Carbon Dioxide and Carbonic Acid The most common source of acidity in water is dissolved carbon dioxide. Carbon dioxide enters the water through equilibrium with the atmosphere CO 2 (aq) « CO 2 (g) and biological degradation/photosynthesis involving organic carbon, {CH 2 O} {CH 2 O} + O 2 (aq) « CO 2 (aq) + H 2 O Aqueous CO 2 (aq) also undergoes a number of important inorganic equilibrium reactions. First, it can dissolve limestone CaCO 3 + CO 2 (aq) + H 2 O « Ca2+(aq) + 2 HCO 3 - (aq) Second, it can react with the water to form carbonic acid CO 2 (aq) + H 2 O « H 2 CO 3 (aq) Only a small fraction exists as the acid and the kinetics to form H 2 CO 3 are relatively slow (on the time scale of seconds). Carbon Dioxide and Carbonic Acid-Base Equilibria Dissolved CO 2 in the form of H 2 CO 3 may loose up to two protons through the acid equilibria H 2 CO 3 (aq) « H+ (aq) + HCO 3 - (aq) HCO 3 - (aq) « H+ (aq) + CO 3 2- (aq) The equilibrium equations for these are labeled as "1" and "2" hence To account for the fact that CO 2 (aq) is in equilibrium with H 2 CO 3 (aq), the first acid equilibrium is normally given by The acid equilibrium equations can be solved to give the fraction of carbonates in a particular form. Relative H 2 CO 3 concentration is really CO 2 (aq) in equilibrium with water. In summary; CO 2 enters water through interface with the atmosphere and the biological processes of organic carbon digestion and photosynthesis. Aqueous carbon dioxide, CO 2 (aq), reacts with water forming carbonic acid, H 2 CO 3 (aq). Carbonic acid may loose protons to form bicarbonate, HCO 3 - , and carbonate, CO 3 2-. In this case the proton is liberated to the water, decreasing pH. The complex chemical equilibria are described using two acid equilibrium equations. The first acid equilibrium constant accounts for the CO 2 (aq) - H 2 CO 3 (aq) equilibrium. It concequently seems to have a high pK a . The fraction of the inorganic carbon in a particular form is call the "alpha" and there are simple equation to describe this alpha. Graphical Results Consequences of the fractional amount or "alpha" equations may be understood by examination of the graphical results. Shown to the left is a plot of the various alpha as a function of pH. Some important points to observe are: For pH well below pK a1 a (H 2 CO 3 ) ~ 1 At pH = pK a1 , a (H 2 CO 3 )= a (HCO 3 -) For 7<pH<10, HCO 3 - is the predominant species

The pH of water (no lime) We now have enough information to calculate the pH of water. First, we calculate the amount of CO 2 dissolved in water under an atmosphere of pressure from Henrys Law Since CO 2 makes up 0.0355% of the atmosphere (on the average) and K CO2 =2x10-3 Since is in equilibrium with H 2 CO 3 (aq), the first acid equilibrium is normally given by is predominant. Also since CO 2 (aq) + H 2 O « H+ (aq) + HCO 3 - (aq) The proton and bicarbonate concentrations are equal. Thus When we substitute the carbon dioxide concentration, and solve for pH, we get pH = 5.65 Since rain is in equilibrium with the atmosphere, this is the pH expected for natural rain. It is also the pH expected if the body of water is in equilibrium with the atmosphere, and does not contact limestone (e.g., CaCO 3 ). This page edited Thursday, December 21, 2006