Defined by the equation: $$k_{\text{d}}=\frac{I_{\text{l}}}{nFcA}$$ where the limiting current $I_{\text{l}}$ 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.