Abstract: An electrochemical cell has an annular flow passage for electrolyte solution. The cell is designed such that, in an intermediate zone, the passage is of constant cross-section (to achieve high electrolytic activity) and, in end zones, the passage progressively increases in cross-section to a maximum value (to lower the current density in regions in which unwanted deposition of salts may preferentially occur). The cross-sectional shape of the passage along its length also assists in reducing turbulence in the flow thereby reducing the rate of deposition of salts. Owing to the low rate of deposition, it is possible to maintain the electrodes relatively clean by current reversal at relatively infrequent intervals. That enables the cell to be used for longer periods between cleaning operations.