Thermodynamic properties of two zinc borates: 3ZnO·3B2O3·3.5H2O and 6ZnO·5B2O3·3H2O

Two pure zinc borates with microporous structure 3ZnO·3B 2 O 3 ·3.5H 2 O and 6ZnO·5B 2 O 3 ·3H 2 O have been synthesized and characterized by XRD, FT-IR, TG techniques and chemical analysis. The molar enthalpies of solution of 3ZnO·3B 2 O 3 ·3.5H 2 O(s) and 6ZnO·5B 2 O 3 ·3H 2 O(s) in 1   mol   ·   dm −3 HCl(aq) were measured by microcalorimeter at T   =   298.15   K, respectively. The molar enthalpies of solution of ZnO(s) in the mixture solvent of 2.00   cm 3 of 1   mol   ·   dm −3 HCl(aq) in which 5.30   mg of H 3 BO 3 were added were also measured. With the incorporation of the previously determined enthalpy of solution of H 3 BO 3 (s) in 1   mol   ·   dm −3 HCl(aq), together with the use of the standard molar enthalpies of formation for ZnO(s), H 3 BO 3 (s), and H 2 O(l), the standard molar enthalpies of formation of −(6115.3   ±   5.0)   kJ   ·   mol −1 for 3ZnO·3B 2 O 3 ·3.5H 2 O and −(9606.6   ±   8.5)   kJ   ·   mol −1 for 6ZnO·5B 2 O 3 ·3H 2 O at T   =   298.15   K were obtained on the basis of the appropriate thermochemical cycles .