Ocean pH

Coccoliths are very small shells of calcium carbonate that form around a number of species of algae. Algae play an important role in the global carbon-oxygen cycle and thus in our ecosystem. Scientists at the Nano-Science Center, University of Copenhagen, have measured how individual coccoliths react to water with different degrees of acidity.

Coccoliths which have a mass of about 500 pg (0.0000000005 g), were weighed before and after they had been immersed in water with different acidities. The results enable the scientists to say something about how important the water acidity is for the marine environment.

The world's oceans are acidifying due to our emissions of carbon dioxide. Over time the pH of the Earth's oceans is decreasing:

Time	pH
18th century	8.179
Recent past (1990s)	8.104
Present levels	~8.069
2050 (estimated)	7.949
2100 (estimated)	7.824

Coccoliths are protected from dissolution by a very thin layer of organic material that the algae form, even though the seawater is extremely unsaturated relative to calcite (calcium carbonate). The protection of the organic material is lost when the pH is lowered slightly. In fact, it turns out that the shell falls completely apart when experiments are done in water with a pH value of 7.8, the pH that many researchers believe will be the found in the world oceans in the year 2100.

Reference:
T. Hassenkam, A. Johnsson, K. Bechgaard, S. L. S. Stipp. Tracking single coccolith dissolution with picogram resolution and implications for CO2 sequestration and ocean acidification. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1009447108\

Further Reading:
Carbon Cycle
Combustion of Hydrocarbons
Acid Rain
Mass Conversions
pH Calculations
Acid Dissociation Constants (Ka)

Study Questions:

1. Write the formula for:
   
   • carbon dioxide
   • calcium carbonate
2. Calculate the percentage composition of
   
   • carbon dioxide
   • calcium carbonate
3. Write a balanced chemical equation to show the production of carbon dioxide from the combustion of methane (a fossil fuel).
4. Assuming 100 tonnes of methane gas were combusted under standard laboratory conditions:
   
   • What is the maximum mass of carbon dioxide that could be produced?
   • What volume would this mass of carbon dioxide occupy?
5. Coccoliths have a mass of about 500 pg. Convert this to a mass in:
   • milligrams
   • micrograms
   • nanograms
   • kilograms
6. The current pH of ocean water is approximately 8.069. Assuming the temperature of the oceans to be 25^oC, calculate the current:
   • hydrogen ion concentration of ocean water
   • hydroxide ion concentration of ocean water
   • pOH of ocean water
   
   
7. In 2100, the pH of ocean water is predicted to be 7.824. Assuming the temperature of the oceans to be 25^oC, calculate the:
   • hydrogen ion concentration of ocean water in 2100
   • the hydroxide ion concentration of ocean water in 2100
   • the pOH of ocean water in 2100
   • the increase in hydrogen ion concentration between now and 2100
     
   
   
   
8. For the reaction: H₂CO₃ HCO₃^- + H⁺ K_a = 4.5 x 10^-7
   
   • Is H₂CO₃ a strong acid or weak acid? Explain your answer.
     
   • Calculate the concentration of H⁺
   • Calculate the pH of the solution.
   • Calculate the pOH of this solution at 25^oC.
   • Calculate the concentration of hydroxide ions at 25^oC.
   • Explain what impact an increase in hydrogen ion concentration would have on this equation.