ABSTRACT

For any electrode reaction at equilibrium current does not flow through the circuit, but there is always a finite exchange of ions and atoms at the interface. For example, for the reaction:

Zn P Zn2+ + 2e-(2.20) some moles of zinc atoms are leaving the surface and entering the electrolyte as zinc ions; at the same time, an equal number of zinc ions from the electrolyte are getting reduced on the electrode surface. Since electron transfer is involved, the rate of exchange can be expressed in terms of current density using Faraday's law:

Toxid = Tra~ = ~~ (2.38) where roxid and rra~ are the equilibrium oxidation and reduction rates, and i0 is the exchange current density. Thus, exchange current density can be defined as the rate of oxidation or reduction at an equilibrium electrode expressed in terms of current density.