Environmental Engineering Reference
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double layer and charge transfer resistance (
R
ct
) at the electrode|solution interface. The
estimated equivalent circuit for the impedance spectra of figure 5 is shown in figure 6 based
on the Randles circuit. The solution resistance (
R
s
) was determined from the equivalent
circuit, and then
D
app
of the Fe(CN)
6
3-
was calculated.
Figure 6. Electrochemical redox system along with its equivalent circuit [3].
The concentration dependence of
D
app
of Fe(CN)
6
3-
determined by EIS is shown in figure
7 a).
D
app
was estimated also by a PSCA method under a potential step from -0.2 (vs.
Ag|AgCl|KCl
sat
) to +0.5 V and is shown in figure 7 (b). It is remarkable that the diffusion of
the Fe(CN)
6
3-
takes place in the solid to the same extent as in an aqueous solution. The
D
app
values from both measurements (figure 7 a) and b)) were almost the same, and were almost
independent of the Fe(CN)
6
3-
concentration. The
D
app
obtained by EIS was slightly lower than
that by PSCA; since in the PSCA measurement direct current is used in the potential step, a
concentration gradient generated in the double layer would cause slightly higher
D
app
value.
The
D
app
of Ru(bpy)
3
2+
in the 2 wt%κ-carrageenan solid (4.5 × 10
-6
cm
2
s
-1
) was about 70% of
the
D
app
in the 2 wt% agarose solid (6.7 × 10
-6
cm
2
s
-1
). This result indicates that the diffusion
of Ru(bpy)
3
2+
in theκ-carrageenan solid is suppressed slightly by the anionic groups of the κ-
carrageenan.
The contact angle of water on the present solid surface was almost zero showing that the
surface is super-hydrophilic. The concentration dependence of the charge transfer resistance
(
R
ct
) at the electrode|solid interface is shown in figure 8 a).
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