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

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 ΔfHmo of a series of alkaline-earth metal borates for MB8O11(OH)4 (M = Ca, Sr, Ba).