Synthesis and thermochemistry of two zinc borates, Zn2B6O11·7H2O and Zn3B10O18·14H2O

Two pure zinc borates, Zn2B6O11·7H2O and Zn3B10O18·14H2O have been synthesized and characterized by XRD, FT-IR, DTA–TG techniques and chemical analysis. The molar enthalpies of solution of Zn2B6O11·7H2O(s) and Zn3B10O18·14H2O(s) in 1 mol dm−3 HCl(aq) were measured to be 15.42 ± 0.03 kJ mol−1 and 57.78 ± 0.11 kJ mol−1, respectively. The molar enthalpy of solution of ZnO(s) in (HCl + H3BO3)(aq) was determined to be −80.01 ± 0.15 kJ mol−1. With incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1 mol dm−3 HCl(aq), and the standard molar enthalpies of formation for ZnO(s), H3BO3(s), and H2O(l), the standard molar enthalpies of formation of −6738.1 ± 4.8 kJ mol−1 for Zn2B6O11·7H2O and −11,786.4 ± 8.0 kJ mol−1 for Zn3B10O18·14H2O were obtained.