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Carbonate System and pH

Carbonate System and pH

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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 • CO2dissolves 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. Can write a dissolution reaction: • g indicates gas partial pressure • aq indicates amount dissolved in water CO2(g) = CO2(aq) KCO2 =

  5. Equilibrium constant: • Here KH is Henry’s Law constant aCO2(aq) KH = fCO2(g)

  6. Once CO2 is dissolved it reacts with the water: CO2(aq) + H2O = H2CO3* aH2CO3* aH2CO3* Keq = ≈ aCO2(aq)aH2O aCO2(aq)

  7. Keq= 2.6 x 10-3 at 25o C • Less than 0.3% of CO2(aq) present is H2CO3*

  8. In most cases the two reactions are combined • Now consider only the control of PCO2 on the amount of carbonic acid in solution: • Here H2CO3o is sum of mCO2(aq) and mH2CO3* CO2(g) + H2O = H2CO3o

  9. Can write an equilibrium constant for dissolution reaction: • In terms of additional carbonate reactions the form doesn’t matter because reaction kinetics are fast enough. aH2CO3o KCO2 = aCO2(aq)