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
作者:Sean Quinlan、Dmitri Chvedov、Liliana N. Trevani、Peter R. Tremaine
DOI:10.1007/s10953-015-0330-4
日期:2015.5
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 $$ \textHPO}}_4}^2 - }} (\textaq}})$$ + 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.
羟基磷酸钠铁Na3FeIII(PO4)2·(Na4/3H2/3O),“SIHP”,是高压蒸汽发生器在磷酸盐pH控制下的一种重要的三价铁腐蚀产物。在498至598 K的温度下,通过在搅拌的哈氏合金和锆合金压力容器中与过量赤铁矿平衡,测定了羟基磷酸钠铁的实验溶解度。确定了溶解反应的平衡常数:3Fe2O3(s) + 26Na+(aq) + 12$$\textHPO}}_4}^2 - }} (\textaq}})$$ + 2OH−(aq) ⇌ 6Na3Fe(PO4)2·(Na4/3H2/3O)(s) + 5H2O。将523 K时Na3Fe(PO4)2·(Na4/3H2/3O)(s)的实验表观吉布斯自由能生成值,∆a G 523 ° (SIHP, s) = –3641.21 ± 5.86 kJ·mol−1,与先前报道的摩尔热容和298.15 K时的生成焓相结合,计算了标准条件下的吉布斯自由能生成值:∆f G 298 ° (SIHP, s) = −3544.77 ± 5.77 kJ·mol−1。报道了SIHP溶解度在实验温度范围内的热力学模型。