  The conductivity of concentrated salt solutions has posed a real puzzle for
theories of electrolytes. Despite a quantitative understanding of highly dilute
solutions, an analytical theory for concentrated ones remains a challenge
almost a century, although a number of parameters and effects incorporated into
theories increases with time. Here we show that the conductivity of univalent
salt solutions can be perfectly interpreted using an extremely simple model
that relies just on a mean-field description of electrostatic effects and on an
adapted for salt ions classical approach to calculating colloid
electrophoresis. We derive a compact equation, which predicts that the ratio of
conductivity at a finite concentration to that at an infinite dilution is the
same for all salt, if it is plotted against a product of the harmonic mean of
ion hydrodynamic radii and the square root of concentration. Our equation fits
very well the experimental data for inorganic salts up to 3 mol/l.
