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 4 3– (aq) and PO 4 3– (aq), demand four Raman active bands which have been subsequently characterized and assigned. For the PO 4 3– (aq) ion, the deformation modes ν 2 (e) and ν 4 (f 2 ) appear at 415 and 557 cm –1 , and these modes are depolarized. In the P–O stretching region, the strongest Raman band appears at 936.5 cm –1 , which is totally polarized with a depolarization ratio (ρ-value) of 0.002. The broad and depolarized mode at 1010 cm –1 constitutes the antisymmetric stretching band ν 3 (f 2 ). The Raman spectrum of VO 4 3– shows two depolarized deformation modes ν 2 (e) and ν 4 (f 2 ) at 327 and 345.6 cm –1 , which are severely overlapped. These bands are very weak. The strongest band in the Raman spectrum of VO 4 3– (aq) is the symmetric stretching mode ν 1 (a 1 ) at 820.2 cm –1 which is totally polarized with a ρ-value at 0.004. The depolarized antisymmetric stretching mode ν 3 (f 2 ) appeared at 785 cm –1 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 4 3– (aq) sufficiently, so, it was possible to characterize the VO 4 3– modes. Quantitative Raman spectroscopy was applied to follow the hydrolysis of PO 4 3– over a wide concentration range from 0.00891 to 0.592 mol/L. The hydrolysis data allowed the calculation of the p K a3 value for H 3 PO 4 to be 12.330 ± 0.02 (25 °C). The hydrolysis of the VO 4 3– ion is ∼21 times larger than that of the PO 4 3– . The p K a3 value for H 3 VO 4 is estimated to be 13.65 ± 0.1 (25 °C).