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

Phosphate (PO43−) 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 PO43−(aq) (Td symmetry) have been assigned and compared to the calculated values derived from the density functional theory (DFT) method for the unhydrated PO43− (Td) and phosphate–water clusters: PO43− · H2O (C2v), PO43− · 2H2O (D2d), PO43− · 4H2O (D2d), PO43− · 6H2O (Td), and PO43− · 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, PO43− · 18H2O (T), has also been calculated. Agreement between measured and calculated vibrational modes is best in the case of the PO43− · 12H2O cluster and the PO43− · 18H2O cluster but far less so in the case of the unhydrated PO43− or phosphate–water cluster with a lower number of water molecules than 12. The asymmetric, broad band shape of ν1(a1) PO43− 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 PO43− is strongly hydrated in aqueous solutions. This has been verified by Raman spectroscopy comparing ν2(H2O), the deformation mode of water, and the stretching modes, the ν1OH and ν3OH 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 (C0 ≥ 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, PO43−(aq) solvent separated ion pairs and doubly solvent separated ion pairs exist, while in very dilute solutions fully hydrated ions are present (C0 ≤ 0.005 mol/L). Quantitative Raman measurements have been carried out to follow the hydrolysis of PO43−(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.