The apparent molal volumes (φV) and apparent molal adiabatic compressibilities [φK(S)] of aqueous solutions of NaH2PO4, Na2HPO4, Na3PO4, KH2PO4, K2HPO4, K3PO4) and H3PO4 in the molality range 0 ≤ m ≤ 1.0 have been determined from precise density and sound speed measurements at 25 °C. The values of φV and φK(S) for Na3PO4 and K3PO4 have been corrected for hydrolysis and the values for H3PO4 have been corrected for ionization. The partial molal volumes and compressibilities have been extrapolated to infinite dilution with the aid of an extended Debye-Hückel equation. Values of V̄° = 47.71 cm3 mol-1 and K°S = -8.6 × 10-4 cm3 mol-1 bar-1 were obtained for H3PO4. Ionic values of V̄° = 36.92, 16.54, and -13.95 cm3 mol-1 and 104K̄° = -55.4, -104.8, and -192.6 cm3 mol-1 bar-1 were obtained, respectively, for H2PO4-, HPO42-, and PO43- ions. The V̄° for the phosphate species decreases by 10.7, 20.4, and 30.5 cm3 mol-1; while the 104K̄° decreases by 46.8, 49.4, and 87.8 cm3 mol-1 bar-1 as the charge is increased from -1 to -3. This decrease in V̄° and K̄° is due to the increase in hydration as the charge is increased (hydration numbers of nH = 5.5, 13.0, and 25.5 have been estimated for H2PO4-, HPO42-, and PO43- ions). The volume and compressibility changes for the first, second, and third ionization of H3PO4 have been determined from the data and used to calculate the effect of pressure on the ionization. The calculated values of KP/K0 for the first ionization are in good agreement with the direct measurements of Disteche and Disteche and Ellis and Anderson.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry