Bjerrum plot

rdf:langString Bjerrum plot

rdf:langString A Bjerrum plot (named after Niels Bjerrum; sometimes also known as a Sillén diagram or a Hägg diagram) is a graph of the concentrations of the different species of a polyprotic acid in a solution, as a function of 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 dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate. In acidic conditions, the dominant form is CO2; in basic (alkaline) conditions, the dominant form is CO2−3; and in between, the dominant form is HCO−3. At every pH, the concentration of carbonic acid is assumed to be negligible compared to the concentration of dissolved CO2, and so is often omitted from Bjerrum plots. These plots are very helpful in solution chemistry and natural water chemistry. In the example given here, it illustrates the response of seawater pH and carbonate speciation due to the input of man-made CO2 emission by the fossil fuel combustion. The Bjerrum plots for other polyprotic acids, including silicic, boric, sulfuric and phosphoric acids, are other commonly used examples.