When carbon dioxide levels increase, causing ocean pH to decrease, how do the concentrations of carbonate species change? Use the Carbon Dioxide and Ocean pH Learning Tool to explore the answer to this question.

First, adjust the concentration of atmospheric CO­­­₂ and note how the pH of the ocean changes. Remember that this occurs because aqueous carbon dioxide reacts with water to produce carbonic acid which, in turn, reacts with water again to produce hydronium ions.

Then, click the ‘Show Graph’ button to view a speciation plot displaying the concentrations of carbonic acid (H­­­₂CO­­­₃), hydrogen carbonate ions (HCO­­­₃^-) and carbonate ions (CO­­­­₃^-2). The speciation plot of carbonate species differs from the speciation plots that you have seen previously because carbonic acid is a diprotic acid and three species are shown. Also, because carbonate species are involved in many reactions in the ocean, the calculations used to create the plot are quite complicated. However, the concepts illustrated by the plot are the same as those you have seen before.

Worked Example

When ocean pH decreases, how do the concentrations of carbonate species change? Use the graph in the Carbon Dioxide and Ocean pH Learning Tool to explore the answer to this question.

Your Turn

The concentration of atmospheric carbon dioxide has important effects on carbon speciation. According to the Carbon Dioxide and Ocean pH Learning Tool, approximately what percentage of total carbonate species exists as carbonate ions, when the concentration of atmospheric carbon dioxide is 395 ppm? What is the approximate percentage of total carbonate species that exists as carbonate, if the concentration of atmospheric carbon dioxide increases to 600 ppm? Hint: Use the Evaluate Single Point tool in the top left corner of the window.