An Introduction to Environmental Chemistry

164 Chapter Five

Box 5.4 Eh-pH diagrams

Acidity (pH) and redox potential (Eh) (see Box

4.3) may determine the chemical behaviour

of elements or compounds in an

environment. In theory, an infinite valley of

Eh-pH combinations is possible, although the

pH of most environments on Earth is

between 0 and 14 and more usually between

3 and 10. Redox potential is constrained by

Redox potential, Eh (volts)

0 2 4 6 8 10 12 14

• 0.6


• 0.4


• 0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

pH

O 2

Fe3+

Fe2+

Fe 2 O 3

(haematite)

FeCO 3

(siderite)

FeCO 3

Fe 3 O 4

(magnetite)

FeS 2

(pyrite)

H 2 O

H 2 O

H 2

Fig. 1Simplified Eh-pH diagram showing stability fields of common iron minerals. The stability fields change

position slightly depending on the activity of the components. In this case Fe^2 +, Fe^3 +and S= 10 -^6 mol l-^1 and

dissolved inorganic carbon=1 mol l-^1. After Garrels and Christ (1965).

(continued)