Debye – Hückel and Onsager Equation

At infinite dilution, ions in an electrolytic solution are so far apart that the electrostatic interaction between them becomes negligible. At any finite, non-zero concentration, however, that interaction is not negligible: each ion of a strong (fully-dissociated) electrolyte is effectively surrounded by an 'ionic atmosphere' of ions of the opposite charge, and this atmosphere exerts a retarding drag on the central ion's motion, lowering the measured conductivity below what it would be for completely free, non-interacting ions.

Debye and Hückel modelled this ion-ion interaction quantitatively for strong electrolytes, assuming complete dissociation, and their treatment was later refined by Onsager into the form used today. For a uni-univalent (1:1) electrolyte, the Debye–Hückel–Onsager equation is written as

where A and B are constants that depend only on the nature of the solvent and the temperature — not on the identity of the electrolyte dissolved in it. Explicitly,

$A = \dfrac{82.4}{(DT)^{1/2}\,\eta} \qquad\qquad B = \dfrac{8.20\times10^{5}}{(DT)^{3/2}}$ …