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.
     

Calcite

The teeth and bones of mammals, the shells of marine organisms, even the sharp spines of sea urchins, are all made of the same material, calcite. Calcite is a carbonate mineral and is the most stable form of calcium carbonate.

Marine organisms use the most readily available elements in the sea water, that is calcium, oxygen and carbon, to produce their teeth, bones or shells. In seawater solutions, organisms can use the available calcium and carbonate ions to precipitate calcium carbonate.

Calcite is also commonly found in sedimentary rocks, and especially in limestone which is formed from the shells of dead marine organisms. Calcite is the mineral found in cave formations such as stalactites and stalagmites. Overlying limestones and marbles are dissolved away by slightly acidic groundwater which then percolates into caverns below. As the water evaporates, calcite is precipitated.


Calcite has some unusual properties.
It becomes less soluble in water as the temperature increases, a property known as retrograde solubility.
Single crystals of calcite display an optical property known as birefringence which causes objects viewed through a piece of calcite to appear to be doubled.

The sea urchin tooth is a biomineral composed of calcite crystals in both plate and fiber forms, arranged crosswise and cemented together with super-hard calcite nanocement. Between the crystals are layers of organic materials which are weaker than the calcite crystals. These weaker organic materials allow pieces of the tooth to break off in predefined places which means that the sea urchin tooth is self-sharpening!

Reference
Christopher E. Killian, Rebecca A. Metzler, Yutao Gong, Tyler H. Churchill, Ian C. Olson, Vasily Trubetskoy, Matthew B. Christensen, John H. Fournelle, Francesco De Carlo, Sidney Cohen, Julia Mahamid, Andreas Scholl, Anthony Young, Andrew Doran, Fred H. Wilt, Susan N. Coppersmith and Pupa U. P. A. Gilbert. Self-Sharpening Mechanism of the Sea Urchin Tooth. Advanced Functional Materials, 2010; DOI: 10.1002/adfm.2010015

Further Reading
Naming Ionic Compounds
Writing Ionic Formulae
Molecular Mass
Percentage Composition
Shapes of Molecules
Lewis Structures
Solubility Product

Study Questions

1. Give the chemical formula for calcium carbonate.
2. List the names and formulae of other compounds that can be made out of carbon, oxygen and calcium.
3. Calculate the percentage composition of calcium carbonate as well as the compounds you listed in question 2.
   
4. Write a balanced chemical equation to show the precipitation of calcium carbonate from ions in sea-water.
5. What shape do you expect the carbonate ion to take? Explain your answer.
6. Draw a Lewis Structure, electron dot diagram, for a "molecule" of calcium carbonate.
7. Why is it inaccurate to describe calcium carbonate as a molecule?
   
8. Describe the process by which calcium carbonate can dissolve in water.
9. Would calcium carbonate more readily precipitate from fresh water or sea-water at the same temperature? Explain your answer.