Chemistry Reference
In-Depth Information
A first disadvange is that when the electrode potential is changed, some
capacitive current will flow, which can influence the measured data.
5.
Owing to the small thickness of the double layer and the fact that the
experiment causes electrochemical reactions by application of a poten-
tial, an electrical field is created over the double layer, which can reach
a magnitude of 1 ¥ 10
9
Vm
-1
.
6.
With every electrochemical reaction, consumption of electroactive
species at the interface is involved. This means that fresh electroactive
species should be transported towards the interface by diffusion, con-
vection and/or migration. Migration, in particular, can cause significant
problems and is very difficult to interpret and quantify. However, migra-
tion can be suppressed by using a supporting electrolyte in the solution.
1.3
Conventions for presentation of
electrochemical data
In the past, different sign conventions were used in electrochemistry, which
led to difficulty in interpretation of experiments and results. Consequently
the electrochemical literature requires an understanding of this problem to
avoid confusion. The approach followed in this topic is summarised in this
section. As pointed out in the previous section, all electrochemical cells are
regarded as a combination of two half cells, with each of the latter repre-
sented by a half reaction written as a reduction:
OeR
+Æ
n
-
[1.6]
As will be explained in more detail further in this topic, each half-reaction
corresponds to a potential and can be calculated by means of the Nernst
equation
3-5
:
a
a
R
T
0
EE
=+
0
ln
[1.7]
n
F
R
where
E
is the potential,
E
0
is the standard potential, R is the gaseous con-
stant (8.317 J mol
-1
K
-1
),
T
is the temperature (K),
n
is the number of elec-
trons exchanged in the electrochemical reaction, F is the Faraday constant
(96 485 C mol
-1
), and
a
is the activity of oxidant (O) and reductant (R). The
half cell with the most positive potential will be the anode, with the other
one the cathode, and the measurable cell potential will be the difference
between the two half-cell potentials.
In addition, in the graphical presentation of data, different conventions
are used, giving rise to problems in the interpretation and comparison of
results. However, a good convention will not cause any problems. In this
topic (Fig. 1.2) positive potentials are directed to the right of the origin, and
anodic (oxidation) currents are taken as positive (directed upward from the
