Carbon Dioxide and Carbonate Species in Water

Since air is only about 0.039% CO2 by volume, the concentration of this gas that dissolves in water from the atmosphere is relatively low. Much of the dissolved CO2 in water comes from the microbial decay of organic matter in water and in soil. If the concentration of free carbon dioxide in water from these or subterranean (volcanic) sources is too high, the respiration and gas exchange of aquatic organisms may be adversely affected and fish and other organisms may even die as a result. The formation of HCOj and CO significantly increases the solubility of carbon dioxide in water. 

The reaction between dissolved carbon dioxide in water and carbonate minerals such as limestone, CaCOj, 

FIGURE 3.10 Carbon dioxide in water reacts with limestone, CaCOj, to produce dissolved Ca (water hardness) and dissolved HCO3 (water alkalinity), a reaction that buffers the water pH at a slightly basic level. The carbon dioxide may come from the atmosphere or from decaying organic matter in water or soil. 

The reaction of HCO3 to produce H and the equilibrium constant expression for that reaction 

From these reactions and equilibrium constant expressions, it may be shown that the reaction between calcium carbonate and dissolved CO2 

---