Compressibilities of aqueous electrolyte solutions

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 Φk in c½ (Φk 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., Sv/(BT + 1), where S, is the Masson slope for apparent molal volumes and BT 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, Vcelect, of the limiting partial molal volume is equal to - K°(BT+ 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 Φk at 200 bar were calculated also and are in good agreement with the corresponding experimental values.