
55-6 Chemical Variables
e classical form of the Nernst equation (Equation 55.10) can be formulated more generally for a
redox reaction. If a
ox
and a
red
are the activities of the oxidized and reduced form of the considered ion,
the Galvani potential dierence is given as
∆ ∆
φ φ
= +
0
RT
zF
a
a
ln
ox
red
(55.12)
In potentiometry, the activity of a certain ion can be determined directly by the measurement of the
equilibrium Galvani potential dierence of a suitable electrode (direct potentiometry). On the other
ha
nd,
ch
anges
of th
e
ac
tivity
of th
e
de
tected
io
n
an
d
eq
uivalence
po
ints
(E
Ps)
ca
n
be de
tected
in ti
tra-
tion
r
eactions
(p
otentiometric endpoint titration ...