Evaluation of the Concentration Changes near the Electrodes in Electrolysis with Natural Convection

Abstract

In electrolytic processes the composition of the solution is not the same in the bulk of the cell and at the electrode-electrolyte interface. The concentration changes near the electrodes are of importance in many applications of electrochemistry. The mass transport to or from the electrode determines the concentration differences which build up at a given current density. The methods for the evaluation of interfacial concentrations are discussed for solutions containing one or several electrolytes. The case of steady state natural convection at plane vertical electrodes is considered in detail. The problem is treated for the boundary condition of a uniform current density distribution over the electrode surface. Von Kármán's approximation is used to calculate the interfacial concentrations of H⁺- ions and Cu²⁺-ions in the electrolysis of a solution of CuSO₄ + H₂SO₄.The results of the computation are in good agreement with experimental data taken from the literature.
For the case of a great excess of indifferent electrolyte the results of the theoretical derivation are presented in generalized form.