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

Phosphate (PO 4 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 PO 4 3− (aq) (T d symmetry) have been assigned and compared to the calculated values derived from the density functional theory (DFT) method for the unhydrated PO 4 3− (Td) and phosphate–water clusters: PO 4 3− · H 2 O (C 2v ), PO 4 3− · 2H 2 O (D 2d ), PO 4 3− · 4H 2 O (D 2 d), PO 4 3− · 6H 2 O (T d ), and PO 4 3− · 12H 2 O (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, PO 4 3− · 18H 2 O (T), has also been calculated. Agreement between measured and calculated vibrational modes is best in the case of the PO 4 3− · 12H2O cluster and the PO 4 3− · 18H 2 O cluster but far less so in the case of the unhydrated PO 4 3− or phosphate–water cluster with a lower number of water molecules than 12. The asymmetric, broad band shape of ν 1 (a 1 ) PO 4 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 PO 4 3− is strongly hydrated in aqueous solutions. This has been verified by Raman spectroscopy comparing ν 2 (H 2 O), the deformation mode of water, and the stretching modes, the ν 1 OH and ν 3 OH of water, in K 3 PO 4 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 K 3 PO 4 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 T d symmetry to C 3v . In the less concentrated solutions, PO 4 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 PO 4 3− (aq) over a very broad concentration range. From the hydrolysis data, the pK 3 value for H 3 PO 4 has been determined to be 12.45 at 23 °C.