Chemistry Reference
In-Depth Information
structure, where the holders of the cell are positioned (Fig. 9.12c), pressure
is applied (to avoid leakage of electrolyte solution), which causes a sup-
pression of the knitted structure. The latter is not obtained in the opening
of the electrochemical cell (to make contact with the electrolyte solution),
and the structure adapts to this opening as shown in Fig. 9.12c. In this case,
the actual distance between the knitted electrodes in the electrochemical
cell is smaller compared with the geometrical distance, explaining the shift
of the calibration curve to somewhat smaller impedance values than the
curve obtained for palladium electrodes. From the shift of about 1.5%, it
was possible to obtain the actual distance between the electrode being
d
ยข=
101 mm instead of
d
= 103 mm.
Study of the electrode surface area
In this section, the influence of the electrode surface area of woven, non-
woven and knitted textile electrodes at the impedance of the electrochem-
ical cell was studied. This was done for various electrolyte concentrations
and a constant distance between the electrodes of 103 mm. Note that this is
the geometrical distance (
d
), which can vary over a range of 1-2% depend-
ing on the type of textile electrode that is used. The data in Fig. 9.13 show
the relationship between the logarithm of the electrolyte resistance, which
is in fact the measured impedance of the cell, and the logarithm of the elec-
trode surface area recorded at palladium and textile electrodes. It can be
seen that good correlation is obtained between the different electrodes,
except for the smallest electrode surface area. This can be explained by the
non-ideal behaviour of textile electrodes with regard to the behaviour of
palladium sheet electrodes. For each electrode, edge effects should be taken
into account, which are much more expressed at textile electrodes, because
of their rough structure. However, if the surface of the electrode is large
enough, the bulk properties determine the overall signal, and edge effects
can be neglected. This is the reason why edge effects only start to play a
role at the smallest electrode surface. The data in Fig. 9.13 (and addition-
ally obtained data) show that edge effects cannot be neglected for the
textile electrodes if the ratio of electrode surface area to loop is < 2.5 mm,
obtained from following equation:
Ar
p
2
r
=
=
[9.9]
loop
2
p
r
2
It should be taken into account that this is only valid for the textile elec-
trode investigated in this work, because this parameter is also dependent
on the roughness of the surface. The roughness is definitely the cause of this
edge effect, because it is absent when using smooth palladium electrodes.
