Thermodynamics of the Sodium-Iron-Phosphate-Water System Under Hydrothermal Conditions II: The Solubility and Gibbs Energy of Formation of Maricite (NaFe^IIPO₄) from 220 to 300 °C

Abstract

Maricite, NaFe^IIPO₄, is an important iron(II) corrosion product in high-pressure steam generators operating under phosphate pH control. Experimental solubilities of maricite, in equilibrium with a large excess of magnetite and a known partial pressure of hydrogen, have been determined in a stirred Hastelloy C pressure vessel at temperatures from 220 to 300 °C. Equilibrium constants have been determined for the solubility reaction,

${\text{Fe}}_{{3}} {\text{O}}_{{4}} \left( {\text{s}} \right) \, + {\text{ 3 Na}}^{ + } \left( {{\text{aq}}} \right) \, + {\text{ 3 HPO}}_{{4}}^{{{2} - }} \left( {{\text{aq}}} \right) \, + {\text{ H}}_{{2}} \left( {{\text{aq}}} \right) \, \rightleftharpoons {\text{ 3 NaFePO}}_{{4}} \left( {\text{s}} \right) \, + {\text{ 3 OH}}^{ - } \left( {{\text{aq}}} \right) \, + {\text{ H}}_{{2}} {\text{O(1)}}$

The resulting molar Gibbs energy of formation of maricite at 250 °C, has been combined with previously reported molar heat capacities and the enthalpy of formation at 25 °C to calculate the Gibbs energy of formation under standard conditions Δ_fG°₂₉₈(NaFePO₄,s) = −1453.81 ± 2.76 kJ·mol⁻¹. A thermodynamic model for the solubility of maricite over the experimental temperature range is reported. These values are combined with previously reported data for iron (III) compounds to provide a self consistent database for the reactions of magnetite protective films in high pressure steam generators operating under sodium phosphate chemistry control.