Chemistry Reference
In-Depth Information
2.4.5
Experimental detection of impedance
Electrical stimulation
As described in section 2.4.2, an impedance experiment involves the
application of an electrical signal (E or O) and the measurement of an
electrical current or a potential. Consider that a potentiostatic experiment
(application of a potential) with three types of input signals can be used for
determination of the impedance
49
.
A first method consists of a measurement of an electrical current as a
function of time, while applying a potential step at
t
= 0:
()
=
Et
E
step
for
for
t
≥
0
()
=
Et
0
t
<
0
The impedance is obtained using Fourier transformations, on the condition
that
E
step
is small enough to obtain a linearly dependent output signal. The
simplicity of this experimental approach is one of its advantages. A second
method is based on the application of a (continuous) noise signal to the
system. Also in this case, Fourier transformations are used to obtain the
impedance. An advantage of this method is its short measuring time
because only one signal must be applied. Despite the advantages of the pre-
vious methods, most researchers use a third method: application of an alter-
nating potential signal with frequency w and measurement of alternating
current. With Equation 2.41 and related equations, the impedance is
obtained. The most important reason to use this method is that the exist-
ing equipment allows measurements in the frequency range of 1 ¥ 10
-4
-
1 ¥ 10
6
Hz. This assumes that the frequency range can be chosen and varied
according to the type of system being studied.
Impedance of two- and three-electrode systems
Before the introduction of potentiostats in the early 1960s, the study of elec-
trode processes was done mainly with two-electrode systems in which the
functions of the reference and the counter electrode were unified in one
simple electrode. Such an electrode is a non-polarisable electrode with a
relatively large surface to be sure that it can conduct a certain amount of
current due to occurring electrochemical processes.
A potentiostatic, three-electrode circuit allows the separation of both
functions physically: for the reference potential, a non-polarisable electrode
is used (a calomel or Ag|AgCl reference electrode), while the electrical-
current conducting electrode is an inert metal electrode. With electro-
chemical, direct-current methods, the effect of this modification is limited
to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by
