A simple formula is proposed to describe the pressure dependence of the variable Φ in El?yanov and Gonikberg's linear free energy relationship for ionization reactions in solution at high pressure. ��� The expression, given in equations (10) and (12), provides a good description of the influence of pressure on ionization equilibria in aqueous solutions. It permits El'yanov's general linear relationships between Φ and ionization free energies, enthalpies and entropies, pH and Hammett's p parameter, to be expressed in terms of the pressure in convenient analytical forms. ��� The formula is shown to be consistent with the simple electrostatic theory of ion hydration, allowing for the effect of pressure on the dielectric constant of water. Combined with the theory, it provides a general means of predicting ionization constants over a wide range of pressures and temperatures simply from knowledge of the changes in molar volume, enthalpy and entropy which accompany the reactions at atmospheric pressure.

A study has been made of the applicability of a linear free energy relationship describing the effect of pressure on the equilibrium and rate constants of chemical reactions. Some properties of the relationship have been analysed and corollaries, including linear dependences, obtained which make it possible to calculate reaction enthalpies and entropies, volume changes and Hammett's p-constants at high pressures. Ionization reactions have been used as examples to illustrate the conclusions arrived at. The linear free energy relationship has been shown to describe the effect of pressure up to 8000 bars on the ionization constants of acids and on the pH of buffer solutions.