Alkalinity, dissolved inorganic carbon and pH buffering

In Section 4.4 we saw that most soilwaters that feed rivers and groundwater have near-neutral pH, with I ICO, as the major anion. This results from the dissolution of C02 in water (see eqn. 4.7) and from the acid hydrolysis of silicates and carbonates. The total concentration of weak acid anions like HCOf in water is referred to as alkalinity. These anions are available to neutralize acidity (H+) in natural waters, consequently it is important to understand their chemical behaviour. 

In continental waters, bicarbonate (I ICO ) and carbonate (CO2-) ions are the most important components of alkalinity, although in seawater other ions also contribute to alkalinity. The relative importance of HCOf and CO2- depends on the pH of the solution and can be calculated from the known dissociation constants (see Box 4.5) of these ions and the solution pH. 

The first dissociation of dissolved carbon dioxide (expressed here as carbonic acid), 

The V denotes activity the formal thermodynamic representation of concentration (see Section 2.6). 

It is simple to demonstrate that the alkalinity in most continental waters is dominated by HCOf by rearranging the above equations at a typical pH value for these waters. For example, many mature rivers have pH values around 8. Rearranging equation 5.9 to solve for aHCO gives, 

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