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 CO 2 ; in basic (alkaline) conditions, the dominant form is CO 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 CO 2, 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 CO 2 emission by the fossil fuel combustion.