  Ion mixtures require an exclusion core to avoid collapse. The Debye Hueckel
theory, where ions are point charges, is accurate only in the limit of infinite
dilution. The MSA is the embedding of hard cores into DH, is valid for higher
densities. In the MSA the properties of any ionic mixture can be represented by
a single screening parameter $\Gamma$. For equal ionic size restricted model is
obtained from the Debye parameter $\kappa$. This one parameter representation
(BIMSA) is valid for complex and associating systems, such as the general
n-polyelectrolytes. The BIMSA is the only theory that satisfies the infinite
dilution limit of the DH theory for any chain length. The contact pair
distribution function of hard ions mixture is a functional of $\Gamma$ and a
small mean field parameter. This yields good agreement with the Monte Carlo
(Bresme et al. Phys. Rev. E {\textbf 51} 289 (1995)) .
