Bjerrum Plot

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 ; in basic (alkaline) conditions, the dominant form is ; and in between, the dominant form is . At every pH, the concentration of carbonic acid is assumed to be negligible compared to the concentration of dissolved , 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 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.

Bjerrum plot equations for carbonate system

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:

: \begin[]
\left textrm_2\right\text &= \frac \times \textrm, \\[3pt]
\left[\textrm_3^-\right]_\text &= \frac \times \textrm, \\[3pt]
\left[\textrm_3^\right]_\text &= \frac \times \textrm,
\end

where the subscript 'eq' denotes that these are equilibrium concentrations, ''K''₁ is the equilibrium constant for the reaction + H⁺ + (i.e. the first acid dissociation constant for carbonic acid), ''K''₂ is the equilibrium constant for the reaction H⁺ + (i.e. the second acid dissociation constant for carbonic acid), and DIC is the (unchanging) total concentration of dissolved inorganic carbon in the system, i.e. [] + [] + []. ''K''₁, ''K''₂ and DIC each have units of a concentration, e.g. mol/ L.

A Bjerrum plot is obtained by using these three equations to plot these three species against , for given ''K''₁, ''K''₂ and DIC. The fractions in these equations give the three species' relative proportions, and so if DIC is unknown, or the actual concentrations are unimportant, these proportions may be plotted instead.

These three equations show that the curves for and intersect at , and the curves for and intersect at . Therefore, the values of ''K''₁ and ''K''₂ that were used to create a given Bjerrum plot can easily be found from that plot, by reading off the concentrations at these points of intersection. An example with linear Y axis is shown in the accompanying graph. The values of ''K''₁ and ''K''₂, and therefore the curves in the Bjerrum plot, vary substantially with temperature and salinity.Mook W (2000) Chemistry of carbonic acid in water. In 'Environmental Isotopes in the Hydrological Cycle: Principles and Applications' pp. 143-165. (INEA / UNESCO: Paris)

Retrieved 30 November 2013.

Chemical and mathematical derivation of Bjerrum plot equations for carbonate system

Suppose that the reactions between carbon dioxide, hydrogen ions, bicarbonate and carbonate ions, all dissolved in water, are as follows:

Note that reaction is actually the combination of two elementary reactions:
: + H⁺ +

Assuming the mass action law applies to these two reactions, that water is abundant, and that the different chemical species are always well-mixed, their rate equations are
: \begin
\frac &= -k_1\left textrm_2\right+ k_\left textrm^+\rightleft textrm_3^-\right \\
\frac &= k_1\left textrm_2\right- k_\left textrm^+\rightleft textrm_3^-\right+ k_2\left textrm_3^-\right- k_\left textrm^+\rightleft textrm_3^\right \\
\frac &= k_1\left textrm_2\right- k_\left textrm^+\rightleft textrm_3^-\right- k_2\left textrm_3^-\right+ k_\left textrm^+\rightleft textrm_3^\right \\
\frac &= k_2\left