Page 54 - Soil and water contamination, 2nd edition

Basic environmental chemistry 41

The oxidation state is indicated by roman numbers: for example, Fe(II) for ferrous iron and
Fe(III) for ferric iron . Since O is highly electronegative and H is electropositive, as a rule of
thumb it may be assumed that O is always in the 2- oxidation state (except in O ) and that
2
H is always in the 1+ oxidation state (except in H ), which simplifies the calculation of the
2
oxidation state of other elements.
Example 2.14 Oxidation state

2-
Derive the oxidation state of chromium (Cr) in chromate (CrO )
4
Solution
Because O is in the 2- oxidation state , the total negative charge due to the 4 oxygens is
(8-). Subtracting the total negative charge of the chromate ion (2-) gives a net charge of
(6-) for the four oxygen atoms. Hence, the oxidation state of the chromium atom must
be 6+.

2.10.3 Redox potential
Natural systems are characterised by multiple equilibria and the tendency of the system to
donate or accept electrons is represented by the redox potential , analogous to the pH, which
represents the tendency of a system to donate or accept protons . The voltage or electromotive
force developed by a redox reaction is related to the free energy of a system:
G
E (2.60)
nF
2 -2
2 -2
-1
where E = the potential [M L T Q ], ΔG = the change in free energy [M L T ], n = the
-1
amount of electrons transferred in the reaction [mol], F = the Faraday constant [Q mol ],
-1
i.e. the charge of 1 mol of electrons (F = 96.490 C mol ). Every redox reaction like Equation
(2.59) can be written in its general form:
aA + bB cC + dD (2.61)
red ox ox red
where the subscripts red and ox denote respectively the reductants and oxidants participating
in the reaction. The change of free energy for this reaction is (see Equations 2.35 and 2.36):
[ C ] c [ D ] d
0
ox
red
G G RT ln a b (2.62)
[ A ] [ B ]
ox
red
Combining Equations (2.60) and (2.62) yields:
RT [ C ] c [ D red ] d
ox
0
E E ln a b (2.63)
nF [ A ] [ B ]
red
ox
2 -2
-1
0
where E = the standard potential [M L T Q ], which is the potential with all substances
present at unit activity at 25 °C and 1 atmosphere pressure. The redox potential for a given
redox system is now defined as the potential relative to the potential of a hydrogen electrode,
which is an electrode at which the equilibrium :
2H + + 2e H (2.64)
2

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Soil and Water.indd 53 10/1/2013 6:44:18 PM