Chemtrol CO2 pH Control System controls carbon dioxide, CO2 is a natural non-corrosive gas that can be used as a substitute for other acids (such as Muriatic acid HCl) in chemical treatment of pools and spas. CO2 is available in standard pressurized gas cylinders. When dissolved in water, it forms carbonic acid H2CO3, a weak acid which can be used to lower the pH of water. and neutralize liquid chlorine.