The length characterizing the decrease with distance of the potential in the double layer = characteristic Debye length in the corresponding electrolyte solution = κ-1:

\[\frac{1}{\kappa } = \sqrt{\frac{\varepsilon_{\text{r}}\varepsilon_{0}RT}{F^{2}\sum _{i}c_{i}z_{i}^{2} }}\]

(rationalized four-quantity system)

\[\frac{1}{\kappa } = \sqrt{\frac{\varepsilon_{\text{r}}RT}{4\pi F^{2}\sum _{i}c_{i}z_{i}^{2} }}\]

(three-quantity electrostatic system)

where ɛ = static permittivity = ɛr.ɛ0, ɛr = relative static permittivity of solution; ɛ0 = permittivity of vacuum, R = gas constant, T = thermodynamic temperature, F = Faraday constant, ci = concentration of species i, ci = ionic charge on species i.