Raman- and infrared-spectroscopic investigations of dilute aqueous phosphoric acid solutions

Abstract

Phosphoric acid in water and heavy water has been studied by Raman and infrared spectroscopy over a broad concentration range (0.00873–1.560 mol kg⁻¹) at 23 °C. The vibrational modes of the PO₄ skeleton (C_3v symmetry) of H₃PO₄(aq) and D₃PO₄(D₂O) have been assigned. In addition to the P–O stretching modes a deformation mode has been detected, δPO–H(D) at 1250 and 935 cm⁻¹, respectively. In addition to the modes of the phosphoric acid and heavy phosphoric acid a mode of the dissociation product H₂PO₄⁻ and D₂PO₄⁻ has been detected at 1077 cm⁻¹ and 1084 cm⁻¹ respectively. H₃PO₄ and D₃PO₄ is hydrated in aqueous solution which could be verified by Raman spectroscopy following the νPO and ν_sP(OH)₃ mode as a function of temperature. These modes show a pronounced temperature dependence inasmuch as νPO shifts to higher wavenumbers with temperature increase and ν_sP(OH)₃ to lower wavenumbers. In the range between 300–600 cm⁻¹ the deformation modes have been observed. In very dilute H₃PO₄ solutions however, the dissociation product is the dominant species. The dissociation degree, α for H₃PO₄(aq) and D₃PO₄(D₂O) as a function of dilution has been measured at 23 °C. In these dilute H₃PO₄(aq) and D₃PO₄(D₂O) solutions no spectroscopic features for a dimeric species of the formula H₆P₂O₈ and D₆P₂O₈ could be detected. Quantitative Raman measurements have been carried out to follow the dissociation of H₃PO₄ and D₃PO₄ over a very broad concentration range and also as a function of temperature. From the dissociation data, the pK₁ value for H₃PO₄ has been determined to 2.14(1) and for D₃PO₄ to 2.42(1) at 23 °C. In the temperature interval from 24.5 to 99.7 °C the pK₁ values for H₃PO₄(aq) have been determined and thermodynamic data have been derived.