The most important buffer system in natural waters is the carbonic acid-bicarbonate-carbonate system.
* Carbonic Acid ($H_2CO_3$) is a weak acid that forms when carbon dioxide ($CO_2$) dissolves in water.
* Bicarbonate ($HCO_3^-$) is the conjugate base of carbonic acid.
* Carbonate ($CO_3^{2-}$) is the conjugate base of bicarbonate.
The equilibrium reactions between these three species can be represented as follows:
$$CO_2_{(g)} + H_2O_{(l)} \rightleftharpoons H_2CO_{3(aq)}$$
$$H_2CO_{3(aq)} \rightleftharpoons H^+_{(aq)} + HCO_{3(aq)}^-{}$$
$$HCO_{3(aq)}^- \rightleftharpoons H^+_{(aq)} + CO_{3(aq)}^{2-}$$
The relative concentrations of these three species depend on the pH of the water. At low pH, the concentration of carbonic acid is highest, while at high pH, the concentration of carbonate is highest. Bicarbonate is the dominant species at a pH of around 8.3, which is the average pH of natural waters.
The carbonic acid-bicarbonate-carbonate system acts as a buffer in natural waters because it can react with both acids and bases to maintain a relatively constant pH.
* When an acid is added to a natural water, the bicarbonate and carbonate ions will react with the hydrogen ions to form carbonic acid, thus reducing the acidity of the water.
* When a base is added to a natural water, the carbonic acid will react with the hydroxide ions to form bicarbonate and carbonate ions, thus reducing the basicity of the water.
