Carbon Dioxide exists in most natural waters. Carbon dioxide content is closely related to pH. PH decreases as carbon dioxide content increases and pH goes up as carbonate alkalinity goes up.
Carbon Dioxide exists at pH levels between 3.6 and 8.4. Carbon Dioxide cannot be found in water with a pH of 8.5 or higher. "The pH value is not a measurement of the amount of carbon dioxide in the water, but rather the relationship of carbon dioxide and bicarbonate alkalinity." (Enting Engineering Handbook.)
Carbon dioxide can be used as a means of controlling the pH of swimming pools, by continuously adding gas to the water, thus keeping the pH level from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. (Wikipedia.)
Although carbon dioxide mainly consists in the gaseous form, it also has a solid and a liquid form. It can only be solid when temperatures are below -78 oC. Liquid carbon dioxide mainly exists when carbon dioxide is dissolved in water. Carbon dioxide is only water-soluble, when pressure is maintained. After pressure drops the CO2 gas will try to escape to air. This event is characterized by the CO2 bubbles forming into water. (Lenntech.)
Carbon dioxide is essential for internal respiration in a human body. Internal respiration is a process by which oxygen is transported to body tissues and carbon dioxide is carried away from them.
Removing Carbon Dioxide from Water: Carbon Dioxide can be easily dissipated by aeration. It can be removed also by two column deionization, or by raising the pH above 8.5 by injecting Soda Ash.