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

Abstract

Dilute aqueous phosphoric acid solutions have been studied by Raman spectroscopy at room temperature and over a broad temperature range from 5 to 301°C. R-normalized spectra (Bose-Einstein correction) have been constructed and used for quantitative analysis. The vibrational modes of H 3 PO 4 (aq) (pseudo C 3v symmetry) have been assigned. The band with the highest intensity, the symmetric stretch ν s {P(OH) 3 }(ν 1 (a 1 )) is strongly polarized while ν 4 (e), the antisymmetric stretch ν as P(OH) 3 ) is depolarized. The stretching mode of the phosphoryl group (-P=O), ν 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, δ{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 H 2 PO − 4 (aq) have been observed. The mode at 1077 cm −1 has been assigned to ν s {PO 2 }, and the mode at 877 cm −1 to ν s {P(OH) 2 } which is overlapped by ν s {P(OH) 3 } of H 3 PO 4 (aq). The modes of H 2 PO − 4 (aq) have been measured in dilute solution and were assigned and presented as well. H 3 PO 4 is hydrated in aqueous solution, which can be verified with Raman spectroscopy by following the modes ν 2 (a 1 ) and ν 1 (a 1 ) as a function of temperature. These modes show a strong temperature dependency. The mode ν 1 (a 1 ) broadens and shifts to lower wavenumbers. The mode ν 2 (a 1 ) on the other hand, shifts to higher wavenumbers and broadens considerably with increases in temperature. At 301°C the phosphoric acid is almost molecular in nature. In very dilute H 3 PO 4 solutions at room temperature, however, the dissociation product, H 2 PO − 4 (aq) is the dominant species. In these dilute H 3 PO 4 (aq) solutions no spectroscopic features could be detected for a hydrogen bonded dimeric species of the formula H 5 P 2 O − 8 (or the neutral dimeric acid H 6 P 2 O 8 ). Pyrophosphate formation, although favored at high temperatures, could not be detected in dilute solution even at 301°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.

Gas diffusion electrode setup for catalyst testing in concentrated phosphoric acid at elevated temperatures

“…The molecule can dissociate into various anions and condensate into polyphosphoric acids at various degrees based on temperature and water concentration. 23,24 The concentrated acid is also hygroscopic and will attract water at low temperatures but will lose water at elevated temperatures. At more extreme temperatures, above 200 • C, the acid will continue to lose water and polymerize.…”

Study of the electro-oxidation of a recreational drug GHB (gamma hydroxybutyric acid) on a platinum catalyst-type electrode through chronoamperometry and spectro-electrochemistry

“…With the exception of the bands corresponding to the adsorbed phosphoric acid at 367 cm -1 [24] and 770 cm -1 (see blank electrolyte spectra in the same conditions), the bands in these spectra correspond to GHB and water adsorbed species being formed/consumed in the potential dependent adsorption processes.…”

Electrodialysis of Phosphates in Industrial-Grade Phosphoric Acid

“…This structure results in a high viscosity fluid, especially at high phosphoric acid concentrations. The dimeric anion (H 5 P 2 O 8 − ) has been found at high acid concentrations, but has not been observed in dilute solutions at room temperature [41,42]. For solutions below 50% phosphoric acid, there are more bonds between the phosphate ions and water than between the phosphate ions themselves [40].…”