Statistical Associating Fluid Theory coupled with Restricted Primitive Model is extended to multivalent ions by relaxing the range of the square-well width parameter, which leads to a new dispersion term approximation and calls for a new set of salt and ion parameters. This new approximation, referred to as SAFT2, requires a single set of parameters derived from the salt (mean ionic) activity coefficients and liquid densities of single-salt solutions for 5 cations (Li⁺, Na⁺, K⁺, Ca⁺², Mg⁺²), 6 anions (Cl^-, Br^-, I^-, NO₃^-, SO₄^-2, HCO₃^-), and 24 salts. These parameters, in turn, are shown to predict the osmotic coefficients for single salt + water solutions.