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Raman and Infrared Spectroscopic Investigations on Aqueous Alkali Metal Phosphate Solutions and Density Functional Theory Calculations of Phosphate–Water Clusters

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Abstract:

Phosphate (PO4 3−) solutions in water and heavy water have been studied by Raman and infrared spectroscopy over a broad concentration range (0.0091–5.280 mol/L) including a hydrate melt at 23 °C. In the low wavenumber range, spectra in R-format have been constructed and the R normalization procedure has been briefly discussed. The vibrational modes of the tetrahedral PO4 3−(aq) (Td symmetry) have been assigned and compared to the calculated values derived from the density functional theory (DFT) method for the unhydrated PO4 3−(Td) and phosphate–water clusters: PO4 3−·H2O (C2v), PO4 3−·2H2O (D2d), PO4 3−·4H2O (D2d), PO4 3−·6H2O (Td), and PO4 3−·12H2O (T), a cluster with a complete first hydration sphere of water molecules. A cluster with a second hydration sphere of 12 water molecules and 6 in the first sphere, PO4 3−·18H2O (T), has also been calculated. Agreement between measured and calculated vibrational modes is best in the case of the PO4 3−·12H2O cluster and the PO4 3−·18H2O cluster but far less so in the case of the unhydrated PO4 3− or phosphate–water cluster with a lower number of water molecules than 12. The asymmetric, broad band shape of v 1(a1) PO4 3− in aqueous solutions has been measured as a function of concentration and the asymmetric and broad band shape was explained. However, the same mode in heavy water has only half the full width at half-height compared to the mode in normal water. The PO4 3− is strongly hydrated in aqueous solutions. This has been verified by Raman spectroscopy comparing v 2(H2O), the deformation mode of water, and the stretching modes, the v 1OH and v 3OH of water, in K3PO4 solutions as a function of concentration and comparison with the same modes in pure water. A mode at ∼240 cm−1 (isotropic R spectrum) has been detected and assigned to the restricted translational mode of the strong hydrogen bonds formed between phosphate and water, P–O…HOH. In very concentrated K3PO4 solutions (C 0 ≥ 3.70 mol/L) and in the hydrate melt, formation of contact ion pairs (CIPs) could be detected. The phosphate in the CIPs shows a symmetry lowering of the Td symmetry to C3v. In the less concentrated solutions, PO4 3−(aq) solvent separated ion pairs and doubly solvent separated ion pairs exist, while in very dilute solutions fully hydrated ions are present (C 0 ≤ 0.005 mol/L). Quantitative Raman measurements have been carried out to follow the hydrolysis of PO4 3−(aq) over a very broad concentration range. From the hydrolysis data, the pK3 value for H3PO4 has been determined to be 12.45 at 23 °C.

Keywords: AQUEOUS PHOSPHATE SOLUTIONS; DENSITY FUNCTIONAL THEORY; DFT; INFRARED SPECTROSCOPY; PHOSPHATE HYDRATION; PHOSPHATE WATER CLUSTERS; PHOSPHORIC ACID; RAMAN SPECTROSCOPY; SOLUTION CHEMISTRY

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/000370207783292037

Affiliations: 1: Institut für Virologie im MTZ, TU Dresden, Fetscherstr. 74, 01307 Dresden, Germany 2: Institut für theoretische Physik, TU Bergakademie Freiberg, Leipziger Str. 23, 09596 Freiberg, Germany

Publication date: December 1, 2007

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