Statistical associating fluid theory coupled with the 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 five cations (Li+, Na+, K+, Ca2+, Mg2+), six 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.