In a simplified way, mass transport occurs from concentrated to diluted fluids until the concentration difference has been balanced out.
With a selective membrane, certain components can be "separated out" from the concentrated fluid by increasing the resistance for other components.
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If bases are to be recovered from old process baths, a cation exchange membrane is used, through which mainly cations (positively charged ions) can diffuse.
It has been shown in practice that recovery of caustic solutions with cation exchange membranes actually works, although the OH- ions are much larger than the co-diffusing H+ ions in acid diffusion dialysis with anion exchange membranes. However, the diffusion velocities are lower than in acid diffusion dialysis.