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Carbonate System and pH

Model. CO2 dissolves when it comes in contact with waterThe amount dissolved depends on fugacity of CO2At atmospheric pressure (low), assume fCO2 = PCO2 (analogous to low dissolved concentrations). . Multiple sources of CO2AtmosphereRespirationRemineralization of organic matterDissolution of

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Carbonate System and pH

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    1. Carbonate System and pH Why study the carbonate system? Involves carbonic acid – an example of an acid-base reaction pH of most water controlled by CO2 Can be generalized to other systems: Phosphoric, Sulfuric, Nitric, Silicic etc.

    2. Model CO2 dissolves when it comes in contact with water The amount dissolved depends on fugacity of CO2 At atmospheric pressure (low), assume fCO2 = PCO2 (analogous to low dissolved concentrations)

    3. Multiple sources of CO2 Atmosphere Respiration Remineralization of organic matter Dissolution of carbonate minerals

    4. For gas phases, can write a dissolution reaction: (g) indicates gas partial pressure (aq) indicates amount dissolved in water

    5. Equilibrium constant: Here KH is Henry’s Law constant Henry’s law: the amount dissolved is constant at constant T and constant f

    6. KH = 10-1.46 at 25ºC = 0.035 or about 3.5% of CO2 in atmosphere is in ocean Show in a minute KH is not used much

    7. Once CO2 is dissolved it reacts with the water: Here H2CO3* is the true amount of carbonic acid in the water

    8. In most cases the two reactions are combined Now consider only the control of PCO2 on the amount of carbonic acid in solution: Note, here it is the fugacity of CO2, not dissolved CO2 Here H2CO3o is sum of mCO2(aq) and mH2CO3*

    9. Can write an equilibrium constant for dissolution reaction: Whether H2CO3º is CO2 or H2CO3* doesn’t matter much: Reaction kinetics are fast, essentially immediate.

    10. Keq = 10-1.47 = 0.033 at 25o C Only about 3% of CO2(g) present is H2CO3º Most of the H2CO3 is as CO2(aq)

    11. CO2 units Units commonly reported as ppm by volume: ppmv Current atmospheric concentration is 0.0383% ppmv Pre-industrial concentation about 278 ppmv Annual variation about 6 ppmv

    12. Keeling Curve

    13. Conversion from ppmv to partial pressure (e.g., atm) Because CO2 is 383 ppmv of 1 Atm 383/106 Atm Partial pressure = .000383 Atm = 10-3.41 Atm Concentration typically given as 10-3.5 Atm = 0.000316 Atm = 316 ppmv

    14. On board: Summarize all dissolution reactions Carbonic acid dissociation

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