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$\ce{CO2}$ dissolves in water and goes through chemical reactions and exists at different concentrations of,

  1. dissolved $\ce{CO2}$,
  2. carbonic acid,
  3. bicarbonate, and
  4. carbonate.

The Bjerrum plot says that a high concentration of carbonate results in an increased pH (i.e., basic). But many diagrams in the Internet about ocean acidification show that as carbonate is produced, it becomes more acidic (although they also shows as carbonate is produced $\ce{H+}$ is also produced). enter image description here Source
I don't know how to reconcile this kind of figure and Bjerrum plot as I feel they are not consistent.

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Bjerrum Plot source

I think I see the confusion. You are right that directly adding carbonate, perhaps in the form of $\ce{Na2CO3}$, would increase the pH. However, ocean acidification, as shown in your figure, is from atmospheric $\ce{CO2}$ dissolving in the ocean. The relevant equilibrium: $$\ce{CO2_{(aq)} + H2O <=> H2CO3_{(aq)} <=> HCO3^{-}_{(aq)} +H+_{(aq)}<=>CO_{3(aq)}^{2-} +H+_{(aq)}}$$

The Bjerrum plot is simply a graphical representation of this equilibrium. As you add $\ce{CO2}$, you push the equilibrium towards the right, forming $\ce{H+}$, lowering the pH. Conversely, when you add $\ce{CO3^{2-}}$, the equilibrium move towards the left, consuming some $\ce{H+}$ and raising the pH. Although adding $\ce{CO2}$ leads to an increase in the absolute concentration of carbonate, the ratio of concentrations goes the other way.

You can use this two ways:

  1. If you have a change in pH, you can move along the x-axis to predict how the concentrations of the carbonate species change.

  2. If you change the ratios of the species, e.g. by adding $\ce{CO2}$, you can predict how the pH will change.

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  • $\begingroup$ I notice now that the equilibrium looks funny inlined like that. Note that the last part is just the products from bicarbonate losing its proton and doesn't need H+ to get there. $\endgroup$ – Michael DM Dryden Sep 5 '14 at 16:54

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