A high-pressure conductivity and laser Raman spectroscopic study of aqueous orthophosphate solutions at 25 °C

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Abstract
Conductivities of aqueous solutions of H3PO4, KH2PO4, 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 H3PO4 were ΔV1=21±4 cm3 mol−1 (conductivity), −18±3 (Raman, 0.1 M H3PO4), −8±2 (Raman, 0.5 M H3PO4), and ΔV3=−36±3 (Raman, 0.1 M Na3PO4). Both Raman and conductivity studies of H2PO4− 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.