The Chemistry of the Solid Earth - Principles of Environmental Chemistry - page 256

Worked example 5.4 - pH of soil solution
Carbon dioxide dissolves in water to form carbonic acid, which is a
weak acid.

H 2 CO 3 K ¼ 10 1.5 CO 2 þ H 2 O " (5.10)

[H 2 CO 3 ]/pCO 2 ¼ 10 1.5 mol L 1 atm 1
Carbonic acid dissociates to produce bicarbonate and hydrogen ions
H 2 CO 3 K ¼ 10 6.4 HCO 3 þ H 1 " (5.11)

½ HCO 3 ½ H þ
½ H 2 CO 3

¼ 10 6 : 4 molL 1

½ HCO 3 ¼½ H þ
½ H þ 2 ¼ 10 6 : 4 ½ H 2 CO 3 ¼ 10 6 : 4 10 1 : 5 pCO 2
pCO 2 in the atmosphere ¼ 10 3.5 atm

½ H þ 2 ¼ 10 6 : 4 mol L 1 10 1 : 5 mol L 1 atm 1
10 3 : 5 atm ¼ 10 11 : 4
½ H þ ¼ 10 5 : 7 mol L 1
pH ¼ 5 : 7
If the partial pressure of CO 2 in the soil atmosphere is considered to
be 10 times greater than the free atmosphere,

½ H þ 2 ¼ 10 6 : 4 mol L 1 10 1 : 5 mol L 1 atm 1
10 2 : 5 atm ¼ 10 10 : 4
½ H þ ¼ 10 5 : 2 mol L 1
pH ¼ 5 : 2
Sulfur dioxide dissolves to form the strong acid, sulfurous acid, so is
strongly acidifying at very low concentrations. For example, for pSO 2
¼ 10 7 atm (a value much higher than concentrations now occurring
owing to air pollution control measures in developed countries. Note,
however, that low-pH rainfall still occurs as a result of incorporation
of sulfuric and nitric acids).
SO 2 þ H 2 O " H 2 SO 3 K ¼ 1 (5.12)

[H 2 SO 3 ]/pSO 2 ¼ 1 mol L 1 atm 1