Thermodynamics of the Sodium–Iron–Phosphate–Water System Under Hydrothermal Conditions: The Gibbs Energy of Formation of Sodium Iron(III) Hydroxy Phosphate, Na3Fe(PO4)2·(Na4/3H2/3O), from Solubility Measurements in Equilibrium with Hematite at 498–598 K

Abstract

Sodium iron hydroxy phosphate, Na3FeIII(PO4)2·(Na4/3H2/3O), “SIHP”, is an important iron(III) corrosion product in high-pressure steam generators operating under phosphate pH control. Experimental solubilities of sodium iron hydroxy phosphate, in equilibrium with a large excess of hematite, have been determined in stirred Hastelloy and Zircalloy pressure vessels at temperatures from 498 to 598 K. Equilibrium constants have been determined for the solubility reaction: 3Fe2O3(s) + 26Na+(aq) + 12\( {\text{HPO}}_{4}^{{2 - }} ({\text{aq}})\) + 2OH−(aq) ⇌ 6Na3Fe(PO4)2·(Na4/3H2/3O)(s) + 5H2O. The experimental apparent Gibbs energy of formation of Na3Fe(PO4)2·(Na4/3H2/3O)(s) at 523 K, ∆a G 523 ° (SIHP, s) = –3641.21 ± 5.86 kJ·mol−1, has been combined with previously reported molar heat capacities and the enthalpy of formation at 298.15 K to calculate the Gibbs energy of formation under standard conditions ∆f G 298 ° (SIHP, s) = −3544.77 ± 5.77 kJ·mol−1. A thermodynamic model for the solubility of SIHP over the experimental temperature range is reported.