Raman and infrared spectroscopic investigations of dilute aqueous phosphoric acid solutions

  • Rudolph W
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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 degree]C. The vibrational modes of the PO4 skeleton (C3v symmetry) of H3PO4(aq) and D3PO4(D2O) have been assigned. In addition to the P-O stretching modes a deformation mode has been detected, small 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 small nu]Pdouble bond, length as m-dash]O and small nu]sP(OH)3 mode as a function of temperature. These modes show a pronounced temperature dependence inasmuch as small nu]Pdouble bond, length as m-dash]O shifts to higher wavenumbers with temperature increase and small nu]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, small alpha] for H3PO4(aq) and D3PO4(D2O) as a function of dilution has been measured at 23 degree]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 degree]C. In the temperature interval from 24.5 to 99.7 degree]C the pK1 values for H3PO4(aq) have been determined and thermodynamic data have been derived.

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  • Wolfram W Rudolph

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