Abstract: In apparatus for separating ions in a solution by countercurrent electrolysis, a generally cylindrical column is constituted by axially juxtaposed annular segments which define respective chambers separated by liquid-permeable partitions. The two axially terminal chambers contain electrodes. Radial bores in the segments permit introduction and discharge of the solution to be tested, of an electrolyte flowing countercurrent to the solution, and of a cooling liquid passing through a cooling coil in each chamber. A shaft axially extending through the column carries agitating blades in each chamber and is magnetically rotated.