Bjerrum plot

A Bjerrum plot is a graph of the concentrations of the different species of a polyprotic acid in a solution, as functions of the solution's pH, when the solution is at equilibrium. Due to the many orders of magnitude spanned by the concentrations, they are commonly plotted on a logarithmic scale. Sometimes the ratios of the concentrations are plotted rather than the actual concentrations. Occasionally H⁺ and OH⁻ are also plotted.

Most often, the carbonate system is plotted, where the polyprotic acid is carbonic acid (a diprotic acid), and the different species are carbonic acid, carbon dioxide, bicarbonate, and carbonate. In acidic conditions, the dominant form is CO₂; in basic (alkalinic) conditions, the dominant form is CO₃²⁻; and in between, the dominant form is HCO₃⁻. At every pH, the concentration of carbonic acid is assumed to be negligible compared to the concentration of CO₂, and so is often omitted from Bjerrum plots. These plots are typically used in ocean chemistry to track the response of an ocean to changes in both pH and of inputs in carbonate and CO
2.

The Bjerrum plots for other polyprotic acids, including silicic, boric, sulphuric and phosphoric acids, can also be constructed.

If carbon dioxide, carbonic acid, hydrogen ions, bicarbonate and carbonate are all dissolved in water, and at chemical equilibrium, their equilibrium concentrations are often assumed to be given by:

...
Wikipedia