Magnesium Pyrophosphate Octahydrate (Mg₂P₂O₇·8H₂O): Structure and Properties

We report the discovery and comprehensive characterization of the octahydrate phase of magnesium pyrophosphate, Mg₂P₂O₇·8H₂O, synthesized via aqueous reactive crystallization of magnesium chloride and sodium pyrophosphate in Tris buffer at pH 8. While Mg₂P₂O₇·3.5H₂O and Mg₂P₂O₇·6H₂O readily form under these conditions, the octahydrate appears only rarely. However, once nucleated, it can be seeded to reproducibly promote its formation and suppress other hydrate phases. The structure was elucidated using a single crystal and further characterized by powder X-ray diffraction, vibrational spectroscopy, and thermal analysis focused on dehydration behavior. Microscopy techniques (optical, SEM, TEM, and Raman microspectroscopy) revealed a pure phase exhibiting a radiating bladed crystal habit. Unlike other known hydrates, which form nanosheet-like crystals, the octahydrate phase instead grows as large, multidirectional micron-sized crystals. Its markedly faster growth rate and distinct morphology suggest that it is a promising candidate as a pyrophosphate scavenger in in vitro transcription reactions.