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

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 degrees 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, delta PO-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 vP=O and VsP(OH)(3) mode as a function of temperature. These modes show a pronounced temperature dependence inasmuch as vP=O shifts to higher wavenumbers with temperature increase and v(s)P(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, alpha for H3PO4(aq) and D3PO4(D2O) as a function of dilution has been measured at 23 degrees 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 pK(1) value for H3PO4 has been determined to 2.14(1) and for D3PO4 to 2.42(1) at 23 degrees C. In the temperature interval from 24.5 to 99.7 degrees C the pK(1) values for H3PO4(aq) have been determined and thermodynamic data have been derived.