Raman-Spectroscopic Measurements of the First Dissociation Constant of Aqueous Phosphoric Acid Solution from 5 to 301 A°C

Dilute aqueous phosphoric acid solutions have been studied by Raman spectroscopy at room temperature and over a broad temperature range from 5 to 301 A degrees C. -normalized spectra (Bose-Einstein correction) have been constructed and used for quantitative analysis. The vibrational modes of H3PO4(aq) (pseudo C-3v symmetry) have been assigned. The band with the highest intensity, the symmetric stretch (s){P(OH)(3)}( (1)( (1))) is strongly polarized while (4)(e), the antisymmetric stretch P-as(OH)(3)) is depolarized. The stretching mode of the phosphoryl group (-P=O), (2)(a(1)) occurs at 1178 cm(-1) and is polarized. In the range between 300 and 600 cm(-1), the deformation modes are observed. The deformation mode, {PO-H}, involving the O-H group has been detected at 1250 cm(-1) as a very weak and broad mode. In addition to the modes of phosphoric acid, modes of the dissociation product have been observed. The mode at 1077 cm(-1) has been assigned to (s){PO2}, and the mode at 877 cm(-1) to (s){P(OH)(2)} which is overlapped by (s){P(OH)(3)} of H3PO4(aq). The modes of have been measured in dilute solution and were assigned and presented as well. H3PO4 is hydrated in aqueous solution, which can be verified with Raman spectroscopy by following the modes (2)(a(1)) and (1)(a(1)) as a function of temperature. These modes show a strong temperature dependency. The mode (1)(a(1)) broadens and shifts to lower wavenumbers. The mode (2)(a(1)) on the other hand, shifts to higher wavenumbers and broadens considerably with increases in temperature. At 301 A degrees C the phosphoric acid is almost molecular in nature. In very dilute H3PO4 solutions at room temperature, however, the dissociation product, is the dominant species. In these dilute H3PO4(aq) solutions no spectroscopic features could be detected for a hydrogen bonded dimeric species of the formula (or the neutral dimeric acid H6P2O8). Pyrophosphate formation, although favored at high temperatures, could not be detected in dilute solution even at 301 A degrees C due to the high water activity. In highly concentrated solutions, however, pyrophosphate formation is observable and in hydrate melts the formation of pyrophosphate is already noticeable at room temperature. Quantitative Raman measurements have been carried out to follow the dissociation of H3PO4(aq) over a very broad temperature range. In the temperature interval from 5.0 to 301.0 A degrees C the p (1) values for H3PO4(aq) have been determined and thermodynamic data have been derived.