Summary
Donnan dialysis utilizes counter diffusion of two or more ions through an ion exchange
membrane to achieve a separation. For example, if a cation exchange membrane
separates two solution, one containing CuSO4 and the other containing H2SO4, the H+
and Cu2+ ions will diffuse in opposite directions across the membrane until an
equilibrium is reached. Since the cation exchange membrane is impermeable to SO4
2-
ions, equivalent amounts of H+ and Cu2+ ions must be transferred through the membrane
to maintain electroneutrality. The process can be described as a continuous ion
exchange process where the regenerating solution is constantly supplying H+ ions to
exchange for the Cu2+ ions so that they can be removed continuously from their original
solution. Differences in the volumes and concentrations of the two solutions can be
exploited to achieve some interesting and useful separations. Donnan dialysis can be
used for water softening, pollution control, and even deionization of a process stream.
The deionization process, called neutralization dialysis, combines Donnan dialysis
through both cation exchange and anion exchange membranes with H+ and OH- ions
exchanging for the cation and anion of a salt.