Coupled fluxes in electrochemistry. Concentration distributions near electrodialysis membranes

Abstract

In electrodialysis the gradient of electrical potential leads to coupling of the flux equations for all the different ionic species. When these ions have the same (numerical) valency, their concentrations close to and within the electrodialysis membrane can be obtained from a closed-form solution, as can the potential. When they do not, a series solution is derived, which requires that all the ionic fluxes be known. If they are not available (e.g. from measurement) they must be guessed at the start of an iterative scheme.

Concentration and potential profiles are discussed for cases with and without “water-splitting”, which is the production of sizeable fluxes of H⁺ and OH^– ions even though the feed solution is effectively neutral. The concept of “limiting current density” is discussed, and is shown to lose much of its usefulness in the presence of water-splitting.