Compressibilities of aqueous electrolyte solutions

Abstract

The Tammann-Tait-Gibson (TTG) model was used to derive and analyse an equation for the isothermal compressibility of aqueous electrolyte solutions as a function of temperature and pressure. The linear equation of Φ_k in c^½ (Φ_k is the apparent molal compressibility, c is in mol 1^-1) is shown to be inadequate. The best function in the square-root of the concentration [either in (mol kg^-1) or c] is degree three. This gives the correct limiting slope predicted by the TTG model, viz., S_v/(B_T + 1), where S, is the Masson slope for apparent molal volumes and B_T is the Tait parameter for pure water. This slope was verified previously by comparison with the limiting slope obtained experi- mentally, and can be predicted from the standard ionic entropies. The difference in the TTG slope and the Debye-Hiickel point-charge slope is proportional to changes in the reorientation motion of water molecules close to the ionic surface.

The electrostriction component, V^c_elect, of the limiting partial molal volume is equal to - K°(B_T+ 1), where K° is the limiting partial molal compressibility. Values of V°_elect calculated from this relationship are compared with values from other models. The TTG model was used to derive internal pressure functions which could be used to analyse deviations of V°_elect from the NIH (non-interacting homomorph) model.

The TTG equations were used to calculate the isothermal compressibilities of 15 electrolytes. The agreement with experimental values is good (deviations are less than 0.1 × 10^-6cm³ g^-1 bar^-1 for βv for the most reliable data). Values of Φ_k at 200 bar were calculated also and are in good agreement with the corresponding experimental values.