Equilibrium in the Exchange of Hydrogen between Phosphine and Water

Abstract

The exchange equilibrium PH₂D(g)+H₂O(l)=PH₃(g)+HDO(l) has been measured at 25°C by the use of dilute sulfuric acid and buffer solutions as a catalyst. From this measurement and the ratio of the vapor pressures of H₂O and HDO the equilibrium constant for the exchange PH₂D(g)+H₂O(g)=PH₃(g)+HDO(g) is calculated to be 1.52 at 25°C. The latter experimental value is combined with a theoretical computation of the temperature independent factor of the equilibrium constant to evaluate the difference in the zero point energies of PH₃ and PH₂D. This leads to the equation, which is valid up to about 500°K, K_g=0.781e^198/T. This equation is in good agreement with calculations based on the spectra of PH₃ and PD₃ and the rule of the geometric mean.