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70 Chapter 4
inert conductor. The standard hydrogen electrode (SHE), discussed
further in the next section, is one example of a gas electrode.
Hydrogen gas is bubbled around the platinum foil, covered with very
finely divided platinum and immersed in a solution of H+ ions.
In order to write the reduction half-reaction for a gas electrode, the
following procedure is adopted:
1. Identify the gas involved, e.g. Hz(~), Clz,,), 02(g), etc.
2. Write down the corresponding ion associated with the gas, i.e.
H + (as), Cl-(aq), etc.
3. For each couple, state the oxidation number of the atom in each

4. From step 3, identify the species with the greatest oxidation
number and place it on the left-hand side of the equation and the
species with the lowest oxidation number on the right-hand side
to show the reduction, i.e.

+ (as) + H2k)
I 0
C12,) -+
0 c1- (as)
-I
5. Balance the charges with the appropriate number of electrons in
each case:

e.g. H+(aq) + e -+ HW
I 0
i.e. 2H+(,) + 2e -+ Hag)
and
Clz,,, + e + c1- (as)
0 -I

(d) Metal-Insoluble Salt Anion Electrode (e.g. Figure 6.6)
This electrode consists of a metal, M, surrounded by a layer of
insoluble salt M+X-, immersed in a solution of X- ions. The typical
example of this type of electrode involves the standard silver chloride
precipitation reaction, i.e. Ag+(aq) + Cl-(aq) -+ AgCl,,). The electrode
consists of a bar of silver metal, A&,), surrounded by a layer of
AgCl,, salt, immersed in a solution of chloride Cl-,,,) anions.