The non-primitive model of perturbation theory with the mixture of charged hard spheres as reference system is used to correlate the osmotic coefficients of twelve 1:1 electrolyte solutions. The various interaction energies between particles (dispersion, electrostatic, dipole, quadrupole and induced dipole) are considered. The relative dielectric constant is put into unity. The parameters of seven ions (Na^+, K^+, Rb^+, Cs^+, Cl^-, Br^-, I^-) are obtained. The total average absolute deviation between calculated and experimental values of osmotic coefficients is 0.041. The ionic parameters can keep contant in different systems. No mixing parameters are introduced in the calculations.

Key words: ELECTROLYTE, PERTURBATION THEORY, OSMOTIC COEFFICIENT