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

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: ΔfH298.15°=-2815.2±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: ΔdissH°=14.47±0.42 kJ·mol-1 and -6.68±0.27 kJ · mol-1, respectively. Enthalpies of mixing ΔmixH∘ have small positive values and can be fitted by a regular solid solution model with a mixing parameter W=4.26±0.32 kJ·mol-1.