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Characterization of Orthophosphate and Orthovanadate in Aqueous Solution Using Polarized Raman Spectroscopy.
1TU Dresden, Dresden, Germany.
Applied Spectroscopy
|September 6, 2024
Summary
This study used polarized Raman spectroscopy to analyze aqueous orthophosphate and orthovanadate solutions. Orthovanadate exhibits significantly more hydrolysis than orthophosphate, with calculated pKa3 values of 13.65 and 12.33, respectively.
Area of Science:
- Inorganic Chemistry
- Spectroscopy
- Solution Chemistry
Background:
- Aqueous solutions of orthophosphate (PO4^3-) and orthovanadate (VO4^3-) are common in various chemical and biological systems.
- Understanding their hydration and hydrolysis behavior is crucial for predicting their reactivity and speciation.
- Polarized Raman spectroscopy offers a powerful tool for characterizing the vibrational modes and interactions of these tetrahedral anions in solution.
Purpose of the Study:
- To characterize the Raman active bands of aqueous orthophosphate and orthovanadate ions.
- To investigate the hydration and hydrolysis of these anions across a wide concentration range.
- To determine the pKa3 values for phosphoric acid (H3PO4) and vanadic acid (H3VO4) in aqueous solution.
Main Methods:
- Polarized Raman spectroscopy was employed to analyze aqueous solutions of sodium orthophosphate and orthovanadate.
- Isotropic scattering profiles and R-normalized spectra were obtained for detailed analysis.
- Quantitative Raman spectroscopy was used to monitor the hydrolysis of PO4^3- and estimate the hydrolysis of VO4^3-.
Main Results:
- Four Raman active bands for both PO4^3- and VO4^3- ions were characterized and assigned.
- PO4^3- exhibited deformation modes at 415 cm^-1 (ν2) and 557 cm^-1 (ν4), and a polarized stretching mode at 936.5 cm^-1 (ν1).
- VO4^3- showed overlapped deformation modes at 327 cm^-1 (ν2) and 345.6 cm^-1 (ν4), and a polarized stretching mode at 820.2 cm^-1 (ν1).
- Both anions displayed significant hydration and hydrolysis; VO4^3- was found to be a much stronger base than PO4^3-.
- The pKa3 value for H3PO4 was determined to be 12.330 ± 0.02, and for H3VO4, it was estimated to be 13.65 ± 0.1.
- The hydrolysis of VO4^3- was approximately 21 times greater than that of PO4^3-.
Conclusions:
- Polarized Raman spectroscopy successfully characterized the vibrational modes of aqueous orthophosphate and orthovanadate.
- The study quantified the significant difference in basicity and hydrolysis between orthovanadate and orthophosphate.
- The determined pKa3 values provide critical thermodynamic data for understanding the speciation of these ions in aqueous environments.
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