, differing from the known polymorph. This discovery reveals distinct sodium-coordination and carboxylate deprotonation patterns in the novel sodium citrate form.
Area of Science:
- Crystallography
- Materials Science
- Solid-State Chemistry
Background:
- Sodium di-hydrogen citrate exists in multiple crystalline forms (polymorphs).
- The previously known crystal structure (NAHCIT) was characterized by Glusker et al. in 1965.
- Commercial samples may contain polymorphs not matching established structures.
Purpose of the Study:
- To solve and refine the crystal structure of a second polymorph of sodium di-hydrogen citrate.
- To compare the structural features of this new polymorph with the known NAHCIT structure.
- To investigate the coordination environment of sodium ions and the deprotonation state of carboxyl groups.
Main Methods:
- X-ray powder diffraction (XRPD) for data collection.
- Laboratory X-ray powder diffraction.
- Density functional theory (DFT) for structural optimization.
Main Results:
- A second polymorph of sodium di-hydrogen citrate (Na(+)·H2C6H5O7 (-)) was identified and characterized.
- The new polymorph exhibits [NaO7] coordination polyhedra forming edge-sharing chains along the a axis.
- This differs from NAHCIT, where [NaO6] octahedra form edge-sharing pairs bridged by hydroxyl groups.
- A key difference is the deprotonation of a terminal carboxyl group in the new polymorph, versus the central carboxylate group in NAHCIT.
Conclusions:
- The existence of a second polymorph of sodium di-hydrogen citrate is confirmed.
- Structural analysis reveals distinct sodium coordination and carboxylate deprotonation compared to the known NAHCIT polymorph.
- This finding highlights the structural diversity possible within sodium citrate crystalline forms.
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