Enthalpy of mixing of solid solution rhomboclase – Indium-substituted rhomboclase (H3O) Fe1−xInx(SO4)2·nH2O

Abstract

Solid solution of rhomboclase [nominally (H₃O) Fe(SO₄)₂·3H₂O] with Fe-In substitution was synthesized by coprecipitation and hydrothermal treatment and investigated by chemical, X-ray diffraction, and thermodynamic analysis in this study. The solid solution is continuous over the whole stoichiometric range despite significant differences in ionic radii between Fe³⁺ and In³⁺. Enthalpy of formation of rhomboclase and enthalpies of mixing of solid solution rhomboclase – indium-substituted rhomboclase were determined by acid-solution calorimetry in 5 mol$·$dm⁻¹ HCl at T = 298.15 K and p = 1 bar. The actual compositions of the studied synthetic end members are (H₃O)_0.798Fe(SO₄)_1.899·3.078H₂O (rhomboclase with molecular mass of 308.8719 g$·$mol⁻¹) and (H₃O)_1.067In(SO₄)_2.034·3.017H₂O (indium-substituted rhomboclase with molecular mass of 384.8377 g$·$mol⁻¹). The enthalpies of mixing were obtained for a series of four indium-incorporating rhomboclase synthetic samples with molar In/(In + Fe) ratios of 0.115, 0.226, 0.537, and 0.733. The enthalpy of formation from elements in their standard state at T = 298.15 K and p = 1 bar obtained in this study by thermochemical cycle for (H₃O)_0.798Fe(SO₄)_1.899·3.078H₂O is: ${\Delta }_f{H}_{298.15}°=-2815.2\pm 2.8$ kJ$·$mol⁻¹. Mean measured enthalpies of dissolution for (H₃O)_0.798Fe(SO₄)_1.899·3.078H₂O and (H₃O)_1.067In(SO₄)_2.034·3.017H₂O are: ${\Delta }_{\mathrm{diss}}H°=14.47\pm 0.42$ kJ$·$mol⁻¹ and $-6.68\pm 0.27$ kJ $·$ mol⁻¹, respectively. Enthalpies of mixing${\Delta }_{\mathrm{mix}}H°$ have small positive values and can be fitted by a regular solid solution model with a mixing parameter $W=4.26\pm 0.32$ kJ$·$mol⁻¹.