Environmental Chemistry

Step 1: Define buffering capacity.
The buffering capacity of water is its ability to resist changes in pH upon the addition of an acid or a base.

Step 2: Identify the primary buffering system in natural waters.
The most important buffering system in natural aquatic environments is the carbonate system. This system involves the equilibrium between dissolved carbon dioxide (CO ), carbonic acid (H CO ), bicarbonate ions (HCO ), and carbonate ions (CO ).

Step 3: Determine the dominant species in this system.
In the typical pH range of natural waters (6.5 to 8.5), the bicarbonate ion (HCO ) is the dominant species in the carbonate system. It can neutralize added acids by accepting H to form carbonic acid, and neutralize added bases by donating H to form carbonate ions. This makes it the primary agent responsible for the water's buffering capacity.