Defined by the equation: \[k_{\rm{d}}=\frac{I_{\rm{l}}}{n\ F\ c\ A}\] where the limiting current \(I_{\rm{l}}\) is assumed to be due to the diffusion species of concentration \(c\) and of diffusion coefficient \(D\). \(n\) is the charge number of the cell reaction written so that the stoichiometric coefficient of this species is unity. \(A\) is usually taken as the geometric area of the electrode, and \(F\) is the Faraday constant.