Gravimetric Analysis

Gravimetric analysis can be used to determine the quantity of an ion present in a solution.
This can be done by adding a reagent that causes the ion under investigation to form an insoluble compound (a precipitate) that precipitates out of the solution.
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Sulfur Cycle

Sulfur, an element found in proteins, is cycled through the Earth's atmosphere, oceans and land, and, as it does, it undergoes chemical changes.
Most of the sulfur found on Earth is found in seawater and in rocks, in particular sedimentary rocks like shales containing pyrite (iron(II) sulfide or iron disulfide) and  in evaporite rocks containing anhydrite (anhydrous calcium sulfate), baryte (barium sulfate) and gypsum (hydrated calcium sulfate). The amount of mobile sulfur is continuously increasing due its release during volcanic activity. Human activities such as the burning of fossil fuels including coal and natural gas are also increasing the amount of mobile sulfur on Earth because fossil fuels contain sulfur as an impurity.

The sulfur cycle can be represented by the following 4 steps:

1. The incorporation of sulfur from organic (carbon) compounds like proteins, into elemental sulfur and inorganic compounds such as hydrogen sulfide, and the sulfide minerals such as pyrite (iron pyrite or fool's gold).
2. Oxidation of hydrogen sulfide, elemental sulfur and inorganic sulfides into sulfate ions.
3. Reduction of sulfate ions to sulfide ions.
4. Incorporation of the sulfur from sulfide ions into organic compounds (including organic compounds that contain metal atoms), such as proteins.

The oxidation of sulfur (step 2 in the cycle above) plays a part in removing oxygen from the atmosphere by incorporating oxygen into sulfate ions. Scientists have thought for a long time that the contribution of the sulfur cycle in removing atmospheric oxygen  is not nearly as important as the role of the carbon cycle in removing atmospheric oxygen. New research is suggesting that the weathering of pyrite and its burial may be more important than originally thought in regards to regulating oxygen.

Reference:
Halevy, S. E. Peters, W. W. Fischer. Sulfate Burial Constraints on the Phanerozoic Sulfur Cycle. Science, 2012; 337 (6092): 331 DOI: 10.1126/science.1220224

Further Reading:
Writing Ionic Formulae
Naming and Writing Formulae for Covalent Compounds
Oxidation Number (oxidation state)
Writing Precipitation Reaction Equations
Balancing Molecular Equations
Oxidation and Reduction Concepts
Carbon Cycle

Suggested Study Questions:

1. Write the chemical formula for each of the following:
   • elemental sulfur
   • hydrogen sulfide
   • iron(II) sulfide (iron disulfide)
   • calcium sulfate
   • barium sulfate
   • sulfate ion
   • sulfide ion
2. Give the oxidation number (oxidation state) for sulfur in each of the following:
   • S₈
   • H₂S
   • CaSO₄
   • BaSO₄
   • CaSO₄.2H₂O
3. Seawater is an aqueous solution which contains ions such as, barium, calcium, and sulfate. As seawater evaporates, calcium sulfate and barium sulfate precipitate out of the solution. Write a balanced molecular equation for:
   • precipitation of barium sulfate from seawater
   • precipitation of calcium sulfate from seawater
4. Write balanced chemical equations for each of the following:
   • combustion of carbon in coal to form carbon dioxide
   • combustion of sulfur in coal to form sulfur dioxide
   • combustion of methane (natural gas) to form carbon dioxide
5. Consider this equation for the oxidation of elemental sulfur :
   2S + 2H₂O + 3O₂ → 2H₂SO₄
   • Determine the oxidation number (oxidation state) of sulfur in elemental sulfur and in H₂SO₄
   • Use this information to explain why this equation can be said to represent an oxidation of sulfur.
   • Identify a species that is being reduced. Explain why this species can be said to be undergoing reduction.
   • Explain why it is appropriate to refer to the equation above as a redox (oxidation-reduction) reaction
   • For the reaction given above give the formula for the oxidant (oxidizing agent) and for the reductant (reducing agent).

Killer Seaweed in Coral Reefs

Studies have shown that several species of seaweed common in both the Pacific and Caribbean Oceans can kill corals upon contact using chemical means.

The study used racks of transplanted coral placed next to different types of common seaweed as well as plastic plants to simulate the effects of shading and mechanical damage. For comparison, other coral racks had neither seaweed not plastic plants near them. In as little as two days the corals in direct contact with the some seaweeds bleached and died.

Chemicals were extracted from the seaweeds causing death. These chemicals were applied to the corals in a gel matrix. As a control, the gel without the chemicals was added to a different group of corals. The results confirmed that these chemicals were responsible for the death of corals in contact with the killer seaweeds.

Overfishing of herbivorous fishes has resulted in more seaweeds being present amongst corals, and some of these seaweeds are responsible for the death of coral. The less coral there is, the fewer fish will be recruited into the area to feed, so the number of seaweeds increase, more coral dies.

Reference:
Douglas B. Rasher, Mark E. Hay. Chemically rich seaweeds poison corals when not controlled by herbivores. Proceedings of the National Academy of Sciences, 2010; DOI: 10.1073/pnas.0912095107