Pitzer Equations and a Model-Free Version of the Ion Interaction Approach for the Activity of Individual Ions

•Pitzer equations and the more general Ion Interaction Approach are critically examined.•A Model Free Ion Interaction Approach is developed based on thermodynamic requirements.•Values of the Ion Interaction Function are evaluated for selected 1:1, 1:2 and 2:1 electrolytes in aqueous solutions.•Plots of the Ion Interaction Function dependence on ionic strength are shown for the first time.•The system of equations to back-calculate the activity of the individual ions from a knowledge of the Ion Interaction Function is presented. This work presents values of the Ion Interaction Function (IIF) which makes the activity coefficient of the cation to be different from the activity of the anion of a 1:1 electrolyte in aqueous solution and, in addition to the Debye-Hückel-type contribution, determines the very different dependence on ionic strength of the activity coefficients of the cation and the anion in 1:2, 2:1 electrolytes. The IIF cancels out in the mean ionic and osmotic coefficients due to the condition of electroneutrality. As was foreseen by K. S. Pitzer, the evaluation of the IIF was possible thanks to the availability of the activity coefficients of the individual ions. Values of the IIF, which were previously invisible, are presented and a self-consistent method to back calculate the activity of individual ions is developed.