CHEMISTRY 1000

Topics of Interest #8:Carbon Dioxide, Coral Killer!

Carbon dioxide is a Lewis acid. It reacts with water to form carbonic acid, a more

conventional acid (i.e. an Arrhenius acid):

In basic solutions, it reacts with hydroxide ions to form bicarbonate ions:

Carbon Dioxide, Carbonates and Oceans

J.C. Orr et al. Nature (2005) 437, 681-686.

OCO

O

HHC

O

O

HOH+

OCO

O

HC

O

OOH+

Oceans are slightly basic environments because of the dissolved salts they contain. Ocean water is saturated with carbonate ions, for example. These basic ions are used by many creatures living in the ocean to build shells, corals, etc.

250 years ago, the average pH of the ocean was 8.179. 15 years ago, it was 8.104. It’s predicted that the pH may get as low as 7.8 by the year 2100.

This decrease might not seem like a lot but, as the pH lowers, The concentration of carbonate ions dissolved in the ocean drops

dramatically as they are converted into bicarbonate ions. Existing carbonate structures (corals, shells, etc.) become more

prone to being redissolved in the ocean water if the pH ever lowers enough that the oceans are no longer saturated with calcium carbonate.

Carbon Dioxide, Carbonates and Oceans

J.C. Orr et al. Nature (2005) 437, 681-686.

Why is the pH of the oceans lowering? The concentration of CO2 dissolved in a solution is

proportional to the pressure of CO2 in the atmosphere above it. As atmospheric CO2 levels increase, so does the concentration of CO2 dissolved in the oceans.

Atmospheric CO2 has increased from about 280 ppmv (parts per million by volume) in the pre-industrial era to about 380 ppmv today.

Note that the oceans don’t actually have to become acidic (pH < 7) for us to observe the consequences of ocean acidification. All that needs to happen is for the pH to lower enough that calcium carbonate no longer saturates the ocean water.

Carbon Dioxide, Carbonates and Oceans

J.C. Orr et al. Nature (2005) 437, 681-686.

The image below shows a pteropod (small shellfish) grown in water not saturated with CaCO3. We can see the degradation of its shell relative to the bottom right image (the same species grown in CaCO3-saturated water).

Carbon Dioxide, Carbonates and Oceans

J.C. Orr et al. Nature (2005) 437, 681-686.

CaCO3-saturated water

unsaturated waterunsaturated water

unsaturated waterunsaturated water