Under normal circumstances, a certain amount of carbon dioxide dissolves in water. After dissolving in water, carbon dioxide reacts with water to form a carbonic acid equilibrium, as shown in the following reaction equation:
CO 2 +H 2 O→HCO 3 – +H + →CO 3 2- +2H +
Since reverse osmosis membranes cannot remove dissolved gases in water, but can remove various ions, the concentration of various ions in the product water is lower than that in the feed water, while the concentration of carbon dioxide is about the same. Therefore, the reaction proceeds to the right, forming more hydrogen ions, which lowers the pH value of the product water.
