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

Two pure zinc borates, Zn2B6O11 center dot 7H(2)O and Zn3B10O18 center dot 14H(2)O have been synthesized and characterized by XRD, FT-IT, DTA-TG techniques and chemical analysis. The molar enthalpies of Solution of Zn2B6O11 center dot 7H(2)O(s) and Zn3B10O18 center dot 14H(2)O(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(1), the standard molar enthalpies of formation of -6738.1 +/- 4.8 kJ mol(-1) for Zn2B6O11 center dot 7H(2)O and -11,786.4 +/- 8.0 kJ mol(-1) for Zn3B10O18 center dot 14H(2)O were obtained. (C) 2008 Elsevier B.V. All rights reserved.