The pair distributions and the osmotic coefficient in binary electrolytes up to concentrations c ≅ 1 mol/l

Referring to the results of a previous paper dealing with the pair distributions in a fully ionized dilute hydrogen plasma it is concluded that the pair distributions in binary electrolytes up to concentrations c ≅ 1 mo1/1 take the form n(r) = exp[-β(Ṽ(r) + V(hs)(r))]. Here Ṽ(r) represents the screened Coulomb potential and V(hs)(r) a hard sphere potential. The contact distances R of the hard sphere potentials are assumed to be equal for ions with the same sign of their charges, that means R++ = R-- = R, and to be smaller than R for ions of opposite charges, that means R+- = qR, 0 < q < 1. With the help of these pair distributions the pressure of the ionic components and the osmotic coefficient are calculated. The numerical evaluation of the osmotic coefficient of hydrous electrolytes at T = 273 K shows that the above pair distributions explain the experimental data of all kinds of 1-1-valent electrolytes, even those of the tetra-alkylammonium halogenids, within a domain of very reasonable values of R and q. No necessity seems to exist for considering additional short range attractive forces. Furthermore, some numerical results for 2-2-valent electrolytes in water are given. It comes out that the osmotic coefficient of all 2-2-valent electrolytes with R ≲ 5 Å is larger than the limiting law in a certain domain of concentration.