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330 ELECTROCHEMISTRY

Measurement with the hydrogen electrode The SHE is the primary reference
electrode, so other half-cell potentials are measured relative to its potential. In practice,
if we wish to determine the value of E O then we construct a cell of the type
M n+ ,M
+
n+
Remember that the Pt|H 2 (a = 1)|H (a = 1)||M (a = 1)|M
proton is always sol-
vated to form a hydro- Worked Example 7.19 We immerse a piece of silver metal into a
solution of silver ions at unit activity and at s.t.p. The potential across
xonium ion, so a (H 3 O + )
= a (H + ) .
the cell is 0.799 V when the SHE is the negative pole. What is the
standard electrode potential E O of the Ag , Ag couple?
+
The words unit activ-
By definition
ity here mean that
the activity of silver
ions is one (so the emf = E (positive electrode) − E (negative electrode)
system perceives the
Inserting values gives
concentration to be
1mol dm −3 ).
0.799 V = E O − E (SHE)
+
Ag ,Ag
Just because a half- 0.799 V = E O
+
reaction appears in a Ag ,Ag
table is no guarantee
because E (SHE) = 0.
that it will actually The value of E in this example is the standard electrode
+
work; such potentials Ag ,Ag
potential because a (Ag )
+ = 1, and s.t.p. conditions apply. We say that
are often calculated.
O
E Ag ,Ag = E Ag ,Ag = 0.799 V versus the SHE. Most of the values of
+
+
E O in Table 7.7 were obtained in a similar way, although some were
The SHE is sometimes
erroneously called a calculated.
We should be aware from Table 7.8 that the SHE is an ideal device,
normal hydrogen elec-
trode (NHE). and the electrode potential will not be exactly 0 V with non-standard
usage.
Secondary reference electrodes
The SHE is experimentally inconvenient, so potentials are often measured and quoted
with respect to reference electrodes other than the SHE. By far the most common
reference is the saturated calomel electrode (SCE). We will usually make our choice
of reference on the basis of experimental convenience.

The SCE
Note that the ‘S’ of
‘SCE’ here does NOT
mean ‘standard’, but By far the most common secondary reference electrode is the SCE:
‘saturated’.
Hg|Hg Cl 2 |KCl(sat’d)|
2