The effect of pressure on the ionization of water at various temperatures from molal-volume data¹

Abstract

The apparent molal volumes of dilute (0.002 to 1.0m) aqueous HCl and NaOH solutions have been determined at 0, 25, and 50°C and NaCl solutions at 50°C. The partial molal volumes ($\bar V$) of HCl, NaOH, and NaCl solutions have been determined from these apparent molal volumes and other reliable data from the literature. The partial-molal-volume changes ($\Delta \bar V_1$) for the ionization of water, H₂O→H⁺+OH⁻, have been determined from 0 to 50°C and 0 to 1m ionic strength from the partial molal volumes of HCl, NaOH, NaCl, and H₂O. The partial molal compressibilities ($\bar K$ for HCl, NaOH, NaCl, and H₂O have been estimated from data in the literature and used to determine the partial molal compressibility changes ($\Delta \bar K_1$) for the ionization of water from 0 to 50°C and 0 to 1m ionic strength. The effect of pressure on the ionization constant of water has been estimated from partial-molal-volume and compressibility changes using the relation$RTln(K^P /K) = - \Delta \bar V(P - 1) + \tfrac{1}{2}\Delta \bar K(P - 1)^2$ from 0 to 50°C and 0 to 2000 bars. The results agree very well with the directly measured values.