Equation for diffusion current $I_{\mathrm{d}}$ at time $t$ at a large planar electrode in contact with a semi-infinite layer of unstirred solution containing excess supporting (inert) electrolyte and an electroactive substance of bulk concentration $c$ when, at the instant $t=0$, the potential of the electrode is suddenly changed to a value at which a mass-transport limited electrochemical reaction occurs and the concentration of the electroactive species at the electrode is zero, $$I_{\mathrm{d}}=zFA{D}^{1/2}c/({\pi }^{1/2}{t}^{1/2})$$ where $z$ is the electron number of an electrochemical reaction in the charge transfer step of the species (being positive for an oxidation and negative for a reduction), $F$ the Faraday constant, $A$ the electrode surface area, and $D$ the diffusion coefficient of the electroactive substance.