Page 53 - Soil and water contamination, 2nd edition

40 Soil and Water Contamination

In comparison, addition of 0.020 mol HCl to 1.0 l distilled water would result in a pH
of 1.70.

2.10 REDOX REACTIONS

2.10.1 Introduction

Oxidation and reduction (redox) reactions constitute a major class of geochemical and
biochemical reactions that control the form of species such as oxygen , iron, sulphur,
nitrogen, and organic materials, and their distribution in water and sediment. Redox
reactions involve the transfer of electrons between atoms. Reductants and oxidants are
defined as electron donors and electron acceptors, respectively, in an analogous way that acids
and bases can be considered as donors and acceptors of protons . For example, the oxidation
2+
3+
reaction of ferrous iron (Fe ) to ferric iron (Fe ) can be written as:
Fe 2 + Fe 3 + + e (2.57)
-
where e represents the electron. There are no free electrons, so oxidation reactions are always
coupled to a simultaneous reduction reaction: for example, the reduction of oxygen (O ):
2
O + 4 H + + e 4 2 H O (2.58)
2 2
Adding the so-called half reaction s (2.57) and (2.58) and balancing them according to the
number of electrons on each side of the equation results in the following full redox equation
of the oxidation of iron (reductant ) by oxygen (oxidant ):

4 Fe 2+ + O 2 + 4 H + 4 Fe 3+ + 2 H 2 O (2.59)

Thus, an oxidant is a substance that causes oxidation by accepting electrons from a
reductant , while being reduced itself. Many redox reactions proceed extremely slowly, except
for those mediated by microorganisms – though these too may be rather slow. This implies
that the concentrations of oxidisable and reducible substances may very well deviate from
equilibrium concentrations predicted thermodynamically and that, besides thermodynamic
equilibrium, the reaction kinetics also have to be considered. This section, however,
concentrates on redox equilibria.

2.10.2 Oxidation state
The electron transfer between atoms causes a change in the oxidation state of the reactants
and products. The oxidation state represents the hypothetical charge that an atom would
have if the molecule or ion were dissociated. This hypothetical assignment of electrons to an
atom is carried out according to the following rules (Stumm and Morgan, 1996):
1. The oxidation state of a monoatomic substance equals its electronic charge
2. In a covalent compound, the oxidation state of each atom is the charge remaining on the
atom if each shared electron pair is fully assigned to the more electronegative atom (i.e. the
atom that has the largest tendency to accept electrons) of the two atoms sharing them. An
electron pair shared by two atoms of the same electronegativity is split between them.
3. For molecules, the sum of the oxidation state s equals zero and for ions it equals their
formal charge.

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