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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 CO 2 Can be generalized to other systems: Phosphoric, Sulfuric, Nitric, Silicic etc. Model.

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carbonate system and ph
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.
model
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)
slide3

Multiple sources of CO2

    • Atmosphere
    • Respiration
    • Remineralization of organic matter
    • Dissolution of carbonate minerals
slide4

Can write a dissolution reaction:

  • g indicates gas partial pressure
  • aq indicates amount dissolved in water

CO2(g) = CO2(aq)

KCO2 =

slide5

Equilibrium constant:

  • Here KH is Henry’s Law constant

aCO2(aq)

KH =

fCO2(g)

slide6

Once CO2 is dissolved it reacts with the water:

CO2(aq) + H2O = H2CO3*

aH2CO3*

aH2CO3*

Keq =

aCO2(aq)aH2O

aCO2(aq)

slide7

Keq= 2.6 x 10-3 at 25o C

  • Less than 0.3% of CO2(aq) present is H2CO3*
slide8

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

slide9

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)