Encyclopedia of Environmental Science and Engineering, Volume I and II

1266 WATER CHEMISTRY

set equal to unity. This means that the activity of the solid phase

is implicitly contained in the solubility equilibrium constant.

Therefore, experimental differences from precisely known

equilibrium constants can be used to deduce information about

the constitution of the solid phase.

There are various factors that affect the activity of the

solid phase: (1) the lattice energy, (2) the degree of hydra-

tion, (3) solid solution formation, (4) the free energy of the

surface and (5) the presence of constituents affecting the

purity of the solid.

The solids occurring in nature are seldom pure sub-

stances. For example, isomorphous replacement by a foreign

constituent in the crystalline lattice is an important factor by

which the activity of the solid phase may be decreased.

Redox Equilibria and Electron Activity

There is a conceptual analogy between acid-base and

oxidation-reduction reactions. In a similar way that acids

and bases have been interpreted as proton donors and proton

acceptors, reductants and oxidants are defined as electron

donors and electron acceptors. Since free electrons do not

exist in solution, every oxidation is accompanied by a reduc-

tion and vice versa. An oxidant is thus a substance which

causes oxidation to occur, while itself becoming reduced.

The oxidation states of the reactants and products change

as a result of the electron transfer which mechanistically may

occur as a transfer of a group that carries one or more elec-

trons. Since electrons are transferred in every redox reac-

tion they can be treated conceptually like any other discrete

reacting species, namely, as free electrons. The following

redox reaction is illustrative:

O 2  4H  4e  2H 2 O reduction

4Fe^2   4Fe^3  4e oxidation

O 2  4Fe^2   4H  4Fe^3   2H 2 O redox

reaction (27)

2

4

6

8

10

(^6810) pH 12
Zn+2
Fe+2
SR+2
CO+2 Ks 0
CT
(^12)
(^12)
(Ks 0 )
Mg+2
CO 3
–log CONC. (M)
–2
FIGURE 7 Solubility of carbonates in solutions of con-
stant total dissolved carbonate carbon. The maximum soluble
metal ion concentration ([Me^2 ]  ks0/[(CO 32 ]  Ks0/a 2 CT)
is shown as a function of pH for CT  10 2.5 M. The acid-
ity constants for H 2 CO 3 * are indicated on the horizontal axis.
Dashed portions of the curves indicated conditions under
which MeCO 3 (s) is not thermodynamically stable due to
the formation of the more stable solid hydroxide or oxide.
Ref.: Stumm, W. and J. Morgan, Aquatic Chemistry, Wiley-
Interscience, New York, 1970, p. 179.
FIGURE 6 Solubility of amorphous Fe(OH) 3 , ZnO, and CuO. The
equilibrium solubility curves for the hydroxo metal complexes and
for the free metal ion have been combined to yield the composite
curve bordered by the cross hatching. The constituent curves were
constructed from the data in Table 3. The possible occurrence of poly-
nuclear complexes, for example Fe 2 (OH) 22 , CU 2 (OH) 22 , has been
ignored. Such complexes do not change the solubility characteristics
markedly for the solids considered. Also ignored is complexing with
other ligands such as NH 3. Ref.: Stumm, W. and J. Morgan, Aquatic
Chemistry, Wiley-Interscience, New York, 1970, p. 173.

• log CONC.


• log CONC.

ZnO(s) CuO(s)

CuOH+

Cu+2

ZnOH+

ZnOH 3 –

am-Fe(OH)* 3 (s)

8

8

8

(^66)
6
4 4
(^22)
Zn(OH)–2 4
7911 pH
Zn+2
(^1012) pH
Cu(OH)–2 4
Cu(OH) 3 –
a b
(M)
(M)
Fe+3 FeOH+2
Fe(OH) pH
2
2 4 6 8 10 12
Fe(OH) 4
2
4
6
8
c

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