Carbonate Equilibria

10/14/25

“How does the interaction between carbon dioxide and water keep pH levels stable?”

Water (H2O) can absorb carbon dioxide (CO2).  Some of this CO2 will react with the H2O to form carbonic acid (H2CO3), and some carbonic acid will reversibly decompose into a hydrogen ion (H+) and bicarbonate (HCO3-), and some of the bicarbonate will further reversibly decompose into a hydrogen ion and carbonate (CO3). The more basic a specific body of water is, the more likely these decompositions will occur. At pH levels below ~6.3, carbonic acid predominates. At pH levels between 6.3 to 10.3, bicarbonate predominates. At levels above 10.3, carbonate predominates. This balance is described as the Carbonate Equilibria. The balance of these chemicals holds a massive influence over a water body’s alkalinity and potential for certain treatment methods. 

• Chemistry