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

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.