Lecture 5

L1 /

CH3041

CARBONATE SYSTEM

CO2, HCO3- (bicarbonate), CO32- (carbonate)

• [CO2(aq)] = KH p(CO2) KH = 3.38 x 10-2 mol l-1 atm-1

• The large value of KH means that natural water always contains appreciable CO2(aq).

(1) CO2(aq) + H2O D H2CO3 K = 2 x 10-3

(2) CO2 + H2O D HCO3- + H+ pKa1 = 6.35

(3) HCO3- D CO32- + H+ pKa2 = 10.33

• The inputs in a natural surface water then are both from the atmosphere and the dissolution of carbonate minerals.