Characterization of Orthophosphate and Orthovanadate in Aqueous Solution Using Polarized Raman Spectroscopy.

Polarized Raman spectroscopy was used to analyze aqueous solutions of sodium orthophosphate and orthovanadate over a wide concentration range (0.00891-0.702 mol/L) at 23 °C. The isotropic scattering profiles were obtained by measuring polarized Raman scattering spectra. Furthermore, R-normalized spectra were calculated and presented. The tetrahedral ions, VO(aq) and PO(aq), demand four Raman active bands which have been subsequently characterized and assigned. For the PO(aq) ion, the deformation modes ν(e) and ν(f) appear at 415 and 557 cm, and these modes are depolarized. In the P-O stretching region, the strongest Raman band appears at 936.5 cm, which is totally polarized with a depolarization ratio (ρ-value) of 0.002. The broad and depolarized mode at 1010 cm constitutes the antisymmetric stretching band ν(f). The Raman spectrum of VO shows two depolarized deformation modes ν(e) and ν(f) at 327 and 345.6 cm, which are severely overlapped. These bands are very weak. The strongest band in the Raman spectrum of VO(aq) is the symmetric stretching mode ν(a) at 820.2 cm which is totally polarized with a ρ-value at 0.004. The depolarized antisymmetric stretching mode ν(f) appeared at 785 cm as a broad and weak band. Both anions are strongly hydrated and showed extensive hydrolysis in an aqueous solution. Orthovanadate is a much stronger base than orthophosphate in aqueous solution. Therefore, a large amount of NaOH was used to suppress the hydrolysis of VO(aq) sufficiently, so, it was possible to characterize the VO modes. Quantitative Raman spectroscopy was applied to follow the hydrolysis of PO over a wide concentration range from 0.00891 to 0.592 mol/L. The hydrolysis data allowed the calculation of the p value for HPO to be 12.330 ± 0.02 (25 °C). The hydrolysis of the VO ion is ∼21 times larger than that of the PO. The p value for HVO is estimated to be 13.65 ± 0.1 (25 °C).