This need not be
always true Nernst showed that for the electrode reaction:
Mn+(aq) + ne–® M(s)
the electrode potential at any concentration measured with respect to
standard hydrogen electrode can be represented by:
(
)
(
)
+
+
=
n
n
o
M
/ M
M
/ M
E
E
 –  RT
nF  ln [M]
[M
]
n+
but concentration of solid M is taken as unity and we have
(
)
(
)
+
+
=
n
n
o
M
/ M
M
/M
E
E
 – 
RT
nF  ln
n+
1
[M
]
(2 8)
(
)
n+
o
M
/ M
E
has already been defined, R is gas constant (8 314 JK–1 mol–1),
F is Faraday constant (96487 C mol–1), T is temperature in kelvin and
[Mn+] is the concentration of the species, Mn+