Conductivity of concentrated salt solutions

The conductivity of concentrated salt solutions has posed a real puzzle for theories of electrolytes. Despite a quantitative understanding of dilute solutions, an analytical theory for concentrated ones has remained a challenge for almost a century, although a number of parameters and effects incorporated into theories increase with time. Here, we show that the conductivity of univalent salt solutions can be perfectly interpreted using a simple model that relies on a modified mean-field description of electrostatic interactions and on a classical approach to calculating colloid electrophoresis. We derive a compact equation, which predicts that the ratio of conductivity to that at an infinite dilution is the same for all salts and depends only on the product of the harmonic mean of ion hydrodynamic radii and the square root of concentration. Our equation fits very well the data for inorganic salts (up to a few mol/l), although at a very high dilution, the relaxation correction seems necessary.