Journal of Membrane Science, Vol.98, No.1-2, 13-25, 1995
Theoretical-Analysis of Neutralization Dialysis in the 3-Compartment Membrane Cell
A theoretical model of the kinetics of neutralization dialysis across ideally selective ion exchange membranes is suggested. The model is based on the use of the Nernst-Planck equations describing ion transport through both the membranes and adjacent unstirred layers (ULs) of solution. Special emphasis is made on the kinetics of change in pH of the desalinated solution. The character of this change with time is shown to depend essentially on the solution concentration. At sufficiently high concentration of the solutions in the three compartments, the presence of the ULs can be neglected in the model, and the pH value in the desalination compartment is determined only by the relationship between the intrinsic permeabilities of the membranes. In this case, the model predicts the pH to be in either the acidic or the alkaline range of values during most of the desalination experiment, so that the passage of the pH through neutrality, if it occurs at all, is "leaplike". In contrast, in solutions of sufficiently low concentration,, the exchange of counterions between the compartments is controlled by mass transfer through the ULs. For this limiting case, a rigorous relationship linking the concentrations in the three compartments is derived under which the pH of the desalinated solution assumes the neutral value. Finally, at intermediate concentration of the solutions, the model is found to describe satisfactorily the data of auxiliary experiments with appropriate choice of parameters.