Thermochemical properties for a series of transition metal borates of M[B12O14(OH)10] (MII = Mn, Zn, Fe, Co, Ni)

A pure zinc borate, Zn[B12O14(OH)10], has been synthesized and characterized by XRD, FT-IR, simultaneous TG-DTA techniques, and chemical analysis. The molar enthalpy of solution of Zn[B12O14(OH)10] (s) in 1 mol·dm−3 HCl(aq) was measured by microcalorimeter at T = 298.15 K. The molar enthalpy of solution of ZnO(s) in the mixture solvent of 2.00 cm3 of 1 mol·dm−3 HCl(aq) and calculated amount of H3BO3 was also measured. With the incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1 mol·dm−3 HCl(aq), together with the use of the standard molar enthalpies of formation for ZnO(s), H3BO3(s), and H2O(l), the standard molar enthalpy of formation of −(9646.6 ± 9.6) kJ·mol−1 at T = 298.15 K was obtained on the basis of an appropriate thermochemical cycle. In addition, the molar enthalpy of formation of −9492.7 kJ·mol−1 for [B12O14(OH)10]2− has also been estimated by a group contribution method, which has been used to predict the ΔfHmθ of a series of transition metal borates of M[B12O14(OH)10] (MII = Mn, Zn, Fe, Co, Ni). These estimated data have been used to further compare the stability of this series of transition metal borates.