Multivalent-electrolyte screening-length ratio (${\lambda }_{D,2}/{\lambda }_{D,1}=\sqrt{I_1/I_2}$)

For fully dissociated electrolytes at the same temperature and solvent permittivity, linear screening theory gives ${\lambda }_D^{-2}\propto {\sum }_i{c}_i{z}_i^2=2I$. A divalent symmetric salt of formula concentration $c$ has $I=4c$, not $c$ or $2c$. ion association and nonideal concentrated-solution effects limit this comparison.