PrintExample 1.1: A closed carbonate system. Imagine we have a closed bottle with clean water except with a head space filled with $\text{C}\text{O}_2$ gas. The system therefore only has carbonate species in the water phase, the pertinent reactions are as follows:
$$\mathrm{H}_2\mathrm{CO}_3^0\Leftrightarrow\mathrm{H}^++\mathrm{HCO}_3^-\quad K_{a1}=\frac{a_{H^+}a_{HCO_3^-}}{a_{H_2CO_3^0}}=10^{-6.35}$$
$$\mathrm{HCO}_3^-\Leftrightarrow H^++\mathrm{CO}_3^{2-}\quad K_{a2}=\frac{a_{H^+}a_{CO_3^{2-}}}{a_{\mathrm{_{ }HCO}_3^-}}=10^{-10.33}$$
$$\mathrm{H}_{2} \mathrm{O} \Leftrightarrow \mathrm{H}^{+}+\mathrm{OH}^{-} \quad K_{w}=a_{H^{+}} a_{O H^{-}}=10^{-14.00}$$
These are the speciation reactions between carbonate species and water species such as $\text{H}^+$ and $\text{OH}^-$. All these reactions are aqueous speciation reactions and occur fast. All Ks are equilibrium constants at standard temperature and pressure conditions. Note that here we are not imposing the condition for charge balance. Reactions in Example 1.1 are examples of aqueous complexation reactions. These reactions occur ubiquitously in water systems and have significant impacts on water chemistry. For example, ligands, dissolved organic carbon (DOC), and other anions can form complexes with cations, including metals, to prevent cations from precipitation, therefore increasing the mobility of cations in water systems. The carbonate complexation and dissociation reactions with $\text{H}^+$ in example 1.1 works as a buffering mechanism to prevent the pH of water from changing rapidly.