Chemistry Reference
In-Depth Information
6.5
y
= -0.6828x + 6.9472
R
2
= 0.9709
6.0
5.5
y
= -0.6808x + 5.8745
R
2
= 0.9676
4
5.0
4.5
y
= -0.6806x + 4.9806
R
2
= 0.9824
3
4.0
3.5
y
= -0.6673x + 3.9933
R
2
= 0.9774
2
3.0
2.5
1
2.0
1
1.5
2.0
2.5
3.0
Log
A
(mm
2
)
9.6
Logarithmic plot of the impedance at zero phase-angle shift as a
function of electrode surface area obtained from the
electrochemical cell
d
=
112 mm,
T
=
298.0 K and an electrolyte
concentration of (1) 1
¥
10
-
1
, (2) 1
¥
10
-
2
, (3) 1
¥
10
-
3
and
(4) 1
¥
10
-
4
mol l
-
1
.
trolyte concentration (1 ¥ 10
-1
-1 ¥ 10
-4
mol l
-1
), values for the impedance
were obtained as a function of electrode surface area, while keeping in mind
that the distance between the electrode was kept constant at a value of
d
=
112 mm. As also mentioned above, the impedance, when no phase-angle
shift is observed, is the most important factor for this research. It is obtained
at the cross-section of the x-axis and the completion of the semi-circle. The
data obtained are shown in Fig. 9.6 as a logarithmic plot for electrode sur-
faces of 19.63, 78.5, 176.6, 314 and 490.6 mm
2
(corresponding to diameters
of 5, 10, 15, 20 and 25 mm, respectively).
It can be seen that linear relationships with a slope around -0.68 are
obtained for all electrolyte concentrations. However, the correlation coef-
ficients are rather poor. This can be explained by the presence of edge
effects, which become more important when the electrode surface decreases
(electrode surface area decreases with
r
2
; edge effects decrease with
r
).
Indeed, not taking into account the data of the smallest electrode surface
area (with
r
= 2.5 mm) results in correlation coefficients in the range of
0.980-0.995 and a slightly lower slope of -0.65. However, in order to cover
a wide range of electrode surface areas, it is preferred to use all the data
given in Fig. 9.6, even for those electrodes with small surface area. The rela-
tionship between impedance and electrode surface area can be inserted in
Equation 9.2 to produce Equation 9.3:
()
◊
RkdA
=
-
068
.
◊
c
-
1
[9.3]
2
