The enthalpies of dilution of aqueous solutions of HCl, H3PO4, NaOH, NaH2PO4, Na2HPO4 and Na3PO4 in the molality range 0.1 to 1.0 mole-kg-1 have been determined at 30°C. The relative apparent molal enthalpies φL of HCl, NaOH, NaH2PO4 and Na2HPO4 have been determined with the aid of an extended form of the Debye-Hückel limiting law. The relative apparent molal enthalpies for Na3PO4 solutions have been corrected for hydrolysis. A value of ΔHHo=9525±150 cal-mole-1 was determined for the heat of hydrolysis of PO4-3. This value gives ΔH3o=3815±150 cal-mole-1 for the ionization of H2PO4-, which is in good agreement with the value of ΔH3o=3500±500 cal-mole-1 determined directly by Pitzer at 25°C. The relative apparent molal enthalpies for H3PO4 solutions have been corrected for ionization. A value of ΔH1o=-1900±150 cal-mole-1 was obtained for the heat of ionization of H3PO4 to H++H2PO4-. This value is in good agreement with the value of ΔH1o=-2031 cal-mole-1 at 30°C determined by Harned and Owen from the temperature coefficient of the equilibrium constant and ΔH1o=-1950±80 cal-mole-1 at 25°C determined from calorimetry by Pitzer.
- Ionization of phosphoric acid
- enthalpy of dilution
- phosphate solutions
ASJC Scopus subject areas
- Molecular Biology
- Physical and Theoretical Chemistry