Thermochemical properties of alkaline-earth metals borates of a series of MB8O11(OH)4·xH2O (M = Ca, Sr, Ba; x = 0, 3)

Highlights: • BaB₈O₁₁(OH)₄·3H₂O has been synthesized and characterized. • Enthalpies of solution of BaB₈O₁₁(OH)₄·3H₂O and Ba(OH)₂·8H₂O were measured. • Δ_fH_m^o for BaB₈O₁₁(OH)₄·3H₂O was obtained by a thermochemical cycle. • Δ_fH_m^o of three MB₈O₁₁(OH)₄ (M = Ca, Sr, Ba) were also estimated by group contribution method. -- Abstract: A pure barium borate of BaB₈O₁₁(OH)₄·3H₂O has been synthesized and characterized by XRD, FT-IR, TGA, and chemical analysis. The molar enthalpies of solution of BaB₈O₁₁(OH)₄·3H₂O in 1 mol dm⁻³ HCl(aq), and Ba(OH)₂·8H₂O in the mixture solvent of {2.00 cm³ of 1 mol·dm⁻³ HCl + H₃BO₃(aq)} were measured by microcalorimeter at T = 298.15 K, respectively. With the use of previously determined enthalpy of solution of H₃BO₃ (s) in 1 mol·dm⁻³ HCl(aq), and the standard molar enthalpies of formation for Ba(OH)₂·8H₂O(s), H₃BO₃(s), and H₂O(l), the standard molar enthalpy of formation of −(7500.6 ± 6.5) kJ·mol⁻¹ at T = 298.15 K was obtained based on an appropriate thermochemical cycle. Moreover, the molar enthalpy of formation of −6091.7 kJ mol⁻¹ for [B₈O₁₁(OH)₄]²⁻ was estimated by a group contribution method, which was used to predict the Δ_fH_m^o of a series of alkaline-earth metal borates for MB₈O₁₁(OH)₄ (M = Ca, Sr, Ba).