Interionic Interactions Interpreted Using Friedman’s Parameters and Their Contribution to the Excess Gibbs Energy of Mixing in Ternary Phosphate Aqueous Solutions at 298.15 K

This study examines interactions in aqueous electrolyte solutions using the equations of the Scatchard and Friedman models. The six mixing parameters of the Scatchard model, bAB(01); bAB(02); bAB(03); bAB(12); bAB(13) and bAB(23), were obtained from the literature and estimated by processing experimental results measured by the isopiestic method for osmotic coefficients of three-component systems: {yKCl + (1 − y)K2HPO4} (aq), {yKBr + (1 − y)K2HPO4} (aq), {yKNO3 + (1 − y)K2HPO4} (aq), {yK2SO4 + (1 − y)K2HPO4} (aq), {yKH2PO4 + (1 − y)K2HPO4} (aq) and {yNaH2PO4 + (1 − y) K2HPO4} (aq) at 298.15 K. The Friedman parameters were calculated from the adopted Scatchard parameters as functions of ionic strength. The effects of pair, triplet, and quadruplet interactions on the excess Gibbs energy of mixing were analyzed, and the total Gibbs energy of the solutions was determined. In the systems {yKCl + (1 − y)K2HPO4} (aq), {yKBr + (1 − y)K2HPO4} (aq), {yKNO3 + (1 − y)K2HPO4} (aq), interactions between different anions of the same charge predominate. Triplet interactions dominate in the system {yK2SO4 + (1 − y)K2HPO4} (aq). The strongest contributions of triplet and quadruplet interactions are observed in the {yKH2PO4 + (1 − y)K2HPO4} (aq) system, whereas pair interactions between the same ion pairs are dominant in the {yNa2HPO4 + (1 − y)K2HPO4} (aq) system.