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

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

6. CO2(aq) + H2O = H2CO3* aH2CO3* aH2CO3* Keq = ≈ aCO2(aq)aH2O aCO2(aq)
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. CO2(g) + H2O = H2CO3o 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)