Defined by the equation: \[k_{\text{d}}=\frac{I_{\text{l}}}{n\ F\ c\ A}\] where the limiting current Il 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.