HIGH-PRESSURE CONDUCTIVITY AND LASER RAMAN-SPECTROSCOPIC STUDY OF AQUEOUS ORTHO-PHOSPHATE SOLUTIONS AT 25-DEGREES-C

Conductivities of aqueous solutions of H3PO4, KH 2PO4, and Na3PO4 (0.001-1 M), and Raman spectra of 0.1, 0.5, 10, and 15.5 M H3PO4, 1 M KH2PO4, and 0.1M Na3PO4, have been obtained at 25°C, and pressures up to 200 MPa. Pressure enhanced the first dissociation of H3PO4 (κp/κ⊥, measured by conductivity vs [H3PO4] goes through a maximum), and reduced the extent of hydrolysis of Na3PO4. Volume changes for the first and third dissociations of H3PO 4 were ΔV1 = -21 ± 4 cm3 mol -1 (conductivity), - 18 ± 3 (Raman, 0.1 M H3PO 4), - 8 ± 2 (Raman, 0.5 M H3PO4), and ΔV3 = - 36 ± 3 (Raman, 0.1 M Na3PO 4). Both Raman and conductivity studies of H2PO 4- were consistent with the presence of hydrogen-bonded phosphate dimers, H4P2O82-, with a formation constant of 2-3 l mol-1 (conductivity), and ΔV≈0 cm3 mol-1 (conductivity). This ΔV value agreed with the Raman study which indicated only a very small decrease in dimerization with increasing pressure. © 1979 American Institute of Physics.