Concentration Profiles and Reaction Fronts in A + B -> C Type Processes: Effect of Background Ions

Diffusion and reaction of initially separated ions A- and B+ in the presence of counter ions A'+ and B'- is studied. The dynamics is described in terms of reaction-diffusion equations obeying local electroneutrality, and the time-evolution of ion-concentrations is determined. We find that, in the absence of reactions, unequal mobility of ions generate nontrivial features in the macroscopically observable concentration profiles. Switching on the reaction (A-) + (B+) -> C leads to the formation of a localized, diffusive reaction front and one finds that the properties of the front (e.g. effective diffusion constant) are affected by the background ions. The consequences of this effect on the formation of Liesegang patterns is discussed.