The activity of an ion species is its thermodynamic equivalent concentration; i.e., the ion concentration corrected for the deviation from ideal behavior caused by the interionic attraction of ions. It is this interaction between ions which tends to cause the electrical conductivity of a solution to be less than that predicted from the number of ions present. Only at infinite dilution are the ions completely free of the influence of other ions in solution.
Ionic activity is used in expressing the variation of electrode potentials and other electrochemical phenomena (e.g., SP). At low concentrations, the chemical activity of a solution of a given salt is roughly proportional to its salt content; i.e., roughly proportional to its conductivity or inversely to its resistivity. In concentrated solutions, the relation between conductivity and concentration is complex and depends on individual solute properties.