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 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)

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

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 =

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

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)

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

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

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)