Saturation Vapor Pressure of Water over Sodium Borohydride Dihydrate

The relative humidity and saturated water vapor pressure resulted from the reversible equilibrium dissociation of crystalline NaBH₄·2H₂O in the temperature range 0–36°C were determined. Based on the data obtained for NaBH₄·2H₂O, the equilibrium water vapor pressure as a function of temperature ln P = –6567.1T^–1 + 23.863, enthalpy ∆Н₀ = 54 ± 0.8 kJ mol^–1 and entropy ∆S₀ =198 ± 2 J mol^–1 K^–1 of NaBH₄·2H₂O dissociation into NaBH₄ and H₂O, standard enthalpy ∆H°= –783 kJ mol^–1 and entropy ∆S° = 281 J mol^–1 K^–1 of formation of NaBH₄·2H₂O were calculated. An equation was derived for the humidity vs. temperature, which determines the regions of NaBH₄·2H₂O formation or its dissociation Rh = 0.334t + 17.5. It was shown that the pressure of saturated water vapor over a concentrated NaBH₄ solution (30 wt %) at temperatures up to 45°C has higher values compared to NaBH₄·2H₂O samples synthesized by two different methods.