Limiting Diffusion Currents at Wire Electrodes

Abstract

THE limiting current for an electrode reaction in aqueous solution, in which diffusion of the depolarizer to the electrode surface is the rate-determining factor, is generally treated¹ by assuming there to be present adjacent to the surface a diffusion layer of fixed thickness across which a linear concentration gradient is set up. On these assumptions, the limiting current is given by the expression : where I is the limiting current (amp.), A the area of the electrode surface (cm.²), D the diffusion coefficient of the depolarizing ion (cm.² sec.^-1), n the number of electrons involved in the electrode reaction, F the faraday, C the bulk concentration of the depolarizer (gm. ion per litre), and δ the thickness of the diffusion layer (cm.). This expression is found to hold satisfactorily for large electrodes, and δ has a substantially constant value for different depolarizers in unstirred solutions at constant temperature. The same relation has generally been considered to hold for wire electrodes, and since such electrodes are very convenient in practice and give well-defined limiting currents, they have been much used in voltammetric work.