Which species dissociates to increase the alkalinity of water when pH is elevated?

When the pH of water increases, it indicates a decrease in the concentration of hydronium ions (H₃O⁺) and an increase in alkalinity, which refers to the water's capacity to neutralize acids. The species that plays a significant role in enhancing alkalinity in these conditions is the carbonate ion (CO₃²⁻).

As pH rises, carbonate ions can engage in reactions with hydrogen ions (H⁺) to form bicarbonate ions (HCO₃⁻). This reaction helps to buffer changes in pH, thus maintaining stability in aquatic environments. The presence of carbonate ions increases the overall alkalinity of water, as they can effectively neutralize excess acids that may be introduced.

While bicarbonate ions also contribute to alkalinity, particularly at lower pH levels, they do so more as a buffering agent rather than as a direct contributor to increased alkalinity when pH is elevated. Therefore, in the context of increasing alkalinity with elevated pH, the carbonate ion is the species that primarily facilitates this increase.