Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 99

lose electrons more readily than hydrogen and, therefore, be negative

with respect to the hydrogen electrode. Table 4.2 gives the standard

redox potentials of some common metals. By convention the

oxidised state is always written on the left-hand side.

Redox half-reactions are often written for brevity as, for example,

Li"" + e' Li with the state symbols omitted. The electrode

system represented by the half-reaction may also be written as

Li+/Li. The standard redox potentials for ion-ion redox systems

can be determined by setting up the relevant half-cell and measuring

the potential at 298 K relative to a standard hydrogen electrode.

For example, the standard redox potential for the half-reactions

can be determined by measuring the potential of a half-cell, made

1 molar with respect to both iron(II) and iron(III) ions, and in which

a platinised platinum electrode is placed, relative to a standard

hydrogen electrode at 298 K.

Mercury(l)

Saturated KCl

solution

Mercury

chloride WJj~~ Hole

^^ sleeve

Ring

Figure 4.5

For many purposes the hydrogen electrode is not convenient and
it can be replaced by another cell of known standard electrode
Potential. A well-known example is the calomel cell shown in
figure 4.5.
A number of redox potentials for ion-ion systems are given in
Table 4.3; here again, state symbols are often omitted.