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Structure Determination of Phosphoric Acid and Phosphate Ions in Aqueous Solution Using EXAFS Spectroscopy and Large Angle X‑ray Scattering
journal contribution
posted on 2018-08-29, 00:00 authored by Ingmar Persson, Mylène Trublet, Wantana KlysubunThe
structures of hydrated phosphoric acid and phosphate ions (H2PO4–, HPO42–, and PO43–) in aqueous solution have
been determined by P K-edge EXAFS and large angle X-ray scattering
(LAXS). The P–O bond distance in all phosphate species studied
is close to 1.53 Å. The P–(O)···Oaq distances have been refined to ca. 3.6 Å from the LAXS data
giving a P–O···Oaq bond angle close
to tetrahedral, suggesting that each oxygen or OH group of phosphoric
acid and dihydrogen phosphate, on average, hydrogen bind three water
molecules. The (P−)O(−H)···Oaq and (P−)O···(H−)Oaq hydrogen
bonds in hydrated phosphoric acid and the H2PO4– ion are shorter than the hydrogen bonds in neat
water. This supports previous infrared spectroscopic studies claiming
that the hydrogen bonds in hydrated phosphoric acid and phosphate
ions are stronger than the hydrogen bonds in neat water. Phosphoric
acid and phosphate ions can therefore be regarded as structure making
solutes. This is the first study applying transmission mode X-ray
absorption spectroscopy (XAS) data collection on the P K-edge. It
shows that XAS spectra collected in transmission mode have a much
better S/N ratio than data collected in fluorescence mode, allowing
accurate determination of P–O bond distances. Furthermore,
P K-edge EXAFS data collected in fluorescence mode display a higher
amplitude at high k than expected due to increasing
radiated volume of the sample with increasing energy as the total
absorption decreases sharply with increasing energy of the X-rays.
As a result, the fluorescence signal becomes nonproportional to the
intensity of the X-ray beam over the EXAFS spectrum. This results
in an increasing amplitude of the EXAFS function with increasing energy
of the X-ray beam resulting in too small Debye–Waller coefficients.