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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

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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.

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Acknowledgments

This work was funded by Ontario Power Generation Ltd. (OPG), the Electric Power Research Institute (EPRI) and the Natural Sciences and Engineering Research Council of Canada (NSERC), and Memorial University of Newfoundland. We are grateful to Mr. Chris Finch and Mr. Peter Haring at the Newfoundland Department of Mines and Energies for carrying out the ICP analyses, and to Mr. Randy Thorne of the Memorial University machine shop for much technical support and construction of customized equipment. Helpful discussions and advice form Mr. Jan Stodola (OPG) and Dr. Barry Dooley are gratefully acknowledged. The majority of the experimental results were carried out as part of Sean Quinlan’s MSc Thesis, supported by a Memorial University Graduate Student Scholarship.

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Author notes

  1. Sean Quinlan

    Present address: Food Safety Research, Canadian Food Inspection Agency, 21 Hallett Crescent, St. John’s, NL, A1B 4C4, Canada

  2. Dmitri Chvedov

    Present address: PricewaterhouseCoopers LLP, 111-5th Avenue SW, Suite 3100, Calgary, AB, T2P 5L3, Canada

  3. Liliana N. Trevani

    Present address: Faculty of Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON, L1H 7K4, Canada

  4. Peter R. Tremaine

    Present address: Department of Chemistry, University of Guelph, 50 Stone Road E., Guelph, ON, N1G 2W1, Canada

Authors and Affiliations

  1. Department of Chemistry, Memorial University of Newfoundland, St. John’s, NF, A1B 3X7, Canada

    Sean Quinlan, Dmitri Chvedov, Liliana N. Trevani & Peter R. Tremaine

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  1. Sean Quinlan
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Correspondence to Peter R. Tremaine.

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Quinlan, S., Chvedov, D., Trevani, L.N. et al. 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. J Solution Chem 44, 1121–1140 (2015). https://doi.org/10.1007/s10953-015-0330-4

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