Chemistry of Ocean Acidification As more and more carbon dioxide is released into the atmosphere, the concentration of CO2(aq) in the oceans also increases. We know that solvated carbon dioxide is in equilibrium with carbonic acid. This weak acid releases protons and decreases the pH of seawater. This, in turn, affects the stability of carbonate biominerals including the shells of mollusks and coral reefs. Outline • Carbon Dioxide as an Acid • Biominerals in the Ocean • Changes in Ocean pH • Homework
Carbon Dioxide as an Acid Henry's Law: Constant Concentration in Water Remember that Henry's Law determines the concentration of dissolved carbon dioxide in an aqueous solution exposed to the atmosphere. The current, average concentration of CO2 is 387 ppm, so the concentration of carbon dioxide in all surface water is:
[CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M
Other equilibria that involve dissolved carbon dioxide DO NOT affect the concentration of CO2 in surface water. If some carbon dioxide is used up, more goes into solution. There is an equilibrium between water and dissolved CO2 that forms carbonic acid.
Carbonic acid, like sulfuric acid, is a diprotic acid. It has two acid protons that it can lose in two separate steps.
First Dissociation and HCO3Because the dissolved carbon dioxide is in equilibrium with carbonic acid and carbonic acid is in equilibrium with bicarbonate and a proton in water, it is convenient to consider CO2(aq) to be the acid.
Chemistry 102
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You know that pH is -log[H+], pOH is -log[HO-], and pKw is -log(Kw). We use the same system for equilibrium constants. In tables, pKa values are listed. These are -log(Ka). For the first acid dissociation constant of CO2(aq), Ka1 = -log(4.25 x 10-7) = 6.37.
Second Dissociation and CO32Bicarbonate is a very weak acid with a Ka2 of 5.0 x 10-11 and a pKa2 of 10.3.
We can combine this with the equation for the first dissociation to find the concentration of carbonate anion in water that is in equilibrium with air.
pH of Surface Water Because Ka2 is so small, the equilibrium concentration of H+ in surface water is dominated by Ka1. We can use this to calculate the pH of surface water when no additional acid or base is present.
[HCO3-][H+] = (4.3 x 10-7)([CO2(aq)]) Without added acid or base, [HCO3-] = [H+] = x x2 = (4.3 x 10-7)(1.32 x 10-5) = 5.7 x 10-12 x = 2.4 x 10-6 pH = -log(x) = 5.6
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Biominerals in the Ocean Biominerals are inorganic compounds deposited in living things. Typically, an organic polymer such as a protein organizes the inorganic materials to promote crystallization of the material. Biominerals can have properties differing from the completely inorganic material. The white cliffs of Dover, England are the result of the build-up of calcium carbonate scales of marine algae.
Calcium Carbonate Biominerals Calcium carbonate in various crystalline forms is found in the exoskeletons of many marine organisms, mollusk shells, and even in some higher plants. Humans and other mammals have hundreds of small crystals of the calcite form of CaCO3 in the inner ear. These help us to detect changes in acceleration by acting as an inertial mass. Polysaccharides and glycoproteins from the framework in the deposition of CaCO3 in mollusk shells. The shell is formed within a membrane-enclosed space. Ion pumps add Ca(II) ions to this space and the formation of crystals (calcite or aragonite) is regulated by the organism.
Calcite
Chemistry 102
Aragonite
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Crystals of CaCO3 are bound together with a protein glue, conchiolin. The biomolecule nacre holds small particles of aragonite together in pearls.
Silicate Biominerals Diatoms have cell walls that are based on silica, SiO2. The cells are constructed of two halves that fit together with the top overlapping the bottom. The forms are intricate and vary widely. The silica shells are formed beginning with silicic acid, H4SiO4. These condense. Partially condensed silicates are stabilized polyamines within the organism.
Solubility and pH Silica is a neutral, network material with a diamond-type structure. It is relatively unaffected by changes on the pH of the ocean. Calcium carbonate, however, is a basic salt. The anion CO3-2 is a base, the conjugate base of HCO3-.
Chemistry 102
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Bicarbonate is a very weak acid so the equilibrium concentration of carbonate ion is small even at neutral pH. Because H+ is a product, any acid protons from other sources will combine with carbonate ion to make more bicarbonate ion. By combining the acid dissociation equilibrium with the dissolution equilibrium, we can see that increasing the concentration of acid protons leads to loss of solid calcium carbonate.
Changes in Ocean pH pH Changes The oceans are becoming more acidic at a surprisingly rapid rate.
Using Henry's law and the equilibrium of dissolved CO2 in water, we can calculate the pH of the water if we know the atmospheric concentration.
Chemistry 102
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Coral Changes Corals are colonial organisms. They excrete calcium carbonate and build up reef structures. Coral reefs harbor 25% of the fish in the ocean and promote great biodiversity. However, since 1980 corals have been reduced by over 30% around the globe. This is due to both increasing ocean temperatures and increased pH. When the aragonite form of CaCO3 is saturated, conditions are right for a build-up of coral. Increasing acid concentrations reduce the level of CO3-2 and so lead to a loss of solid calcium carbonate. The figure below shows how the level of carbonate ion has been reduced over time due to increased H+ concentration in the oceans.
Chemistry 102
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Effects on Other Organisms Crustaceans (such as shrimp), echinoderms (such as starfish), large calcareous algae, and some phytoplankton also make calcium carbonate biominerals. These organisms are major food sources for fish that we consume: salmon, mackerel, and cod. Loss of these calcifying organisms could greatly disrupt the ocean food chain.
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