Abstract
Two pure hydrated sodium borates, α-Na2B5O8(OH)·2H2O and β-Na2B5O8(OH)·2H2O, have been synthesized under mild hydrothermal conditions and characterized by X-ray diffraction, infrared spectral, and thermogravimetric analysis and chemical analysis. The molar enthalpies of solution of α-Na2B5O8(OH)·2H2O and β-Na2B5O8(OH)·2H2O in 1 mol·dm-3 HCl(aq) were measured by solution calorimetry to be (20.36 ± 0.33) kJ·mol-1 and (14.27 ± 0.20) kJ·mol-1, respectively. With the incorporation of other thermodynamic data, the standard molar enthalpies of formation were found to be −(4725.1 ± 4.0) kJ·mol-1 for α-Na2B5O8(OH)·2H2O and −(4719.0 ± 4.0) kJ·mol-1 for β-Na2B5O8(OH)·2H2O. The result shows that α-Na2B5O8(OH)·2H2O is more stable than β-Na2B5O8(OH)·2H2O, which is consistent with their thermal behavior.
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