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.