Fig. 3.

Time-lapse images of decarbonation H-cells using platinum electrodes and 1 M NaNO₃ in deionized water as electrolyte. Each cell contains a few drops of pH indicator dye, for which the color scale is shown at the bottom. (A–E) Cell containing CaCO₃ powder in the anode (left) chamber and with no porous separator between chambers. Electrolysis at 2.5-V cell voltage (∼6-mA current) produces color gradient showing acidic solution at anode (left) and alkaline solution at cathode (right). Close examination of cross-tube shows stratification of the solutions, attributed to density-driven convection. (F–I) Decarbonation cell in which porous fiber separators are used at both chambers to restrict convection, and CaCO₃ powder source is contained within a removable cup so that weight loss can be monitored. Note absence of stratification. (J) Ca(OH)₂ precipitated in cross-tube after 12 h of electrolysis at a high cell voltage of 9 V to accelerate reaction.