A thermodynamic study of the palladium–hydrogen system at 245–352 °C and at pressures up to 34 atm

The thermodynamic properties of the solutions of hydrogen in palladium at 245, 297, and 352 °C have been studied by a combined calorimetric–equilibrium method at pressures up to 34 atm. For hydrogen/palladium ratios up to x?0.35, the following equation within experimental precision represents the experimental data at 297 °C, i.e., near the critical temperature: Δ?_H(cal mol⁻¹) =Δ?_H−TΔ?_H =−1750−8250x+12 000x³+T{11.52+R ln[x/(1−x)]}.

Using this expression we obtained the following values of the critical parameters: T_c=567 K; P_c=19.35 atm; x_c =0.241₄; we have also calculated the limits of the (α+β) two‐phase region x_α(max) and x_β(min) at 245 °C to be 0.131 and 0.362, respectively. These calculated values are in good agreement with approximate values obtained directly from discontinuities in the observed partial enthalpies. Calorimetric measurements of the partial enthalpy in absorption and desorption measurements at 245 °C showed little evidence of hysteresis, and yielded Δ?_H=−4.6 kcal mole⁻¹ for the partial heat of absorption. This result is in good agreement with the average of earlier derived data from quasiequilibrium measurements.