  We propose an analytical approximation for the dependence of the effective
charge on the bare charge for spherical and cylindrical macro-ions as a
function of the size of the colloid and salt content, for the situation of a
unique colloid immersed in a sea of electrolyte (where the definition of an
effective charge is non ambiguous).
  Our approach is based on the Poisson-Boltzmann (PB) mean-field theory.
Mathematically speaking, our estimate is asymptotically exact in the limit
$\kappa a\gg 1$, where $a$ is the radius of the colloid and $\kappa$ the
inverse screening length. In practice, a careful comparison with effective
charges parameters obtained by numerically solving the full non-linear PB
theory proves that it is good down to $\kappa a\sim 1$. This is precisely the
limit appropriate to treat colloidal suspensions. A particular emphasis is put
on the range of parameters suitable to describe both single and double strand
DNA molecules under physiological conditions.
