. . . "For example, carbon dioxide may be used to increase solution pH and ammonia may be used to decrease solution pH. In one embodiment, a pH control gas may include a combination of carbon dioxide, ammonia, or other gases to control (e.g., increase or decrease) pH. In another embodiment, the pH of a reaction fluid is controlled by a first sparger containing an agent that increases pH (e.g., CO2) and a" .