CHEMISTRY 1000
Topics of Interest #7:
Carbon Dioxide, Coral Killer!
Carbon Dioxide, Carbonates and Oceans

Carbon dioxide is a Lewis acid.

It reacts with water to form carbonic acid, a more conventional acid
(i.e. an Arrhenius acid):
O
C
+
H
O
H
O
H
O

O
C
O
H
In basic solutions, it reacts with hydroxide ions to form bicarbonate
ions:
O
C
O
+
H O
O
J.C. Orr et al. Nature (2005) 437, 681-686.
H
O
C
O
Carbon Dioxide, Carbonates and Oceans



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.
J.C. Orr et al. Nature (2005) 437, 681-686.
Carbon Dioxide, Carbonates and Oceans

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.
J.C. Orr et al. Nature (2005) 437, 681-686.
Carbon Dioxide, Carbonates and Oceans

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).
unsaturated water
unsaturated water
unsaturated water
unsaturated water
CaCO3-saturated water
J.C. Orr et al. Nature (2005) 437, 681-686.