Page 59 - Soil and water contamination, 2nd edition

46 Soil and Water Contamination

Box 2.I Basic principles for the construction of pH–Eh diagrams

The field of the stability of water is confined between the conditions where H O is
2
oxidised:
2H O (l ) O (g ) + 4H + + 4e (2.Ia)
2 2
and where H O is reduced:
2
2H + + 2e H 2 (g ) (2.Ib)
The Nernst equation applied to Reaction 2.Ia written as a reduction equation for
standard conditions is (see Equation 2.63):
. 0 0592 1
0
Eh E log (2.Ic)
n P [H 4
]
O 2
The standard redox potential for the reaction 2.Ia is calculated from the standard change
in free energy (see Equation 2.58), which in turn is calculated from thermodynamic data:
0
0
0
G 0 2 G H (l ) G O ( ) g 4 G H
R 2 O 2 (2.Id)
474 . 36 0 0 474 . 36 kJ mol 1
Thus,
G 0 474 . 36
0
E R . 1 229 V (2.Ie)
nF 4 96 . 490
The maximum partial pressure of oxygen permissible in the system as defined is 1
atmosphere, so Equation 2.Ic becomes:
Eh . 1 229 . 0 0592 pH (2.If)

Thus, Equation 2.If defines the upper stability boundary for H O. The standard change
2
in free energy for reaction 2.Ib equals 0 by definition, and the lower boundary calculated
similarly as above is:

Eh . 0 0592 pH (2.Ig)
The boundaries between the stable species can be calculated in a similar way using
the basic thermodynamic data and the Nernst equation. Sections 5.6 and 5.7 further
elucidate the pH–Eh diagrams for iron and manganese species.

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