Synthesis, characterization and determination of standard molar enthalpies of formation of two zinc borates with higher ratio of ZnO/B2O3 by solution calorimetry

Abstract

Two pure phases of zinc borates with high ratio of ZnO/B2O3, Zn8[(BO3)3O2(OH)3] and Zn6O(OH)(BO3)3, have been synthesized and characterized by XRD, FT-IR, TG and chemical analysis. The molar enthalpies of solution of Zn8[(BO3)3O2(OH)3](s) and Zn6O(OH)(BO3)3(s) in 1 mol dm−3 HCl(aq) were measured at 298.15 K by using a heat conduction microcalorimeter, respectively. With the incorporation of the previously determined enthalpies of solution of H3BO3(s) in 1 mol dm−3 HCl(aq) and ZnO(s) in the mixture solvent of [1 mol dm−3 HCl(aq) + H3BO3(aq)], together with the standard molar enthalpies of formation for ZnO(s), H3BO3(s) and H2O(l), the standard molar enthalpies of formation of −(5243.8 ± 3.9) kJ mol−1 for Zn8[(BO3)2O3(OH)3](s) and −(4248.5 ± 3.3) kJ mol−1 for Zn6O(OH)(BO3)3(s) at 298.15 K were obtained on the basis of the appropriate thermochemical cycles.