Phase relations for reactions of hydrogen with sodium oxide between 500 and 900°C☆

The equilibrium pressure technique, commonly used for investigating the phase relations in binary gas-condensed phase equilibria, was applied to the Na-Na2O-NaOH-NaH corner of the ternary Na-O-H system. Measured amounts of hydrogen were reacted with Na2O (sealed in vacuo in a thin-walled nickel crucible) and the equilibrium hydrogen pressure (0.03 < P < 100 kPa) was determined as a function of temperature(510 < T < 879°C) and hydrogen atom fraction(0 < XH < 0.275). Discontinuities encountered in the isothermal(√P vs XH) or fixed-hydrogen-concentration (InP vs 1/T) plots were used to delineate the condensed-phase boundaries that occur within theP-T-XH ranges studied. Data were fitted to analytical equations which permitted construction of a portion of the phase diagram and yielded pertinent thermodynamic information. Limitations of the technique in applications to other similar systems are discussed.