Thermochemistry of rubidium calcium octaborate dodecahydrate

The enthalpies of solution of Rb2Ca[B4O5(OH)4]2·8H2O in approximately 1 mol dm−3 aqueous hydrochloric acid and of RbCl in aqueous (hydrochloric acid+boric acid +calcium oxide) were determined. From these result and the enthalpies of solution of H3BO3 in approximately 1 mol dm−3 HCl (aq), and of CaO in aqueous (hydrochloric acid+boric acid), the standard molar enthalpy of formation of −10,308.06±6.57 kJ mol−1 for Rb2Ca[B4O5(OH)4]2·8H2O was obtained from the standard molar enthalpy of formation of CaO (s), RbCl (s), H3BO3 (s) and H2O (l). The standard molar entropy of formation of Rb2Ca[B4O5(OH)4]2·8H2O was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of Rb2Ca[B4O5(OH)4]2·8H2O computed from a group contribution method.