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Structure Determination of Phosphoric Acid and Phosphate Ions in Aqueous Solution Using EXAFS Spectroscopy and Large Angle X-ray Scattering
Department of Molecular Sciences, Swedish University of Agricultural Sciences.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.ORCID iD: 0000-0002-4533-3920
Synchrotron Light Research Institute.
2018 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 122, no 37, p. 7413-7420Article in journal (Refereed) Published
Abstract [en]

The 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 dihydrogenphosphate, on average, hydrogen bind three water molecules. The (P-)O(-H)···Oaq and (P-)O···(H-)Oaq hydrogen bonds in hydrated phos-phoric acid and the H2PO4- ion are shorter than the hydrogen bonds in neat water. This supports previous infrared spectro-scopic studies claiming that the hydrogen bonds in hydrated phosphoric acid and phosphate ions are stronger than the hy-drogen 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 non-proportional 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.

Place, publisher, year, edition, pages
Washington: American Chemical Society (ACS), 2018. Vol. 122, no 37, p. 7413-7420
National Category
Physical Chemistry
Research subject
Chemistry of Interfaces
Identifiers
URN: urn:nbn:se:ltu:diva-70697DOI: 10.1021/acs.jpca.8b05641ISI: 000445711900017PubMedID: 30156411Scopus ID: 2-s2.0-85053767264OAI: oai:DiVA.org:ltu-70697DiVA, id: diva2:1244277
Note

Validerad;2018;Nivå 2;2018-09-26 (inah)

Available from: 2018-08-31 Created: 2018-08-31 Last updated: 2019-04-24Bibliographically approved

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